One of the overlooked items is the compensation capacitor value - as this web page by Dr. Paul Webster VK2BZC elucidates: Power supply design with the LM723.   The average Astron has NO compensation cap at all (leaving out that cap is one of the cost-cutting methods the Astron designer used).   The 723 data sheet (which you can click on below) has a number of sample circuits, and some show no capacitor, some show 15pF, some show 100pF, the largest is 500pF.   Some design notes rate the capacitors in NanoFarads (1,000pf=1nf).   If your supply has a compensation cap you will find it connected from pin 4 to pin 13 on the DIP package (common), or from pin 9 to pin 2 on the TO‑5 round metal can (rare).   If you supply does NOT have a cap from pin 4 to pin 13, ADD ONE - a 470pf or 500pf tacked across the back of the IC socket is all that is needed.   By the way, the "this country" that Paul refers to on his web page is Australia.

That said, if your Astron is going into current limit at random times, this comment from an email sent by Ron Rogers WW8RR is relevant:

A VERY common cause of random current limit shutdown in linear Astron supplies is due to the manufacturing process: a bad solder joint on the collector tab of one of the pass transistors.   During manufacture they solder the buss wire from collector to collector.   If you take a pair of pliers and grab the buss wire next to the solder joint at each transistor and pull on it, you will most likely find one that will simply pull off.

All it takes is for one of those transistors all wired in parallel to have a bad collector‑to‑buss solder joint and all of the source current from the supply tries to flow through the Emitter-Base junction of that one transistor that has the bad solder joint. Bingo !!! Immediate current limit mode !!

From yet another email to repeater-builder:

Another common failure (these supplies are full of them) is due to the use of parallel‑wired diode bridge rectifiers. A good example is on the RS‑35 supply schematics. They use two packaged bridge rectifiers that are wired in parallel with pieces of #10 wire tying them together with no attempt at current equalization. To use a Martha Stewart term, this is A Bad Thing, engineering-wise; at least the designer used current balancing resistors on the pass transistors. Only the positive halves of the bridge rectifiers are used; the negative ends are left floating. Unless the two bridges are absolutely identical, or at lease very closely balanced, one will hog most/all of the current and the other will just sit there and watch. Eventually one diode will short or open, and that often takes the other one along with it. This causes the supply's primary fuse to blow. If your supply instantly blows the primary fuse, unsolder the two heavy transformer wires (frequently colored yellow) from the bridge rectifier terminals and then see if the fuse blows. If not, check for a shorted main filter capacitor and shorted pass transistors. If all checks out OK, chances are high that one or more diodes have shorted. They have to be completely unwired to be properly checked. I bought several 1,000V 50A diode bridges for about US$5 each as replacements and just use one of those in place of the two parts in parallel in RS‑35s. The ones I have here are labeled KEST KBPC 5010 or KEST KBPC‑5010 and are still being sold on eBay as of this writing (October 2007).   Click here for the KBPC‑5010 data sheet.

Here's a photo of the inside of a dual-rectifier Astron RS‑35.	Here's a photo of the 50a replacement. It's the same physical size as one of the existing rectifiers.
Both photos above taken by WA1MIK to illustrate the above situation.

Astron used different transistors at different times in their history.   Don't be surprised if the ones in your supply are not listed in the table below (and if they are not, please let us know what you find).   Just Google the part number and you will find the info somewhere on the web.

From an email to repeater-builder from an amateur that works in the power supply design field...

Another fault in the Astron is that they under-engineered the crowbar circuit - they used a design appropriate for a 7 amp or 12 amp supply in every size supply up to and including 50 amps !!!   The active part is a 2N681 that is rated at 25 amps peak.   They later went to an SO565J device, but that is rated at only 50 amps or 70 amps (it depends on which manufacturers data sheet you read)... and on a crowbar you ALWAYS go for at least two times the maximum current expected, if not four or even ten.   After all, the crowbar triggers because the regulator failed (or somehow exceeded the crowbar trip voltage), and the purpose fo the crowbar circuit is to blow the fuse and shut down the supply.   To do that the SCR has to simultaneously short the full current of the supply AND dump the stored energy contained in the fully charged caps, and doing it without comitting suicide by overcurrent !   Using an undersized SCR or small diameter wiring (i.e. current limiting) is defeating your own purpose. Personally I'd use a heftier device and bigger wire if I was repairing or rebuilding a high-current Astron.

