The Astron Power Supply Index Page at Repeater-Builder.com

Back to Home Modifications,Schematics and Service Information on Astron™ Power Supplies
Originally Compiled, HTML'd and Maintained by Mike Morris WA6ILQ
Formerly Maintained by Robert Meister WA1MIK (SK)
Currently Maintained by Mike Morris WA6ILQ

www.astroncorp.com
Click here or on the link or logo above to go to Astron's web site
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Contact information:
Astron Corporation
9 Autry
Irvine, CA 92618 USA
949-458-7277 from 8am to 5pm Pacific Time
Fax: 949-458-0826
Sales or purchase questions: sales /at/ astroncorp /dot/ com
Questions or Tech support: astron /at/ astroncorp /dot/ com

The above two email addresses are deliberately NOT links
to avoid having Astron being bombarded by spambot email. Note 1: There is (was?) an Astron Wireless Technologies, Inc. (at one point it was called Astron Antenna Company) in Virginia, USA that makes (made?) some nice antennas. There is no connection between that company and the Astron Corporation that makes power supplies.

Note 2: There are prices mentioned in a few places on these Astron web pages. Please use them as guidelines only as they were relevant at the time of the initial writing of the particular page (this page was created around 2000). Some prices were updated, some were not. We'd appreciate an email if you discover a significant change.

Astron has been making and selling linear power supplies since 1975, DC-DC converters since 1980 and switching supplies since 1996. The basic design of the linear supplies is straight out of the Fairchild Semiconductor (the original chip manufacturer) 1968 Applications Note for the 723 regulator chip. (opens in a new browser tab)

The Pyramid Gold Series linear power supplies are similar in design and capacity to the Astron supplies and have their own web page at this web site. The Pyramids are also based on the popular 723 voltage regulator.

DONATIONS OF ADDITIONAL INFORMATION FOR THIS PAGE WOULD BE GREATLY APPRECIATED
(actually any page at this web site)

You should read this Astron Introductory Information article before any of the other articles here in the Astron section.
It has some very useful information on the Astron linear and switching power supplies, with background, history, model-specific information, both repair and modification suggestions, photos, and more.

Quick Jump to: (the Modification Articles section also includes the battery backup modifications)

Linear Power Supplies Slim-Line Linear Power Supplies Switching Power Supplies Modification Articles Repair and Operation Articles Power Supply Load Testing Power Supply Model Information Reset Circuits Regulated Switching Voltage Converters (DC-DC converters) 28 Volt Linear Power Supplies
The Astron factory says that their "28 volt" supplies will adjust down to 22-23 volts for use with equipment that operate on 24 volts.

Repair and Operation Articles:

Repairing Astron 13.8 volt Linear Supplies by Robert W. Meister WA1MIK (SK)
General linear power supply circuit explanation, followed by the most common problems, diagnostic techniques, and repairs for these units.

Annotated RS35M Schematic by Jim Larsen AL7FS (169 KB PDF)
The December 2005 issue of QST had an article in their "Hands-On Radio" series titled "Power Supply Analysis" You can download a PDF of the QST article - it explains how a linear power supply works, and uses an Astron RS‑35 as the "victim". Jim Larsen, AL7FS (the ARRL Alaska Section Manager) took that QST article and went one step better — he produced an annotated schematic that takes advantage of the annotation feature of the PDF file architecture. The QST article and the annotated PDF are really worth studying. Just mouse over the yellow folder symbols and they will pop up. If your browser does not pop the notes then download the PDF and use the free downloadable Adobe Acrobat Reader to open it.

Troubleshooting Astron Linear Supplies by Jim Ussailis W1EQO (630 KB PDF)
From the Yankee Clipper Contest Club's "Scuttlebutt" newsletter Issue 232, August 2015.

Installing a New Regulator Board in an Old Astron Power Supply by By Tom Dailey WØEAJ
After his regulator board burned up, Tom bought a new one from Astron. Seems they've made a few improvments in the design since his supply was made. This article describes what he discovered and how to deal with putting a new board into an old supply.

Refreshing / Maintaining Astron Linear Supplies by Robert W. Meister WA1MIK (SK)
Common maintenance things you can do to keep your Astron supply running for many years to come.

An evaluation of a 2009 Astron RS35M supply by Stu Martin K2QDE (2.5 MB PDF)
Stu opened his brand-new supply and took several photos, showing what's been added or changed. He scanned then redrew and corrected the schematic so it matched the actual unit; this drawing can be found as a PDF file below.

Restoring / Rebuilding an Astron RS35M by John Keith W5BWC (1.4 MB PDF)
While John titled this as a "restoration" it's actually a complete gut job starting with the chassis, transformer, filter capacitors, the meter and a few other parts remaining. Everything else gets replaced with a completely different design.

Crowbar SCRs in Astron 13.8V Linear Supplies by Robert W. Meister WA1MIK (SK)
A summary of SCRs used by Astron and some available replacements.

Main Capacitors and AC Line Fuses in Astron 13.8V Linear Supplies by Robert W. Meister WA1MIK (SK)
A summary of main filter capacitors and AC line fuses found in Astron supplies.

LM723 Regulator Operation by Robert W. Meister WA1MIK (SK)
A short explanation of what's going on inside the regulator IC in an Astron power supply.

Understanding and Using 723 voltage Regulators by William C. Cloninger, K3OF (SK) 1.8 MB PDF
This is an article out of Ham Radio (hr) Magazine for March 1989. He shows several homebrew supplies as he walks you through the basics. Worth reading.

Parallel Operation of Astron Linear Power Supplies by Robert W. Meister WA1MIK (SK)
The secrets of that "For Parallel Operation Only" binding post on the rear panel.

Adjusting the Output voltage of Astron Linear Supplies by Robert W. Meister WA1MIK (SK)
So easy, a visually handicapped (blind) person can do it!

Astron BB-30M Data Sheet and schematic 346 KB PDF Photo 1 Photo 2 Photo 3 Photo 4
The BB-30M is an interesting and unique device in the Astron product line. It is a 30 amp stand-alone battery backup module that works with any 12 volt or 24 volt system. It switches the load from a DC voltage source to a battery upon power failure without any dropout. It also can maintain the charge of the battery with a constant-voltage trickle charging circuit (of about 3.5 amps max and the charger can be disabled by lifting one leg of R28, the only 22 ohm 10 watt resistor inside the box). Internally this unit is a lot more complex than I ever imagined it would be. Note that there is about a 0.4 to 0.5 volt drop across the unit.

RS-50A / RS-50M / RM-50A / RM-50M Service Manual by Astron, transcribed by Bob WA1MIK (SK) (130 KB PDF)
Specifications, circuit description, theory of operation, servicing, adjustments, parts list, schematic for the 50 Amp rack-mount and desk-top power supplies.

If your linear power supply regulator board is beyond repair, you can buy a brand new one from Astron for about US$25 plus shipping. They will even ship it to you and let you remit payment after you receive it. Tell them the supply model number and they will customize it for you. Here are 300kB photos of the component side and the solder side of a brand new regulator board ordered in 2014 for a VS-35 power supply, courtesy of Bob McKinlay VE3DJ. The blue adjustable potentiometer marked "1K" on the board (visible in the component side photo above and labeled "R5" on the solder side photo above) adjusts the power supply's maximum output voltage, typically from 11 to 15 volts (22 to 32 volts on 24 volt LS and VLS power supplies). See the article above for a more detailed procedure.

Mike WA6ILQ reports that he's found a few cases where the screws that mount the voltage regulator board to the filter capacitors were not tight. Suspecting thermal cycling he now adds star washers under the screw heads on both the + and - capacitor connections before he puts the supply into service. Another incident had one hole of the filter capacitor not drilled to the correct depth before tapping / threading, and therefore the screw bottomed out and did not adequately "pinch" the circuit board and make good contact with the copper pad. The added star washer between the screw head and the board corrected that issue as well.

Stu K2QDE reports "I recently learned that very light to unreadable front panel silk-screening existed in a number of production Astron power supplies around the time I purchased mine in mid-2017. I searched for vendors that could make a new stick-on label. I sent along a laser printout of an SS-55M front panel as a sample. These are plastic, self-adhesive strips, slightly larger and brighter than factory silk-screening. The Astron logo is slightly sharper than the silk-screened version. Perfect when refurbishing your Astron supply. Click here to see a photo. Contact sonia [ at ] kellylaserworks [ dot ] com for more information.

As of 2020 Astron is now selling many of the more common parts that can be used to repair or upgrade your power supply such as the power switch, meters, illumination kits, pass transistors, bridge rectifiers, capacitors, and the crowbar SCR. Check the Astron web site for pricing and availability. Don't hesitate to send them an inquiry email if you don't see the part you need.

Crowbar Reset Circuits: Hint: Add this to any remotely located / hilltop Astron!

The Astron power supply design includes a crowbar circuit, so-called because it essentially drops a dead short across the output of the supply. This short is accomplished by triggering a high current SCR. Astron says the overvoltage protect circuit is reset by turning the power off for between 5 and 10 seconds. The few seconds allows the output voltage bleed down to zero and that is what actually resets the SCR.

Unfortunately this requires you to actually be physically next to the Astron to power cycle it, which can be very inconvient if the Astron is at the repeater site and you are miles away.

Astron later developed an electronic reset circuit that detects the loss of output voltage when crowbar circuit activates and provides automatic crowbar reset of a temporary over-voltage condition. This circuit accomplishes this by shutting down the LM723 voltage regulator chip for several seconds by momentarily grounding pin 13. This removes the output voltage of the Astron for the duration of the momentary ground. This zero output voltage resets the crowbar circuit. The normal output voltage is then restored when the momentary ground is removed. This circuit is packaged as an add-on module called an OVPR-12V.