If you have to replace all of the pass transistors in a supply, and the old ones are 2N3055s or 2N3771s (also known as the ECG181) do yourself a favor and buy something larger ‑ at least 2N3771s and preferably 2N5686s.   All the devices listed in the table below are drop‑in replacements with better performance for a very reasonable price.

For those that want details, here you are:

Transistor Part Number	IC Collector Amps (see note 1)	BVCEO Breakdown Voltage	hFE Gain min‑max at IC	PD Power Dissipation (watts) at 25°C or 77° F (see note 2)	DigiKey Price each (mid 2006)
2N3055	10	60	20-70 @ 4a	75	$2
2N3771 ECG-181	30	40	15-60 @ 15a	150	$2.50
2N3772	20	60	15 @ 10a	150	$2.10
2N3773	16	140	50-60 @ 8a	150	$2.25
2N5301	30	40	15-60 @ 15a	200	$4
2N5302	30	60	15-60 @ 15a	200	$4
2N5686	50	80	15-60 @ 25a	300	$8

Note 1: The maximum collector current specified above is only valid as long as the internal transistor temperature is less than the rated maximum temperature.   Also note that the higher-numbered 2N3773 has a max current of about 1/2 that of the lower-numbered 2N3771.

Note 2: As the temperature goes up, the power dissipation ability goes down.   During long key-down sessions the internal chip temperature of the transistor will be a lot hotter than the case, which will be hotter than the heat sink.   The characteristic that causes a time lag between internal chip temperature rising and the heat sink temperature rising is called thermal resistance.   Think of the situation as including a "heat pipe" between the source (the transistors) and the destination (the heat sink) - like water, the larger the diameter of the pipe, the greater the quantity flowing, and the lower the thermal resistance, the faster the transistor can dump the heat into the heat sink.   You can't do anything about the thermal resistance between the internal chip and the case of the power transistor (i.e. the internal construction of the power transistor), but you can lower the thermal resistance between the transistor case and the heat sink with an application of the proper type and amount of heat sink compound.   And you can add a fan blowing air across the heat sink to help it shed the heat. Note that due to air density a fan on a heat sink at a mountaintop site at 5,000 feet won't cool as well as at sea level, fortunately mountaintop sites are generally cooler than at sea level.

One of the two pass transistors on the heat sink of an RS‑20.
The "9549" is a date code and indicates that the
transistor was made in the 49th week of 1995.

The 2N3055s, 2N3771s and 2N3772s have been found in known-stock Astrons.   The 2N3773s, 2N5301s, 2N5302s and ECG181s have been found in used ones bought at swap meets.   The 2N3773s may have been stock (if they were replacements they were very nicely done, and I couldn't tell), the 2N5301s and 2N5302s were obvious field replacements.

I use the 2N5686 exclusively as a replacement in large supplies as not one supply I have rebuilt with them (over 20) has EVER come back to haunt me (at least for a pass transistor problem).   Yes, they are overkill and cost more, but the price difference on four new devices for an RS‑35 is around $25, even if the originals are 2N3055s.   If you decide to save the $25 and use the 2N3771s what is a future failure, the down time, a round trip to the repeater site (price of vehicle wear and tear plus the gasoline) and ANOTHER power supply rebuild going to cost?   What is your time worth?   If you find 2N3055s in your used supply and don't want to go to 2N5686s at least replace them with 2N3773s.   That provides 60% more current capacity and twice the power dissipation for an additional 25 cents each.   If you use 2N3771s that's triple the current for an additional 50 cents each (and that's what Astron uses anyway).

Don't mix the transistor types !!!   The emitter ballast resistors do their balancing act only when the transistors are identical.   Always replace a dead pass transistor with an identical part number, or if you can't find an exact match, then replace all of them as a group.   And don't go down in ratings - if you find 2N3771s do not replace them with 2N3055s, 2N3772s or 2N3773s.   A short rule of thumb: Use only the 2N3771s or 2N5686s.

If you find one bad pass transistor measure the resistance of all of the emitter ballast resistors (the current balancing resistors) and compare each against the others.   If even one is different, replace ALL of them with new 5% (or better) units - that way you have identical values (after all, they are supposed to evenly distribute or balance the current, and can't if they are not absolutely identical in resistance).  Using all new from the same batch is the simplest practical way to get identical values.

Don't forget to use some good beryllium based thermal compound (the thick white stuff that is the consistency of axle grease), but don't go overboard - you want just enough to put a thin layer between the transistor and the insulator, and again between the insulator and the heat sink.   All you are doing is eliminating any air pockets.   Note that beryllium compounds are known to be human carcinogens when inhaled, fortunately the greasy consistency of the heat sink compound prevents any airborne dust, but you still want to keep it off your skin (i.e. use a surgical glove or gloves for protection, and maybe a popsicle stick to spread it thin).   See this web page for more details.   See this Wikipeida page on the results.

If you can't find the beryllium based thick white stuff then use the Arctic Silver that is made for use between the CPU chips and the heat sinks in high-end PCs.

As long as I am inside the Astron I also do the following:

• Some Astron models use stud mount diodes, others use one or two rectifier blocks in parallel.   The ones that use two rectifier blocks get them replaced with one large one rated at least 2 times the maximum current of the supply, if not 3 or 4 times.   Yes, I prefer to use a 50 amp bridge in a 12 amp supply. It's cheap and I won't have to worry about it ever again. Some of the larger supplies that use a pair of rectifier blocks get converted to 75 amp or 100 amp stud‑mount diodes.   Astron uses two 40a diodes in parallel on each side of the transformer in an RS‑70, personally I feel that's not enough headroom, and besides, there aren't any current balancing components.   In the last one I rebuilt I used two 200 amp diodes, and because I used one on each side of the transformer there was no balancing required.
  
  
• I add a 0.1 uf capacitor rated at 100 volts (or better) from the positive terminal of the large bridge rectifier to ground.
  
  
• I add three 0.01 uF 600 volt (or better) bypass caps - one across the AC line and one more from each side of the AC line to ground, and I use the gap-cap version of the bypass cap if I can find them (see http://www.vishay.com/docs/28521/gapkap.pdf). I've found a bypass cap across the line in some of the Astrons I've worked on, most don't have it.   And since the Astron is on my bench because it has problems, the existing bypass cap gets trashed.
  
  
• I add three 150 volt Metal-Oxide Varistors (MOVs) - one in parallel with each of the gap-caps mentioned above (areas with 220 volt AC mains will need 250, 260 or 270 volt devices).   Some Astrons already have one MOV (across the power line) already in place. Due to the normal failure mode of a varistor (see the "Regarding MOVs" paragraph below), and due to the reason that the Astron is on my bench is because it has problems, I assume the existing varistor is toasted and trash it.
  
  
• I add the missing compensation capacitor (470 pf or 500 pf) - see the 723 data sheet and the text above.
  
  
• I scrape away the paint and add a star washer under the power cord green wire lug.   I've worked on several Astrons that arrived with a non-functional safety ground wire in the power cord due to the paint between the lug and the case.   In other words, the housing / chassis was floating above mains power safety ground.   Use your ohm meter to test between the case to the ground (round prong) of the AC power plug.   It should show zero ohms.   Remember the power plug green wire is a SAFETY ground, and your survivors will appreciate it if you don't scrimp on the safety.   Make it a good solid ground.   And remember that the "green" wire may be green with a yellow stripe or stripes.
  