The OVPR-12V an internal reset circuit for the Astron Linear series A reset circuit provided by The Astron Corporation – they call this board an OVPR-12V – by Kevin K. Custer W3KKC
This circuit shoould have been included in the original Astron regulator board design. If you are inside your Astron for some other reason it's worth adding this circuit as long as you have the cover off.
Just remember to print the page and leave a copy inside the case so the next guy will know what the little circuit board is for.

Astron's own writeup on the OVPR-12V donated by Joe Orrico WB6HRO (52 KB PDF)
Astron's documentation shows an extra LED indicator, which is not actually part of the OVPR-12V.

A photo of Astron's OVPR-12V board (found on the web).

An external reset circuit for the Astron Linear series A reset circuit by Kevin K. Custer W3KKC
You can use this if you don't want to (or can't for political reasons) open up the Astron.

Some of the later supplies have a capacitor across the the SCR gate and ground (C102 on the RM-60 diagram), This cap reduces false tripping of the SCR in RF environments. If your supply does not have it I'd add it.

Load Testing / Testers:
Once you've repaired your power supply you need to test it. Do you really want to risk your expensive HF, VHF or UHF radio as a test load?

A Tutorial On Power Supply Load Testing By Eric Lemmon WB6FLY (SK)
Worth reading… Eric was a regular poster on the repeater-builder mailing list for over a decade. He did a number of power supply tests and posted his results. Someone asked him what tools he used and how did he perform the tests and this document was the result.

Power Supply Dummy Load by Tony King W4ZT (SK) (125 KB PDF)
Tony designed and built a test load made up of common 12 volt automobile stoplight bulbs. You can get them cheap from a do-it-yourself (sometimes called a "pick-your-part", "pick-a-part" or "you-pull-it") auto junkyard. Don't go cheap and leave off the top protection plate, the stoplight bulbs get very hot very quickly and can give you serious burns. This is the voice of experience of your page maintainer.

High Power DC Load for Power Supply and Battery Evaluation by Phil Salas AD5X (1.53 MB PDF)
This is from the October 2006 QST. Take this article as a source of ideas, an AC fan may be cheaper. And this unit has a lot of heat generating ability, you might want to consider using open construction like the unit Tony King built.

Your page maintainer once saw a test load made up of a number of old automobile headlights, with one pull-push headlight switch per bulb (like the switch used in a 1960s-1970s Ford or Chevy. The owner had bought the bulbs, sockets and switches from an antomobile junkyard. Each switch controlled one headlamp - when pulled half way out the low beam came on and when pulled all of the way out both the low beam and the high beam were connected.

While the article is an evaluation of an Astron supply there is an interesting homebrew test load shown here that uses automobile halogen light bulbs (look at the second photo from the top). Be careful, halogen bulbs can get much hotter than tungsten bulbs. (Off-site pointer, the link opens in a new browser tab)

Another test load that your page maintainer witnessed was used on an RM-50 or RM-70 supply. The load was a long length of galvanized 1/2 inch water pipe in a "U" shape and the owner used a set of automobile jumper cables. He clamped the negative cable to one end of the pipe, and then slid the positive jumper cable along the pipe until he had the desired amps of current flow.

Over a decade ago your page maintainer saw a homebrew test load made up of several low-ohm / high-wattage resistors which were bolted to a length of railroad rail two and one-half feet long. It had a large heavy knife switch for switching the load on and off. Yes, it was very heavy! (over 100 pounds / 45.36 kg of steel!) North american railroad rail weight is measured in pounds per yard, and most USA main line rail is "130 pound rail" which works out to just under 44 pounds per foot).
There are eBay sellers that have metal body power resistors in values of 0.1 ohm, 0.22 ohm, 0.33 ohm, 0.47 ohm, 0.68 ohm, 1 ohm, 2 ohms, 3 ohms and many higher values and in 5 watts ($2.17), 10 watts ($2.18), 25 watts ($2.30), 200 watts (US$14.60), 300 watts ($28.31), and 500 watts ($36). As an example, look at this resistor and look at the options on ohms and watts. (off-site pointer, the link opens in a new browser tab) You'd mount them to a large heat sink with thermal compound (just like a power transistor).

At 13.8 volts for an RS-35 you could use a 0.68 ohm and a 1 ohm resistor in parallel. They would provide a load of 34.1 amps. Close enough! There is about 470 watts spread across both resistors (add a fan!), but the 0.68 unit is dissipating 280 watts so I'd use the 500 watt units for a total of $72… And I'd use at least 10 gauge connecting cable, if not 8 gauge.

To test an RS-50 you could use a 0.68 ohm and a 0.47 ohm resistor in parallel provide a load of 49.6 amps and 685 watts. Close enough! But that's 405 watts in the 0.47 ohm resistor so you'd want to use a pair of 500 watt resistors for a total of $72… (add a fan!) And I'd use at least 8 gauge cable, if not 6 gauge.

An RM-60 could use two 0.47 ohm resistors in parallel creating a 58.72 amp load. Close enough! However that's 405 watts in each resistor and you'd want to use the 500 watt units for a total of $72… (add a fan!) And I'd use at 6 gauge cable.

To test an RS-70 you could use five 1 ohm resistors in parallel… They would provide a load of 69 amps. Close enough! Each resistor is dissipating 190.44 watts, you would be too close using 200 watt resistors, 300 watt units ($141.55) would be OK for intermittent testing, 500 watt units ($180) would give you some headroom. (add a fan!) And I'd use at 6 gauge cable.

None of the above cases would be considered continuous duty. Especially if you didn't have some forced air cooling! In each case you would need a good sized heat sink… Ask around and see if anyone has a dead HF, VHF or UHF RF power amplifier - you could salvage the heat sink from it. Another alternative is to duplicate what the page maintainer observed at a shop vsit: See if there is a rail yard near you then find the foreman and politely ask if there is a short length of scrap rail available (if you never ask the answer is always No! The obverse of that is if you never ask they can't say Yes!). It could even be worn out rail. The one that your page maintainer saw had the rail fastened to a piece of wood about three feet long and had casters on the bottom.

Modification Articles:

Astron Linear Supply Battery Backup Modifications
All the schematics and modifications necessary to add a battery backup to your Astron linear power supply.

Float Modification for the RS/RM-35A/S Schematic of the Astron RS-35A/M with float modification (the yellow highlighted area)
If you add a toggle switch to the front panel that shorts out the added resistor you can label it "Float" (when off / open) and "Normal" (when on / shorted).
Make sure you print the mod sheet and tuck it inside the supply for the next guy.

Adding Anderson PowerPole Connectors to an SS-30 supply By Robert Schulz KC6UDS
This is a very nicely done modification that extends the usability of the SS-25 or SS-30 supply.
Kyle Yoksh KØKN took Robert's idea and modified a Daiwa PS-304 power supply in a similar fashion.

Adding Anderson PowerPole Connectors and Binding Posts to an SS-25 supply By Neil Schwanitz V73NS
Other modifications to these small supplies, similar to the ones above.

Reducing Inrush (Surge) Current on Astron Power Supplies by Robert W. Meister WA1MIK (SK)
This modification adds an inexpensive thermistor in series with the transformer, reducing the surge current and eliminating annoying circuit breaker trips on the larger linear power supplies. This same modification could be done to other high-power supplies. Total cost was under four dollars.

Replacing Astron Meter Lamps by Larry Lockard N7FM (offsite link)
A very well-written article that shows how he replaced the "unreplaceable" internal meter lamps, which seem to be a pair of small 14 volt light bulbs wired in series and powered from the raw unregulated power supply voltage.

Upgrading the Astron Over-Voltage Crowbar Circuit by Rich Post KB8TAD (offsite link) (local copy)
Repairing a supply, analysis of key circuits, and replacing the SCR crowbar with MOSFETs. Very well written and chock-full of troubleshooting information.

Fixing False Tripping of the SCR Crowbar Circuit by Ray Maynard NØLGR
Ray figured out why some of his supplies were falsely triggering the SCR crowbar and modified the supply to stop this from happening.

Installing Digital Meters in an Astron Supply by Tony King W4ZT (SK) (833 KB PDF)
Tony bought a used Astron that had both of the stock meters "stuck" and not repairable. He replaced them with digital meters and created a web page about installing them. The ammeter he bought was a type that required an isolated power source, and the article includes a schematic and photo of the isolated +5 volt DC source that he built on perfboard.

Replacing or Adding Digital Meters to Astron 13.8V Linear Supplies by Robert W. Meister WA1MIK (SK)
An overview of the types of meters you can buy and which ones seem to work the best in popular Astron supplies.

Adding Voltage and Current Adjust Pots to an Astron 13.8V Linear Supply by Robert W. Meister WA1MIK (SK)
Turn an RS-supply into a VS-supply. Simple to do and easily reversible. Implement one or both.

One useful modification: Relocate that crowbar SCR anode to the collector of the pass transistors (the output of the rectifier stage, and after a new high current fuse or circuit breaker). If the SCR trips and the supply does not go into current limiting, you can kiss the current meter and expensive current sense resistor / ballast resistor goodbye.
And this goes hand-in-hand with the above: If the crowbar is located before the series pass transistors, it is a good idea to put a large (high current) reverse biased diode across the series transistors (CR7 on the RM-60 diagram), which would discharge any large capacitors on the regulated side through the crowbar. Without the diode, the collector side would be at zero volts after the crowbar tripped, but the emitters could still be at at +12 V due to any large capacitors on the regulated side (perhaps inside the supplied device).

A second useful modification: Add a fuse between the filter cap and crowbar. Better yet, put in a resettable circuit breaker, as murphys law says you will never find a replacement fuse, and they ALWAYS blow at the worst possible moment.....

Power Supply Model Information:

Please realize that you will find multiple different schematics listed below for the same supply as the designs changed over the years due to parts availability, circuit improvements, etcetera. For example, the early supplies use discrete stud-mounted diodes instead of half of a bridge rectifier (switching to an epoxy bridge module, despite the fact that only half is used, is one of the tricks that the designer at Astron used to lower the parts cost and manufacturing labor cost). You may have to download more than one schematic to get the one that matches your supply, and you may not find your schematic at all (as we only have the ones that were donated to us). If you have one that we don't, please consider emailing us a high resolution scan. Or just send us the schematic by postal / snail mail and we'll scan it and return it. Send it to the maintainer listed at the top of this page.

When (or if) you find the schematic that matches your unit I suggest you print it and stuff a copy inside a plastic page protector, and tape it to the underside of the lid of the power supply cabinet! Several folks have mentioned in emails and on mailing lists that you can call Astron on the phone and you will hear them tell you that they don't have electronic copies of their drawings and they don't know how to email them. Trust me, the person that answers the phone will be amazed when you tell them that the drawings from different years for the same model power supply show some different component IDs and values. Unfortunately this is important because if one chooses to buy replacement parts (from Astron) they (according to Astron) need only supply the model and component IDs. Fortunately everything but the filter caps, transformer, the bare regulator circuit board and the metal cabinet are common Mouser or DigiKey parts, and I bet you could find the capacitors if you tried hard enough. Astron has to get them from somebody.

Update: The above paragraph was written about 2000-2004. Things are much better now.

Variable output supplies are identical to the non-variable supplies except for the two front panel adjustment controls. Similarly, supplies with meters are identical to those that don't have meters except for the meters, calibration pots, and possibly a DPDT switch used on supplies that have just one meter for amps and volts. So while you may not find an exact schematic for your particular unit, if you find one for a variable or metered supply, use that one.

Low Current Output Terminals (Binding Posts and Block Terminals): Supplies rated for 12 Amp or less (SS-10, SL-11, or RS-4, RS-5, RS-7 and RS-12) have either two two 4 mm, 15 Amp, 3-way binding posts or block terminals with setscrews on the rear for output terminals.
The binding posts are mounted in 1/4 inch holes with keyways and are made by Cliff Electronic Components as their Model TP1. They are available from www.Newark.com and other suppliers.
The setscrews in the block terminals are brass with slots (i..e not allen or phillips head) and if you turn them too hard you will break off one side of the slot. This is the voice of experience of your page maintainer. Astron sells replacements on their web site for about US$4 each (in 2020). Just take the cover off the supply, disconnect the block then remove it and replace it – but first replace the slotted brass setscrews with allen head! Click for photo.

Hex bolt outputs: Supplies rated for higher current (the RS-20, for example) usually use 1/4-20 threaded hex bolts with flat washers, split lockwashers, and hex nuts for output terminals. Swapping the hex nuts for wingnuts is a popular and inexpensive modification. The wing nuts are available at almost any hardware store, (even Home Depot). Your author / page maintainer found them at his local Ace Hardware.

Replacement shoulder washers: One or both of the output terminals are insulated from the chassis with a fiber shoulder washer that fits into a 5/16 inch hole (Keystone #4711). Mouser stocks them as item 534-4711, DigiKey as a 36-4711-ND.

Some of the Slim-Line (SL) supplies, especially those models that have a metal sleeve that holds a specific model radio have a cable and connector appropriate for the radio that it would normally power. The supplies built for Motorola frequently have a 120 / 240 volt AC switch and other model spacific differences… the Motorola HPN9041 is an SL-15M variant and has no binding posts, instead it has two pigtail cables, one with a 6-pin Molex connector, the other with a 2-pin SAE (trailer) connector. It also has insulating plastic caps on the two pass transistors.

Notes about Serial Numbers:

For years the year of manufacture was the first two digits, the month was the next two digits, and the remaining digits are a sequential number that's assigned to each unit. We're not sure if it's model-specific or the total number of all units made that month by Astron. This date code information was confirmed by Astron. However…
Recently it was pointed out that serial numbers like 202050xxx were showing up. Now you and I both know that there isn't a 20th month… We've dropped a note to Astron and are awaiting a reply…

Notes about Model Numbers:

The model number is constructed from a prefix (letters), a peak current value (digits), and a suffix (letters). Look for a schematic based on the current value first. The circuitry is similar for the different prefixes and suffixes. For example, the Variable power supplies with front panel controls just add two potentiometers. Any supply could have meters added, if they'll fit on the front panel. These don't affect the basic circuitry.

Prefixes:

An "RS-" prefix is a standard desk-top Regulated Supply.
An "RM-" prefix instead of "RS-" prefix indicates a Rack-Mount power supply (i.e. packaged to mount in a 19 inch rack). This packaging is only offered on the larger supplies. If you need to rack mount a smaller one, just use a rack shelf. Other than the meters and the attached adjustment trimpots there is little electronic difference between the RS- series and the similar RM- model.
Change the first letter from R to V and you have one with front panel adjustments (V = Variable). Converting a RS or RM to a VS or VM usually involves adding two potentiometers to the front panel and making the minor wiring changes shown in the schematic. See the article by Bob Meister WA1MIK (SK) elsewhere on this page.
SL- series are Slim-Line or Small Linear power supplies. Their schematics are further down the page. These are often found inside a larger (vertically) enclosure that has room for a two-way radio to be mounted above the supply. The LMR (commercial two way radio) world calls a mobile configured as a base a "control station". The SL-series power supplies are also found in some small desktop repeaters that use a pair of mobile radios.
Like the other Astrons the suffix characters indicate the peak current capacity and/or the make and frequently the manufacturer of the radio that fits the enclosure. Check the catalog pages on Astron's web site for the relevant details.
LS- and VLS- series are Linear Special-voltage and Variable Linear Special-voltage power supplies, usually 28 volt. They will adjust down to 22-23 volts for use with 24 volt equipment.
LSRM- and VLSRM- series are identical to the LS- and VLS- supplies except are Rack-Mount.
SS- series are Small Switching power supplies. Their schematics are further down this page.
SRM- series are SS switching power supplies in a Rack Mount configuration.
SLS- are Switching Special-voltage power supplies, usually 28 volt.
SLSRM- are Switching Special-voltage power supplies, usually 28 volt, in a Rack Mount configuration. Like the LS- and VLS- series they will adjust down to 22-23 volts for use with 24 volt loads.

Suffixes:

A trailing "A" seems to be the default if there's no other suffix. Some models have this, some don't.
A trailing "R" seems to be used for power supplies that were sold with an enclosure that could accept a mobile radio, usually only found on the SL-series.
A trailing "M" indicates front panel Meters. If the supply has the "M" suffix, those under 20 amps usually only have room for just one meter. 20 amp and 35 amp supplies are physically large enough that the front panel could have one or two meters.
50 amp and larger supplies almost always have two meters.
Slim-line (SL) supplies usually don't have room for meters.
Switching supplies (SS) 25 amp and larger usually have two meters.
Newer supplies (starting around 2008) could have illuminated meters. We don't know what the determining factor is for a model to have illuminated meters…
The newest "M" switching supplies have a digital meter that displays both voltage and current.
A trailing "L" indicates there's a cigarette Lighter socket on the front.
A trailing "S" indicates there's a Speaker inside the case.
A trailing "BB" indicates the Battery Backup option.
A trailing "RM" indicates Rack Mount, often for high amperage, occasionally for dual supplies.
A trailing "AP" indicates a pair of Anderson PowerPole connectors on the front panel. These seem to be available (so far) on a few fixed-voltage desktop models: RS-20M, RS-35A, RS-35M, and as an option on the SS-25, SS-30, SS-50.
Of course there's nothing preventing you from adding Andersons to the front, rear or both of your Astron.
A trailing "220" (or "240") indicates the supply operates on 220-240 volt line voltage.

Astron builds supplies specifically for several manufacturers of radios - some with minor differences (like adding a 120 / 240 AC switch)… for example a Kenwood KPS-12 is based on the Astron RS-12, the Motorola RRDN6933A is an RM-35A-BB and the Motorola HPN9041 (45 KB PDF) is an SL-15M variant. Astron also builds custom supplies for GE, Harris, Icom, E.F. Johnson, Kenwood, Motorola, Uniden and Vertex. For example, the Astron SL-11RRA is a 13.8 Volt 11 Amp unit specifically designed with a metal sleeve mounted on top of the cabinet. Multiple sleeves are available, one that the author has is specifically sized so that a MaxTrac, Radius LRA or GM300 mobile can slide into it; (click here for a photo). Astron sells the sleeves separately on their web site.

A conversion list of 65 different Motorola power supplies to Astron model numbers can be found here.

Schematics:

If you don't find the schematic for your Astron below, then we were not given it. If you find a PDF somewhere else that we don't have, we'd appreciate an email with it. If you find a paper copy we'd appreciate a scan (at the highest resolution your scanner can do) sent via email. Or just send us the schematic by postal / snail mail and we'll scan it and return it. Send it to the maintainer listed at the top of this page.