  
• I add more split or star washers under the pass transistor mounting screws.   I've seen thermal cycling loosen them.
  
  
• Most of the time I relocate the voltage adjustment pot to the front panel, mounted under the voltmeter (if present).   In most Astron supplies the voltage adjustment is a 1K carbon linear pot.   I simply mount a screwdriver-adjust pot (of the type that has a locking nut on the adjustment), and run wires to the pads where the regulator board voltage pot used to be.   I use a three conductor shielded wire for this, and ground the board end of the shield.   By the way, the vertical tab on the left side of the pot in the photo is an anti-rotation trick.   When you mount that style of pot you need to drill two holes - one for the center shaft and a second smaller one for the anti-rotation tab.
  
  
• On the VS series, and on the ones I relocate the voltage adjust pot I add ferrite beads to the leads of the front panel potentiometer(s) right at the PC board.   A drop of hot melt glue holds each one in place.   Some of my Astrons are at broadcast sites, and I don't need high RF levels confusing the voltage regulators, the overcurrent foldback circuit, or the crowbar circuit.
  
  
• I add an IEC style power cord fitting (available for free from a dead PC power supply).   Yes, it requires a bit of sheet metal work, but it's worth it to prevent tripping over a dangling power cord again.   I still have scars from the rough concrete on the steps of the repeater building.   The MOVs and gap-caps mentioned above can be soldered right to the pins on the back of the IEC socket.   The IEC may not be appropriate for an RS‑50 or RS‑75 unless you can find a #12 or #10 wire IEC power cord... the last thing you want is for someone to slip up and use a leftover wimpy #20 or #18 wire PC monitor-grade cord.
  
  
• Make sure the fuse holder is wired properly - the center pin is the hot pin and is fed by the switched power line input, the outer sleeve (nearest the case) goes to the transformer.
  
  
• Most of the Astrons use a rocker-style power switch with an internal neon lamp to indicate power on.   After a while the neon lamp dies from old age.   I add a diffused (for wide viewing angle) bright green LED to the front panel, with an appropriate series resistor, and wired across the +12vDC output.   Or you can do as one friend did and carefully take apart the switch and replace the dead neon lamp with a red LED, then reassemble it.   Add the series resistor and you are done.

Regarding MOVs, since the failure mode is an open circuit remember that they have a finite life and do wear out.   This is because when the voltage across a MOV reaches the breakover point, the MOV avalanches and conducts therby shorting the excess impulse energy into heat within the body of the MOV itself.   The problem is, the internal heat affects the MOVs internal characteristics - its breakover voltage (threshold voltage) increases with each hit it takes.   The next impulse comes along and less of it gets shunted.   If a MOV sees enough action, it opens, but physically it still looks perfectly good.   When the MOV is open the equipment protection is compromised to zero and you won't know it (this failure mode is why the cheap PC "surge protection" power strips are a joke and a delusion - usually all that is inside is a single MOV).   Tripp-Lite "Isobars" are much, much, much better - and I use one as the rack cabinet power entry protection at every repeater site.   MOVs are cheap, however, and better than no protection at all.   Just plan on replacing them every so often, especially if the power line feeding the power supply has taken a lightning strike (at which point you replace all three, plus the gap-caps).   I've seen MOVs reduced to two bare leads waving in the breeze and bits of red plastic scattered around the inside of the case.   But the supply was repairable, and is still in service.

If the Astron I'm rebuilding is going to be powering a continuous duty load (i.e. at a repeater site) I add a voltmeter, an ammeter, and if it's a high duty cycle load (i.e. a busy repeater) a 24vDC fan (or two 12vDC fans in series) blowing air across the heat sink...   If it's a busy repeater with large supply (with a wide heat sink) I'll use definitely use two.   A 24vDC fan run at 12vDC moves enough air to keep the supply heat sink cool and will last a lot longer than a 12v fan running at normal speed.   To save the fans from running 24x7 you could use a PTT-triggered timer to run the fans when the repeater is actually in use.   Many repeater controllers have a digital output that can control a fan or a set of fans.   If yours does, and the programming supports it, you could have it ignore a kerchunk (i.e. a transmission of less than 30 seconds), yet turn othe fan(s) on when the repeater becomes active (more than 60 seconds).

Don't forget that the power transformer and rectifier block need cooling as well.   Depending on the duty cycle you may want to add an additional fan blowing air through the box.   Note that the bridge rectifier is going to drop about 0.7v to 0.8v at anything up to the full load current. Let's say that you have an RS‑50 loaded to 50% rating.   25 amps times 0.7 volts is 17.5 watts, and that's only at half of the maximum current.   17 watts is a LOT of heat for a small-package bridge rectifier to dissipate, especially with nothing but convection cooling inside an effectively sealed case.   As easy as it is to do, add a little white beryllium thermal grease under the rectifier block and let the bottom plate of the supply shed some of the heat, or remove the epoxy case unit and install a couple of heavy duty stud-mount diodes into the backside of the heat sink.
The power transformer is not 100 per cent efficient so it will generate some heat as well.   If the supply is going to be running a continuous duty load it's worth punching a 3 inch or 4 inch diameter hole in the top and bottom of the power supply case, put copper screening over the holes and use a 24v fan to force some air flow over the internal components (punching one of the holes in the left side of the lid and the other in the right side of the bottom results in cross-wise air flow at zero cost).   If cabinet clearances are tight (i.e. no room above or below, like in most rack mounted repeater systems), I'll punch both ends of the Astron cabinet, add the copper screening and mount the internal fan on the outside over the intake hole (in general, pushing air into a cabinet cools better than sucking air out).   Copper screening is stocked by most model airplane / hobby stores and you will find that it is easy to solder a grounding pigtail to it.

In some cases I modify the supply for remote voltage sensing, especially if it's a high power repeater (high current load).

On the above-mentioned topic of upgrading the pass transistors, note that on some models it may not be worth the parts or the effort to install them .   The RS‑7 uses a single 2N3771, a 30 amp device.   The RS‑12 uses (used?) a pair of 2N3771s.   Obviously the RS‑12 was designed back in the days when 2N3055s ruled and it took two devices to get over 10 amps, and when they started stocking 2N3771s they just switched to that device across the entire product line.   Why they didn't strip the RS‑12 back to one transistor and save some parts cost is beyond me - one 30 amp rated device would have been plenty of overkill on a (rated) 12 amp supply that's only good for 6 amps in real life.   On the RS‑12, at least, save your modification time and money.

On the above-mentioned topic of IEC power connectors, make sure you wire the connector properly - see this drawing.   Make sure you have continuity from the hot pin to the center of the fuse holder and via the on-off switch.   Some IEC connectors have color dots on the pins: brown is the international standard for hot, blue for neutral, and green or yellow for ground.   Some manufacturers use molded-in lettering, "L" for line, "N" for neutral, "G" or "E" for ground or earth.   The sequence of flow for the power should be from the hot pin of the IEC connector, to the switch, to the tip of the fuse holder, and from the barrel of the fuse holder to the transformer.

Note that on some of the Astrons the DC output is floating from the case and on others the negative output is connected to the case.   A good example of this is to look at the RS35 schematics below from 1987 and compare them to 1991.   Personally, any supply that goes to a repeater site or any other location that has a transient technical crew gets a P-Touch label stating either "Negative side is floating from the case" or "Negative side is grounded to the case".
Eric Lemmon WB6FLY ran into this situation and wrote about it:
It has come to my attention that Astron has a built-in design flaw that may cause problems for some repeater operators.

I discovered this when I replaced a suspect Pyramid power supply at my mountaintop 220 MHz repeater with an Astron SS-12 switching power supply. When I got back home, and in a very quiet environment, I was shocked to hear a very prominent 60 Hz hum on the 220 carrier. Since a switching power supply uses a switching frequency up in the 40 kHz range, I could not understand how there could be 60 Hz hum!