Astron model numbers indicate the peak (intermittent) current that can be drawn from the supply. Depending on the model the continuous current is somewhere between 40 percent and 80 percent of the peak current. For example, an SL-15M is rated 14 amps peak but only 7 amps continuous, yet an RS-20A is rated 20 amps peak but only 16 amps continuous. The absolute maximum current where foldback limiting occurs is usually 10 percent to 30 percent above the peak current rating.
The newer switching supplies have a higher continuous rating, for example a SS-50 is rated at 50 amps intermittent, 40 amps continuous. However I'd add a fan…

The available output current on the VS-series depends on the heat sink dissipation and therefore the output voltage. You get much less deliverable current at a lower output voltage. This derating is due to the power dissipation capability of the heat sinks and transistors on the back or sides of the supply. At lower output voltages, there's more voltage across these transistors, so they get much hotter. Current foldback may also occur earlier at these lower voltage ratings, so beware. The Astron catalog has these derated current specs. If you plan on using a VS-series at a low voltage you'd be smart to add a thermostatic-switch controlled fan blowing across the heat sink(s).
Specific Examples: A VS-50M supply is rated for 37 amps continuous at 13.8 volts DC but only 22 amps continuous at 10 volts DC and only 10 amps continuous at 5 volts DC output. The other VS-series supplies must be derated similarly.

Important Notes: Numbers in many of the schematics below may be blurry (particularly the pin numbers on the 723 regulator IC). The scans are what we were given.

Here's what to look for and how to resolve it:
A blurred pin "3" could look also look like a "6" or an "8".
A blurred "3" could look like an "8" but pin 3 of the 723 chip always goes to the "+" output connection of the supply.
A blurred pin 6 can look like an "8". Luckily there is nothing connected to pin 8 of the 723 chip as it is a NC (unused, no connection) pin and is almost never shown on the schematic.
If you see something that looks like an "8" it's most likely a "6" where the top got closed up by the blur.

The voltage charts on most of the schematics follow this scheme. You can always check some other schematic(s) if you can't read the one for your particular supply as they're very similar, especially around the 723 chip.

If you happen to have a cleaner copy of a blurry schematic please send us a good clean scan (at the highest resolution that your scanner can do), or send us the schematic by postal / snail mail and we'll scan it and return it. Send it to the maintainer listed at the top of this page.

The resistor in the schematic marked R3x or Rx (in parallel with R3) sets the maximum current allowed before the fold-back current limiting comes into play. The value is unique to each model and depends upon the actual maximum current handling capabilities of the model.
There is more discussion of R3x or Rx in the currrent control section of the article titled "Adding Voltage and Current Adjust Pots to an Astron 13.8V Linear Supply" that is linked above.

Linear Power Supply Schematics:

Donations of additional schematics for the library below are always welcome !! Send them to the maintainer listed at the top of this page.

RS-3A and RS-4A 300 KB, dated 12-1989 donated by Oscar Ramsey NV3G
The RS-5A is most likely the same. The regulator board in a 2010 RS-5A supply seems to be used in the 3A, 4A, 5A, and 7A supplies, however there are no crowbar components installed in the 3A, 4A, and 5A supplies. (The draftsman obviously started with the RS-7 schematic; he forgot to adjust the voltage table. It shows a 7 amp load.)

RS-7A 70 KB, dated 10-1994 donated by Mike Morris WA6ILQ

RS-10A, RS-10S 105 KB, dated 09-1981 donated by Bill Netzlof KL7IGB

RS-10A, RS-10S 368 KB, dated 09-1988 donated by George Franklin WØAV

RS-12A 199 KB, dated 11-1983 donated by Richard Reese WA8DBW

RS-12A, RS-12M 127 KB, dated 06-1988

RS-12A, RS-12M 31 KB PDF, also dated 06-1988 donated by Mike Morris WA6ILQ

RS-12A-BB 119 KB PDF, dated 01-2000 donated by John Lund.
See the comments above on the battery backup feature.

RS-12A, RS-12M 71 KB PDF, dated 11-2009 donated by Greg Shaw N4GOS

RS-20A 71 KB, dated 11-1978 donated by Gary Eldridge KC8UD

RS-20A, RS-20S 79 KB, dated 09-1988 donated by Kevin Custer W3KKC

RS-20A, RS-20S 185 KB, dated 09-1988 donated by Mike Morris WA6ILQ

RS-20A 44 KB PDF, dated 01-2000 donated by Ron N8HXR

RS-20M 117 KB PDF, dated 01-2000 donated by Ron N8HXR

RS-20A-BB 330 KB, dated 01-2000 donated by Rick Williams N8EDR
See the comments above on the battery backup feature.

VS-20M 26 KB PDF, dated 11-1978 donated by Bob Burchett WB6SLC

VS-20M 80 KB PDF, dated 09-1986 donated by Doug Marston WB6JCD (SK)

VS-20M 78 KB PDF, dated 01-2000 donated by Larry Horlick VYØHL
This supply is also known as a Motorola RRDN6082A. The Moto invoice said "VS-20ML".

RS-35A 135 KB, dated 01-1978 donated by Joe McIntyre W4DEX

RS-35M 46 KB, dated 04-1987 donated by Kevin Custer W3KKC

RS-35A, RS-35M 216 KB, also dated 04-1987 but different than the above RS-35M. Donated by Mike Morris WA6ILQ

RS-35M 47 KB, dated 04-1987, again different. Donated by Mike Morris WA6ILQ
Thanks go to Ed Lambert K1ZOK who pointed out in an email that there was an important error in this schematic file (WA6ILQ edited the image file and corrected it). There was an extra connection line drawn between the collector of the TIP29 and the base connections of the pass transistors. This wire shorted out the regulator driver transistor (Q2). If you downloaded this schematic in the past you may want to download a fresh copy and replace your incorrect copy.

RS-35A, RS-35M 555 KB PDF, dated 09-1988 donated by Jim Bacher WB8VSU

RS-35A, RS-35M 159 KB PDF, dated 05-1991 donated by Mike Morris WA6ILQ

RS-35A, RS-35M 46 KB PDF, dated 05-1995 donated by Mike Morris WA6ILQ

RS-35M 498 KB, dated 01-2000 donated by Neil McKie WA6KLA (SK)

RS-35M 78 KB PDF, dated 10-2009 redrawn, corrected, and donated by Stu Martin K2QDE
Includes meter illumination.

RS-35M 127 KB PDF, dated 02-2017, drawn by Larry Joy WN8P
This is an actual as-built schematic, drawn from scratch to current IEEE specifications, of an older (1978) RS-35M supply with one meter on the front, stud rectifiers inside, and the TIP29 driver transistor and the crowbar SCR mounted to the chassis (current models have those parts on the regulator board). Astron has since added some parts to the design; these are NOT reflected in this schematic or the parts list below.
RS-35M Detailed Parts List 56 KB PDF, goes along with Larry's schematic diagram above. Lists every piece of wire and hardware.

VS-35M 37 KB PDF, dated 01-1987 donated by Steve Duncan WA4ITA

VS-35M 262 KB PDF, dated 01-2000 donated by Bob Burchett WB6SLC

RM-35A, RM-35M 33 KB PDF, dated 01-2013 donated by John D'Errico N1ERF

RM-35A-BB 80 KB PDF, dated 01-1993 donated by Larry Horlick VYØHL
This is a factory Battery Back-Up supply that is also known as a Motorola RRDN6933A. See the comments in the battery backup section above concerning this particular supply.

RM-35A-BB 300 KB PDF, created 10-2020 by N. Eric Jorgensen W1NEJ
This is a factory Battery Back-Up supply purchased in October 2020. Eric reports:
I acquired a new RM-35M-BB recently. However the schematic I found on-line is not correct for my unit. It appears the rack mount version of the 35 amp power supply has been upgraded. The differences are:
1. My unit contains an extra 2N3771 transistor between Q2 and the pass transistors.
2. My unit contains six 2N3771 pass transistors, not four.
3. My unit has only two battery isolation diodes, not two sets in parallel (total of four).
4. It has an extra diode in parallel with CR7.
I took the non battery-backup version of the RM-35M schematic and drew in the battery isolation, charging, and protection diodes to create a new schematic for a new RM-35M-BB. I note there are slight differences in the parts used for D5, D6, and SCR1 between the schematic that came with my unit and the schematic I have supplied with my changes for the battery backup circuit. I have not verified the exact parts in my unit. I have also not verified the part number of the battery isolation diodes, but they appear to be the same part as the rectifier diodes, D101 and D102. The main rectifier diodes are mounted on the rear panel, and the battery isolation diodes are mounted on a heat sink inside the power supply next to the power transformer and they appear to be identical. The protection diode is wired directly across the output terminals. My unit also contains six pass transistors. By the way, I do not, and will not, use the battery backup circuit as installed in my unit, as it would probably overcharge and ruin the battery.

RS-50M 181 KB PDF, dated 01-2000 donated by Kevin Custer W3KKC

RS-50A, RS-50M, RM-50A, RM-50M 36 KB PDF, dated 03-1996 donated by Tom Allinson WB6DGN

RS-50A, RS-50M, RM-50A, RM-50M 154 KB PDF, dated 01-2013 donated by David Bent

VS-50M 25 KB PDF, dated 08-1982 donated by Bob Shields KA9TYL

VS-50M 74 KB PDF, dated 11-1995 donated by Tim Bovard

RS-50A-BB, RS-50M-BB 127 KB, dated 07-1995 donated by Robert Burton KD4YDC
See the comments above on the battery backup feature.

RS-50M-BB, RM-50M-BB 45 KB PDF, dated 07-2011 donated by Roger Gray N5QS

RM-60A, RM-60M 43 KB PDF, dated 08-1988 donated by Tom Allinson WB6DGN

RS-70A, RS-70M 73 KB, unreadable date (probably 08-1988) donated by Mike Morris WA6ILQ

RS-70A, RS-70M Service Manual 1.58 MB PDF, undated, donated by Hugo Loranger VE2UGO

RS-70A, RS-70M 227 KB, dated 06-1997 donated by Avent Lane

RS-70A, RS-70M 74 KB PDF, dated 01-2007 donated by Tom Allinson WB6DGN

VS-70M 54 KB PDF, dated 06-1995 donated by Matt Trull KX4GG

VS-70M 154 KB PDF, dated 04-2020 donated by Hugo Loranger VE2UGO

Slim-Line (Low-Profile) Linear Power Supply Schematics:

SL-11A 99 KB PDF, dated 04-1990

SL-11A, SL-11R 172 KB, dated 11-2009 donated by Paul Mandel W4PGM

SL-15A 120 KB PDF, dated 04-1990 donated by Ed Lambert K1ZOK

SL-15M 42 KB PDF, dated 12-1998
This is also the Motorola HPN9041 that is found in several low power applications including the GR400 rack mount, GR500 wall mount and GR1225 tabletop Repeaters. It can be used with an HLN9455 Battery Revert accessory. More details on the HLN9455, including a manual (with a schematic) can be found on the R1225 page at this web site.

SL-15R 184 KB, dated 11-2009 donated by Dave Christensen KD7UM

28 volt Linear Power Supply Schematics:

The Astron factory says that their "28 volt" supplies will adjust down to 22-23 volts (for use with "24 volt" loads).

LS-25A 28 volt, 25 A intermittent, 18 A continuous, linear power supply 39 KB PDF, dated 04-2004 donated by Astron.
A no-frills 28V power supply similar to the RS-50 supply but with twice the output voltage and half the output current. This supply, in a rack-mount configuration (LSRM-25A), is also sold under the Uniden name as their model ARX‑330.

LS-35M 28 volt, 35 A intermittent, 25A continuous, linear power supply 197 KB, dated 07-1993
Found on the web. Has some voltage measurements on it.

VLS-10M 28 volt, 10 A intermittent, 7A continuous, linear power supply 41 KB PDF, dated 01-1990 donated by Roger Gray N5QS.

VLS-35M 28 volt, 35 A intermittent, 25 A continuous, linear power supply 58 KB PDF, dated 06-1995 donated by Ron Vincent KF4D
Similar to the VS-70M supply except for the output voltage (variable from 5 to 32 Volts) and current.

VLS-35M 28 volt, 35 A intermittent, 25 A continuous, linear power supply 321 KB PDF, dated 04-2016 donated by David Bent.

Switching Power Supply Schematics:

SS-10 4 MB PDF, early 1990s
Schematic plus parts list for SS-10, SS-12 and SS-18. Not truly accurate, the parts list says that these have a "Banana Socket" for the output.

SS-12 42 KB PDF, dated 08-1996 donated by Eric Lemmon WB6FLY
No parts values other than what Eric noted.

SS-18 47 KB PDF, dated 11-1998

SS-25M, SS-30M 483 KB PDF, dated 09-2000
Shows meters and a 120V / 240V input power selection switch.

SS-25M, SS-30M 1.8 MB PDF, dated 09-2000
Schematic plus parts list.

SS-25M, SS-30M 1.7 MB PDF, dated 11-2010
Four pages long, no voltage selector switch, but shows illuminated meters. It also has a very detailed parts list at the end.

SS-30 71 KB PDF, dated 09-2000 donated by Eric Lemmon WB6FLY
There is minimal parts info on the schematic. Further info is available here.

SS-30 157 KB PDF, dated 09-2000
A second copy of the same schematic, but a little more readable.

SS-25M, SS-30M with enhancements 1.1 MB PDF, dated 09-2000 donated by Alan Sewell N5NA
Alan had to repair a couple of these; in the process he corrected the schematic, added LOTS of parts values and other notes, and took extensive voltage readings at various points, complete with photographs. Excellent documentation! It does seem that a meter calibration pot is missing from the ammeter circuit in the schematic, but since he left no contact info, this will have to go uncorrected until he sees this and contacts the page maintainer. Read his full story here. A local copy can be found here.

A mod for the Astron SS-25 and SS-30 power supplies (and maybe other SS-series) from NU4G:
(This mod may or may not apply to other models of Astron switching power supplies)
These two power supplies use a simple normally open "click" thermostat to switch a very noisy (acoustic noise, not RF noise) fan on and off. The fan noise can be annoying at times, so I've modified my supplies to make it less so. Open the supply after first disconnecting AC power and allowing for DC voltage to bleed down. Looking inside you will see two sets of heatsinks near the rear of the unit. Each heatsink has a thermostat attached to it, the left side heatsink has a normally closed 50 degree C thermostat for the AC input - don't bother it. The right side heatsink has the fan thermostat mounted to it. The fan thermostat switches DC from the output to power the fan. If you bridge the thermostat with a two watt resistor of 75 to 100 ohms the fan will be on continuously, but very slowly. This slow speed is enough to keep the supply cool with very little noise. Before installing this mod simply running my Icom 756PII HF station just on receive was enough to turn on the fan every 5 minutes or so. With the mod I've only had the fan go to full speed (i.e. the thermostat closed) while operating RTTY for an extended time on a very warm day.

Switching Regulated Voltage Converter Schematics: (Also known as DC-to-DC converters)

1212-18 199 KB PDF donated by Tom Allinson WB6DGN
This unit allows you to use a negative ground +12 volt device (like a two-way radio) in a 12 volt positive ground vehicle (i.e. with a - 12 volt battery system). The positive side of the DC input and negative side of the DC output are common. Another way of saying this is that this is a ground inverting unit. The schematic shows that it is based on a 556 chip and has a note saying 14 amps continuous duty, 18 amps surge.

2412 49 KB PDF donated by Ed Lambert K1ZOK
This unit is also based on a 556 chip and allows you to use +12 volt devices (like two-way radios) on +24 volt battery systems where the negative side is common (like those found in many Kenworth, Peterbilt and other large commercial vehicles such as earthmovers and fire trucks). This schematic does not have amy notes as to current capacity.

2412-24 V3a Dated 2011 91 KB PDF donated by Mike Collis WA6SVT. This unit also has a comon negative.
Mike reports: This unit has a "Low Voltage Disconnect" built in but it's set to drop out at 18 volts. This may work great for an 18-wheeler truck driver wanting the unit to provide 13.8 volts out on a nearly dead battery, however you might not want to run your radio site battery down so low, especially at a solar powered site. A slight modification will raise the dropout point to save a battery bank at a solar site. Simply change the 62k resistor (R17) on the voltage divider between the input voltage and ground that supplies voltage to pin 16 (UVLO) of the LM5025B chip. When this pin is below 2.5 volts, the converter shuts down; above 2.5 volts, it turns on. There is hysteresis on this pin to prevent chattering (about 2 volts difference between off and on). Changing R17 to 91k will move the Low Loltage Dropout to 22.75 volts and re-connect at just under 25 volts. If you desire an adjustable dropout, add a 30k trim pot to the board in series with the existing 62k resistor.

2412-12 Rev E 320 KB PDF donated by Tom KN4ONE
Like the other 2412 units above, this unit allows you to use +12 volt devices (like two-way radios) on +24 volt battery systems that have a common negative. This model uses a TL494 switch-mode regulator IC to do the work.

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This page originally created in August 2000 by Kevin Custer W3KKC
Totally rewritten and a number of schematics added on 10-14-2004 by Mike Morris WA6ILQ
Copyright © 2000 and and date of last update by Repeater-Builder.com

A large number of people contributed information to this web page, some of whom are no longer with us. The list became too long to manage. Several contributions were sent in either anonymously or the person asked not to be identified with a particular submission. We are grateful to all those who took the time to send us information.

The Astron logo/image is a registered trademark and is used within this page with permission from the Astron Corporation.

The schematic images are copyright © Astron Corp. Most are dated on the individual drawing. No copyright infringement is intended. If Astron had the schematic library on their web site we wouldn't need to.

This web page, the hand-coded HTML on it, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.

	Repairing Astron 13.8 volt Linear Supplies by Robert W. Meister WA1MIK (SK) General linear power supply circuit explanation, followed by the most common problems, diagnostic techniques, and repairs for these units.
	Annotated RS35M Schematic by Jim Larsen AL7FS (169 KB PDF) The December 2005 issue of QST had an article in their "Hands-On Radio" series titled "Power Supply Analysis" You can download a PDF of the QST article - it explains how a linear power supply works, and uses an Astron RS‑35 as the "victim". Jim Larsen, AL7FS (the ARRL Alaska Section Manager) took that QST article and went one step better — he produced an annotated schematic that takes advantage of the annotation feature of the PDF file architecture. The QST article and the annotated PDF are really worth studying. Just mouse over the yellow folder symbols and they will pop up. If your browser does not pop the notes then download the PDF and use the free downloadable Adobe Acrobat Reader to open it.
	Troubleshooting Astron Linear Supplies by Jim Ussailis W1EQO (630 KB PDF) From the Yankee Clipper Contest Club's "Scuttlebutt" newsletter Issue 232, August 2015.
	Installing a New Regulator Board in an Old Astron Power Supply by By Tom Dailey WØEAJ After his regulator board burned up, Tom bought a new one from Astron. Seems they've made a few improvments in the design since his supply was made. This article describes what he discovered and how to deal with putting a new board into an old supply.
	Refreshing / Maintaining Astron Linear Supplies by Robert W. Meister WA1MIK (SK) Common maintenance things you can do to keep your Astron supply running for many years to come.
	An evaluation of a 2009 Astron RS35M supply by Stu Martin K2QDE (2.5 MB PDF) Stu opened his brand-new supply and took several photos, showing what's been added or changed. He scanned then redrew and corrected the schematic so it matched the actual unit; this drawing can be found as a PDF file below.
	Restoring / Rebuilding an Astron RS35M by John Keith W5BWC (1.4 MB PDF) While John titled this as a "restoration" it's actually a complete gut job starting with the chassis, transformer, filter capacitors, the meter and a few other parts remaining. Everything else gets replaced with a completely different design.
	Crowbar SCRs in Astron 13.8V Linear Supplies by Robert W. Meister WA1MIK (SK) A summary of SCRs used by Astron and some available replacements.
	Main Capacitors and AC Line Fuses in Astron 13.8V Linear Supplies by Robert W. Meister WA1MIK (SK) A summary of main filter capacitors and AC line fuses found in Astron supplies.
	LM723 Regulator Operation by Robert W. Meister WA1MIK (SK) A short explanation of what's going on inside the regulator IC in an Astron power supply.
	Understanding and Using 723 voltage Regulators by William C. Cloninger, K3OF (SK) 1.8 MB PDF This is an article out of Ham Radio (hr) Magazine for March 1989. He shows several homebrew supplies as he walks you through the basics. Worth reading.
	Parallel Operation of Astron Linear Power Supplies by Robert W. Meister WA1MIK (SK) The secrets of that "For Parallel Operation Only" binding post on the rear panel.
	Adjusting the Output voltage of Astron Linear Supplies by Robert W. Meister WA1MIK (SK) So easy, a visually handicapped (blind) person can do it!
	Astron BB-30M Data Sheet and schematic 346 KB PDF Photo 1 Photo 2 Photo 3 Photo 4 The BB-30M is an interesting and unique device in the Astron product line. It is a 30 amp stand-alone battery backup module that works with any 12 volt or 24 volt system. It switches the load from a DC voltage source to a battery upon power failure without any dropout. It also can maintain the charge of the battery with a constant-voltage trickle charging circuit (of about 3.5 amps max and the charger can be disabled by lifting one leg of R28, the only 22 ohm 10 watt resistor inside the box). Internally this unit is a lot more complex than I ever imagined it would be. Note that there is about a 0.4 to 0.5 volt drop across the unit.
	RS-50A / RS-50M / RM-50A / RM-50M Service Manual by Astron, transcribed by Bob WA1MIK (SK) (130 KB PDF) Specifications, circuit description, theory of operation, servicing, adjustments, parts list, schematic for the 50 Amp rack-mount and desk-top power supplies.
	If your linear power supply regulator board is beyond repair, you can buy a brand new one from Astron for about US$25 plus shipping. They will even ship it to you and let you remit payment after you receive it. Tell them the supply model number and they will customize it for you. Here are 300kB photos of the component side and the solder side of a brand new regulator board ordered in 2014 for a VS-35 power supply, courtesy of Bob McKinlay VE3DJ. The blue adjustable potentiometer marked "1K" on the board (visible in the component side photo above and labeled "R5" on the solder side photo above) adjusts the power supply's maximum output voltage, typically from 11 to 15 volts (22 to 32 volts on 24 volt LS and VLS power supplies). See the article above for a more detailed procedure.
	Mike WA6ILQ reports that he's found a few cases where the screws that mount the voltage regulator board to the filter capacitors were not tight. Suspecting thermal cycling he now adds star washers under the screw heads on both the + and - capacitor connections before he puts the supply into service. Another incident had one hole of the filter capacitor not drilled to the correct depth before tapping / threading, and therefore the screw bottomed out and did not adequately "pinch" the circuit board and make good contact with the copper pad. The added star washer between the screw head and the board corrected that issue as well.
	Stu K2QDE reports "I recently learned that very light to unreadable front panel silk-screening existed in a number of production Astron power supplies around the time I purchased mine in mid-2017. I searched for vendors that could make a new stick-on label. I sent along a laser printout of an SS-55M front panel as a sample. These are plastic, self-adhesive strips, slightly larger and brighter than factory silk-screening. The Astron logo is slightly sharper than the silk-screened version. Perfect when refurbishing your Astron supply. Click here to see a photo. Contact sonia [ at ] kellylaserworks [ dot ] com for more information.
	As of 2020 Astron is now selling many of the more common parts that can be used to repair or upgrade your power supply such as the power switch, meters, illumination kits, pass transistors, bridge rectifiers, capacitors, and the crowbar SCR. Check the Astron web site for pricing and availability. Don't hesitate to send them an inquiry email if you don't see the part you need.

	The OVPR-12V an internal reset circuit for the Astron Linear series A reset circuit provided by The Astron Corporation – they call this board an OVPR-12V – by Kevin K. Custer W3KKC This circuit shoould have been included in the original Astron regulator board design. If you are inside your Astron for some other reason it's worth adding this circuit as long as you have the cover off. Just remember to print the page and leave a copy inside the case so the next guy will know what the little circuit board is for.
	Astron's own writeup on the OVPR-12V donated by Joe Orrico WB6HRO (52 KB PDF) Astron's documentation shows an extra LED indicator, which is not actually part of the OVPR-12V.
	A photo of Astron's OVPR-12V board (found on the web).
	An external reset circuit for the Astron Linear series A reset circuit by Kevin K. Custer W3KKC You can use this if you don't want to (or can't for political reasons) open up the Astron.
	Some of the later supplies have a capacitor across the the SCR gate and ground (C102 on the RM-60 diagram), This cap reduces false tripping of the SCR in RF environments. If your supply does not have it I'd add it.

	A Tutorial On Power Supply Load Testing By Eric Lemmon WB6FLY (SK) Worth reading… Eric was a regular poster on the repeater-builder mailing list for over a decade. He did a number of power supply tests and posted his results. Someone asked him what tools he used and how did he perform the tests and this document was the result.
	Power Supply Dummy Load by Tony King W4ZT (SK) (125 KB PDF) Tony designed and built a test load made up of common 12 volt automobile stoplight bulbs. You can get them cheap from a do-it-yourself (sometimes called a "pick-your-part", "pick-a-part" or "you-pull-it") auto junkyard. Don't go cheap and leave off the top protection plate, the stoplight bulbs get very hot very quickly and can give you serious burns. This is the voice of experience of your page maintainer.
	High Power DC Load for Power Supply and Battery Evaluation by Phil Salas AD5X (1.53 MB PDF) This is from the October 2006 QST. Take this article as a source of ideas, an AC fan may be cheaper. And this unit has a lot of heat generating ability, you might want to consider using open construction like the unit Tony King built.
	Your page maintainer once saw a test load made up of a number of old automobile headlights, with one pull-push headlight switch per bulb (like the switch used in a 1960s-1970s Ford or Chevy. The owner had bought the bulbs, sockets and switches from an antomobile junkyard. Each switch controlled one headlamp - when pulled half way out the low beam came on and when pulled all of the way out both the low beam and the high beam were connected.
	While the article is an evaluation of an Astron supply there is an interesting homebrew test load shown here that uses automobile halogen light bulbs (look at the second photo from the top). Be careful, halogen bulbs can get much hotter than tungsten bulbs. (Off-site pointer, the link opens in a new browser tab)
	Another test load that your page maintainer witnessed was used on an RM-50 or RM-70 supply. The load was a long length of galvanized 1/2 inch water pipe in a "U" shape and the owner used a set of automobile jumper cables. He clamped the negative cable to one end of the pipe, and then slid the positive jumper cable along the pipe until he had the desired amps of current flow.
	Over a decade ago your page maintainer saw a homebrew test load made up of several low-ohm / high-wattage resistors which were bolted to a length of railroad rail two and one-half feet long. It had a large heavy knife switch for switching the load on and off. Yes, it was very heavy! (over 100 pounds / 45.36 kg of steel!) North american railroad rail weight is measured in pounds per yard, and most USA main line rail is "130 pound rail" which works out to just under 44 pounds per foot). There are eBay sellers that have metal body power resistors in values of 0.1 ohm, 0.22 ohm, 0.33 ohm, 0.47 ohm, 0.68 ohm, 1 ohm, 2 ohms, 3 ohms and many higher values and in 5 watts ($2.17), 10 watts ($2.18), 25 watts ($2.30), 200 watts (US$14.60), 300 watts ($28.31), and 500 watts ($36). As an example, look at this resistor and look at the options on ohms and watts. (off-site pointer, the link opens in a new browser tab) You'd mount them to a large heat sink with thermal compound (just like a power transistor). At 13.8 volts for an RS-35 you could use a 0.68 ohm and a 1 ohm resistor in parallel. They would provide a load of 34.1 amps. Close enough! There is about 470 watts spread across both resistors (add a fan!), but the 0.68 unit is dissipating 280 watts so I'd use the 500 watt units for a total of $72… And I'd use at least 10 gauge connecting cable, if not 8 gauge. To test an RS-50 you could use a 0.68 ohm and a 0.47 ohm resistor in parallel provide a load of 49.6 amps and 685 watts. Close enough! But that's 405 watts in the 0.47 ohm resistor so you'd want to use a pair of 500 watt resistors for a total of $72… (add a fan!) And I'd use at least 8 gauge cable, if not 6 gauge. An RM-60 could use two 0.47 ohm resistors in parallel creating a 58.72 amp load. Close enough! However that's 405 watts in each resistor and you'd want to use the 500 watt units for a total of $72… (add a fan!) And I'd use at 6 gauge cable. To test an RS-70 you could use five 1 ohm resistors in parallel… They would provide a load of 69 amps. Close enough! Each resistor is dissipating 190.44 watts, you would be too close using 200 watt resistors, 300 watt units ($141.55) would be OK for intermittent testing, 500 watt units ($180) would give you some headroom. (add a fan!) And I'd use at 6 gauge cable. None of the above cases would be considered continuous duty. Especially if you didn't have some forced air cooling! In each case you would need a good sized heat sink… Ask around and see if anyone has a dead HF, VHF or UHF RF power amplifier - you could salvage the heat sink from it. Another alternative is to duplicate what the page maintainer observed at a shop vsit: See if there is a rail yard near you then find the foreman and politely ask if there is a short length of scrap rail available (if you never ask the answer is always No! The obverse of that is if you never ask they can't say Yes!). It could even be worn out rail. The one that your page maintainer saw had the rail fastened to a piece of wood about three feet long and had casters on the bottom.

	Astron Linear Supply Battery Backup Modifications All the schematics and modifications necessary to add a battery backup to your Astron linear power supply.
	Float Modification for the RS/RM-35A/S Schematic of the Astron RS-35A/M with float modification (the yellow highlighted area) If you add a toggle switch to the front panel that shorts out the added resistor you can label it "Float" (when off / open) and "Normal" (when on / shorted). Make sure you print the mod sheet and tuck it inside the supply for the next guy.
	Adding Anderson PowerPole Connectors to an SS-30 supply By Robert Schulz KC6UDS This is a very nicely done modification that extends the usability of the SS-25 or SS-30 supply. Kyle Yoksh KØKN took Robert's idea and modified a Daiwa PS-304 power supply in a similar fashion.
	Adding Anderson PowerPole Connectors and Binding Posts to an SS-25 supply By Neil Schwanitz V73NS Other modifications to these small supplies, similar to the ones above.
	Reducing Inrush (Surge) Current on Astron Power Supplies by Robert W. Meister WA1MIK (SK) This modification adds an inexpensive thermistor in series with the transformer, reducing the surge current and eliminating annoying circuit breaker trips on the larger linear power supplies. This same modification could be done to other high-power supplies. Total cost was under four dollars.
	Replacing Astron Meter Lamps by Larry Lockard N7FM (offsite link) A very well-written article that shows how he replaced the "unreplaceable" internal meter lamps, which seem to be a pair of small 14 volt light bulbs wired in series and powered from the raw unregulated power supply voltage.
	Upgrading the Astron Over-Voltage Crowbar Circuit by Rich Post KB8TAD (offsite link) (local copy) Repairing a supply, analysis of key circuits, and replacing the SCR crowbar with MOSFETs. Very well written and chock-full of troubleshooting information.
	Fixing False Tripping of the SCR Crowbar Circuit by Ray Maynard NØLGR Ray figured out why some of his supplies were falsely triggering the SCR crowbar and modified the supply to stop this from happening.
	Installing Digital Meters in an Astron Supply by Tony King W4ZT (SK) (833 KB PDF) Tony bought a used Astron that had both of the stock meters "stuck" and not repairable. He replaced them with digital meters and created a web page about installing them. The ammeter he bought was a type that required an isolated power source, and the article includes a schematic and photo of the isolated +5 volt DC source that he built on perfboard.
	Replacing or Adding Digital Meters to Astron 13.8V Linear Supplies by Robert W. Meister WA1MIK (SK) An overview of the types of meters you can buy and which ones seem to work the best in popular Astron supplies.
	Adding Voltage and Current Adjust Pots to an Astron 13.8V Linear Supply by Robert W. Meister WA1MIK (SK) Turn an RS-supply into a VS-supply. Simple to do and easily reversible. Implement one or both.
	One useful modification: Relocate that crowbar SCR anode to the collector of the pass transistors (the output of the rectifier stage, and after a new high current fuse or circuit breaker). If the SCR trips and the supply does not go into current limiting, you can kiss the current meter and expensive current sense resistor / ballast resistor goodbye. And this goes hand-in-hand with the above: If the crowbar is located before the series pass transistors, it is a good idea to put a large (high current) reverse biased diode across the series transistors (CR7 on the RM-60 diagram), which would discharge any large capacitors on the regulated side through the crowbar. Without the diode, the collector side would be at zero volts after the crowbar tripped, but the emitters could still be at at +12 V due to any large capacitors on the regulated side (perhaps inside the supplied device).
	A second useful modification: Add a fuse between the filter cap and crowbar. Better yet, put in a resettable circuit breaker, as murphys law says you will never find a replacement fuse, and they ALWAYS blow at the worst possible moment.....

	RS-3A and RS-4A 300 KB, dated 12-1989 donated by Oscar Ramsey NV3G The RS-5A is most likely the same. The regulator board in a 2010 RS-5A supply seems to be used in the 3A, 4A, 5A, and 7A supplies, however there are no crowbar components installed in the 3A, 4A, and 5A supplies. (The draftsman obviously started with the RS-7 schematic; he forgot to adjust the voltage table. It shows a 7 amp load.)
	RS-7A 70 KB, dated 10-1994 donated by Mike Morris WA6ILQ
	RS-10A, RS-10S 105 KB, dated 09-1981 donated by Bill Netzlof KL7IGB
	RS-10A, RS-10S 368 KB, dated 09-1988 donated by George Franklin WØAV
	RS-12A 199 KB, dated 11-1983 donated by Richard Reese WA8DBW
	RS-12A, RS-12M 127 KB, dated 06-1988
	RS-12A, RS-12M 31 KB PDF, also dated 06-1988 donated by Mike Morris WA6ILQ
	RS-12A-BB 119 KB PDF, dated 01-2000 donated by John Lund. See the comments above on the battery backup feature.
	RS-12A, RS-12M 71 KB PDF, dated 11-2009 donated by Greg Shaw N4GOS
	RS-20A 71 KB, dated 11-1978 donated by Gary Eldridge KC8UD
	RS-20A, RS-20S 79 KB, dated 09-1988 donated by Kevin Custer W3KKC
	RS-20A, RS-20S 185 KB, dated 09-1988 donated by Mike Morris WA6ILQ
	RS-20A 44 KB PDF, dated 01-2000 donated by Ron N8HXR
	RS-20M 117 KB PDF, dated 01-2000 donated by Ron N8HXR
	RS-20A-BB 330 KB, dated 01-2000 donated by Rick Williams N8EDR See the comments above on the battery backup feature.
	VS-20M 26 KB PDF, dated 11-1978 donated by Bob Burchett WB6SLC
	VS-20M 80 KB PDF, dated 09-1986 donated by Doug Marston WB6JCD (SK)
	VS-20M 78 KB PDF, dated 01-2000 donated by Larry Horlick VYØHL This supply is also known as a Motorola RRDN6082A. The Moto invoice said "VS-20ML".
	RS-35A 135 KB, dated 01-1978 donated by Joe McIntyre W4DEX
	RS-35M 46 KB, dated 04-1987 donated by Kevin Custer W3KKC
	RS-35A, RS-35M 216 KB, also dated 04-1987 but different than the above RS-35M. Donated by Mike Morris WA6ILQ
	RS-35M 47 KB, dated 04-1987, again different. Donated by Mike Morris WA6ILQ Thanks go to Ed Lambert K1ZOK who pointed out in an email that there was an important error in this schematic file (WA6ILQ edited the image file and corrected it). There was an extra connection line drawn between the collector of the TIP29 and the base connections of the pass transistors. This wire shorted out the regulator driver transistor (Q2). If you downloaded this schematic in the past you may want to download a fresh copy and replace your incorrect copy.
	RS-35A, RS-35M 555 KB PDF, dated 09-1988 donated by Jim Bacher WB8VSU
	RS-35A, RS-35M 159 KB PDF, dated 05-1991 donated by Mike Morris WA6ILQ
	RS-35A, RS-35M 46 KB PDF, dated 05-1995 donated by Mike Morris WA6ILQ
	RS-35M 498 KB, dated 01-2000 donated by Neil McKie WA6KLA (SK)
	RS-35M 78 KB PDF, dated 10-2009 redrawn, corrected, and donated by Stu Martin K2QDE Includes meter illumination.
	RS-35M 127 KB PDF, dated 02-2017, drawn by Larry Joy WN8P This is an actual as-built schematic, drawn from scratch to current IEEE specifications, of an older (1978) RS-35M supply with one meter on the front, stud rectifiers inside, and the TIP29 driver transistor and the crowbar SCR mounted to the chassis (current models have those parts on the regulator board). Astron has since added some parts to the design; these are NOT reflected in this schematic or the parts list below. RS-35M Detailed Parts List 56 KB PDF, goes along with Larry's schematic diagram above. Lists every piece of wire and hardware.
	VS-35M 37 KB PDF, dated 01-1987 donated by Steve Duncan WA4ITA
	VS-35M 262 KB PDF, dated 01-2000 donated by Bob Burchett WB6SLC
	RM-35A, RM-35M 33 KB PDF, dated 01-2013 donated by John D'Errico N1ERF
	RM-35A-BB 80 KB PDF, dated 01-1993 donated by Larry Horlick VYØHL This is a factory Battery Back-Up supply that is also known as a Motorola RRDN6933A. See the comments in the battery backup section above concerning this particular supply.
	RM-35A-BB 300 KB PDF, created 10-2020 by N. Eric Jorgensen W1NEJ This is a factory Battery Back-Up supply purchased in October 2020. Eric reports: I acquired a new RM-35M-BB recently. However the schematic I found on-line is not correct for my unit. It appears the rack mount version of the 35 amp power supply has been upgraded. The differences are: 1. My unit contains an extra 2N3771 transistor between Q2 and the pass transistors. 2. My unit contains six 2N3771 pass transistors, not four. 3. My unit has only two battery isolation diodes, not two sets in parallel (total of four). 4. It has an extra diode in parallel with CR7. I took the non battery-backup version of the RM-35M schematic and drew in the battery isolation, charging, and protection diodes to create a new schematic for a new RM-35M-BB. I note there are slight differences in the parts used for D5, D6, and SCR1 between the schematic that came with my unit and the schematic I have supplied with my changes for the battery backup circuit. I have not verified the exact parts in my unit. I have also not verified the part number of the battery isolation diodes, but they appear to be the same part as the rectifier diodes, D101 and D102. The main rectifier diodes are mounted on the rear panel, and the battery isolation diodes are mounted on a heat sink inside the power supply next to the power transformer and they appear to be identical. The protection diode is wired directly across the output terminals. My unit also contains six pass transistors. By the way, I do not, and will not, use the battery backup circuit as installed in my unit, as it would probably overcharge and ruin the battery.
	RS-50M 181 KB PDF, dated 01-2000 donated by Kevin Custer W3KKC
	RS-50A, RS-50M, RM-50A, RM-50M 36 KB PDF, dated 03-1996 donated by Tom Allinson WB6DGN
	RS-50A, RS-50M, RM-50A, RM-50M 154 KB PDF, dated 01-2013 donated by David Bent
	VS-50M 25 KB PDF, dated 08-1982 donated by Bob Shields KA9TYL
	VS-50M 74 KB PDF, dated 11-1995 donated by Tim Bovard
	RS-50A-BB, RS-50M-BB 127 KB, dated 07-1995 donated by Robert Burton KD4YDC See the comments above on the battery backup feature.
	RS-50M-BB, RM-50M-BB 45 KB PDF, dated 07-2011 donated by Roger Gray N5QS
	RM-60A, RM-60M 43 KB PDF, dated 08-1988 donated by Tom Allinson WB6DGN
	RS-70A, RS-70M 73 KB, unreadable date (probably 08-1988) donated by Mike Morris WA6ILQ
	RS-70A, RS-70M Service Manual 1.58 MB PDF, undated, donated by Hugo Loranger VE2UGO
	RS-70A, RS-70M 227 KB, dated 06-1997 donated by Avent Lane
	RS-70A, RS-70M 74 KB PDF, dated 01-2007 donated by Tom Allinson WB6DGN
	VS-70M 54 KB PDF, dated 06-1995 donated by Matt Trull KX4GG
	VS-70M 154 KB PDF, dated 04-2020 donated by Hugo Loranger VE2UGO

	SL-11A 99 KB PDF, dated 04-1990
	SL-11A, SL-11R 172 KB, dated 11-2009 donated by Paul Mandel W4PGM
	SL-15A 120 KB PDF, dated 04-1990 donated by Ed Lambert K1ZOK
	SL-15M 42 KB PDF, dated 12-1998 This is also the Motorola HPN9041 that is found in several low power applications including the GR400 rack mount, GR500 wall mount and GR1225 tabletop Repeaters. It can be used with an HLN9455 Battery Revert accessory. More details on the HLN9455, including a manual (with a schematic) can be found on the R1225 page at this web site.
	SL-15R 184 KB, dated 11-2009 donated by Dave Christensen KD7UM

	LS-25A 28 volt, 25 A intermittent, 18 A continuous, linear power supply 39 KB PDF, dated 04-2004 donated by Astron. A no-frills 28V power supply similar to the RS-50 supply but with twice the output voltage and half the output current. This supply, in a rack-mount configuration (LSRM-25A), is also sold under the Uniden name as their model ARX‑330.
	LS-35M 28 volt, 35 A intermittent, 25A continuous, linear power supply 197 KB, dated 07-1993 Found on the web. Has some voltage measurements on it.
	VLS-10M 28 volt, 10 A intermittent, 7A continuous, linear power supply 41 KB PDF, dated 01-1990 donated by Roger Gray N5QS.
	VLS-35M 28 volt, 35 A intermittent, 25 A continuous, linear power supply 58 KB PDF, dated 06-1995 donated by Ron Vincent KF4D Similar to the VS-70M supply except for the output voltage (variable from 5 to 32 Volts) and current.
	VLS-35M 28 volt, 35 A intermittent, 25 A continuous, linear power supply 321 KB PDF, dated 04-2016 donated by David Bent.

	SS-10 4 MB PDF, early 1990s Schematic plus parts list for SS-10, SS-12 and SS-18. Not truly accurate, the parts list says that these have a "Banana Socket" for the output.
	SS-12 42 KB PDF, dated 08-1996 donated by Eric Lemmon WB6FLY No parts values other than what Eric noted.
	SS-18 47 KB PDF, dated 11-1998
	SS-25M, SS-30M 483 KB PDF, dated 09-2000 Shows meters and a 120V / 240V input power selection switch.
	SS-25M, SS-30M 1.8 MB PDF, dated 09-2000 Schematic plus parts list.
	SS-25M, SS-30M 1.7 MB PDF, dated 11-2010 Four pages long, no voltage selector switch, but shows illuminated meters. It also has a very detailed parts list at the end.
	SS-30 71 KB PDF, dated 09-2000 donated by Eric Lemmon WB6FLY There is minimal parts info on the schematic. Further info is available here.
	SS-30 157 KB PDF, dated 09-2000 A second copy of the same schematic, but a little more readable.
	SS-25M, SS-30M with enhancements 1.1 MB PDF, dated 09-2000 donated by Alan Sewell N5NA Alan had to repair a couple of these; in the process he corrected the schematic, added LOTS of parts values and other notes, and took extensive voltage readings at various points, complete with photographs. Excellent documentation! It does seem that a meter calibration pot is missing from the ammeter circuit in the schematic, but since he left no contact info, this will have to go uncorrected until he sees this and contacts the page maintainer. Read his full story here. A local copy can be found here.
	A mod for the Astron SS-25 and SS-30 power supplies (and maybe other SS-series) from NU4G: (This mod may or may not apply to other models of Astron switching power supplies) These two power supplies use a simple normally open "click" thermostat to switch a very noisy (acoustic noise, not RF noise) fan on and off. The fan noise can be annoying at times, so I've modified my supplies to make it less so. Open the supply after first disconnecting AC power and allowing for DC voltage to bleed down. Looking inside you will see two sets of heatsinks near the rear of the unit. Each heatsink has a thermostat attached to it, the left side heatsink has a normally closed 50 degree C thermostat for the AC input - don't bother it. The right side heatsink has the fan thermostat mounted to it. The fan thermostat switches DC from the output to power the fan. If you bridge the thermostat with a two watt resistor of 75 to 100 ohms the fan will be on continuously, but very slowly. This slow speed is enough to keep the supply cool with very little noise. Before installing this mod simply running my Icom 756PII HF station just on receive was enough to turn on the fan every 5 minutes or so. With the mod I've only had the fan go to full speed (i.e. the thermostat closed) while operating RTTY for an extended time on a very warm day.

	1212-18 199 KB PDF donated by Tom Allinson WB6DGN This unit allows you to use a negative ground +12 volt device (like a two-way radio) in a 12 volt positive ground vehicle (i.e. with a - 12 volt battery system). The positive side of the DC input and negative side of the DC output are common. Another way of saying this is that this is a ground inverting unit. The schematic shows that it is based on a 556 chip and has a note saying 14 amps continuous duty, 18 amps surge.
	2412 49 KB PDF donated by Ed Lambert K1ZOK This unit is also based on a 556 chip and allows you to use +12 volt devices (like two-way radios) on +24 volt battery systems where the negative side is common (like those found in many Kenworth, Peterbilt and other large commercial vehicles such as earthmovers and fire trucks). This schematic does not have amy notes as to current capacity.
	2412-24 V3a Dated 2011 91 KB PDF donated by Mike Collis WA6SVT. This unit also has a comon negative. Mike reports: This unit has a "Low Voltage Disconnect" built in but it's set to drop out at 18 volts. This may work great for an 18-wheeler truck driver wanting the unit to provide 13.8 volts out on a nearly dead battery, however you might not want to run your radio site battery down so low, especially at a solar powered site. A slight modification will raise the dropout point to save a battery bank at a solar site. Simply change the 62k resistor (R17) on the voltage divider between the input voltage and ground that supplies voltage to pin 16 (UVLO) of the LM5025B chip. When this pin is below 2.5 volts, the converter shuts down; above 2.5 volts, it turns on. There is hysteresis on this pin to prevent chattering (about 2 volts difference between off and on). Changing R17 to 91k will move the Low Loltage Dropout to 22.75 volts and re-connect at just under 25 volts. If you desire an adjustable dropout, add a 30k trim pot to the board in series with the existing 62k resistor.
	2412-12 Rev E 320 KB PDF donated by Tom KN4ONE Like the other 2412 units above, this unit allows you to use +12 volt devices (like two-way radios) on +24 volt battery systems that have a common negative. This model uses a TL494 switch-mode regulator IC to do the work.