The very next day, I took a known-good DuraComm switching power supply with me and returned to the repeater site and exchanged the two units. This time, I moved some distance away from the repeater building and tested with a handheld to ensure that the carrier was hum-free, and it was. I could not detect the hum on the first trip because electrical equipment next to the repeater is quite noisy.

Once I got the Astron power supply on the bench, the cause of the hum was obvious: The negative output terminal was grounded internally! Although most large Astron power supplies such as the RS-20, RS-35, SS-25, and SS-30 have a black jumper wire between the negative terminal and the case, the SS-12 uses a trace on the PC board to make the connection. I then e-mailed Astron Tech Support and received a schematic of the unit, along with advice as to where the offending trace was located. A quick bit of work with an Xacto-style hobby knife cut the trace, and floated the negative output lead. Problem solved!

Astron seems to be the only power supply brand that routinely grounds the negative lead; none of my units made by DuraComm, Samlex, Astec, or Pyramid have this connection. The hum was caused by a ground loop injecting 60 Hz into the DC source feeding the repeater. Since the radio, duplexer, and antenna feedline is always solidly grounded for surge protection, that means that the DC power source is grounded in more than one place- a very bad idea. I have modified all my Astron power supplies- both linear and switching- to remove any internal connections to ground at the negative DC output.

I had a similar problem several months ago on my 6m repeater, which had a recurring problem with controller lockup. After I swapped the Astron RS-35M power supply and put in a DuraComm supply, the problems went away. As you might expect, the Astron RS-35M was causing a ground loop, but this time it didn't cause audible hum. It did, however, corrupt some of the data signals going to the controller.

I strongly suggest that owners of an Astron power supply make a simple test with a V-O-M. With the output connections open and the power supply unplugged, measure the resistance between the grounding prong of the AC plug and the negative DC output terminal. If the reading is in the megohms, fine. If it is a short, you know what to do...

73, Eric Lemmon WB6FLY
Eric provided some photos of the PC board in the SS-12, and the board is probably similar to other Astron switching supplies:           Photo 1           Photo 2           Photo 3

All Electronics is a good reputable surplus source for a wide variety of goodies, including #12 IEC cords, voltmeters, ammeters and fans.   If you are going to add a fan (or two) to a repeater site power supply make sure you use a new name-brand ball bearing fan - cheap fans use brass or bronze bushings, cheaper fans use plastic bushings, good fans use ball bearings and are worth the extra money - what is a service call to replace a fan going to cost you?   Fans with needle bearings are even better (i.e. mil-spec quality) but not too common and when they are found are usually expensive.

Don't even bother asking Astron if the factory metering option can be added later on...   One day I was visiting a client in Irvine and used the opportunity to stop in at Astron to pick up a RS‑20A and RS‑35M schematic, and casually asked if I could buy the lower half sheet metal of an RS‑20M plus the meters to upgrade my RS‑20A to an M series.   Yes, but the price was over 2/3 of the cost of a new supply - plus shipping ! ("Sorry, you'll have to pay in advance, and we'll have to ship them, they aren't in stock")   For that price I can buy a matching pair of surplus meters and cut the holes myself.   By the way, adding the metering to most of the Astron designs is not hard - acquiring matching voltmeters and ammeters (All Electronics, mentioned above, has decent imported meters), and cutting the meter holes is the hardest part.   For the wiring just refer to the RSnn‑M version of your supply or of a similar model.

Regarding power supply metering, from another email to repeater-builder:

...the typical Astron RS‑nnM power supply ammeter is NOT a load current meter in the classic sense - it is calibrated in amps but is wired as a voltmeter, and actually measures the voltage drop across one of pass transistor emitter ballast resistors (the load balancing resistors).   This technique only works properly if all of the pass transistors are absolutely identical (not just the same part number) AND all of the emitter ballast resistors are exactly the same resistance (not just the same marked value).   Then and only then is the voltage drop across the one resistor directly proportional to the entire load.   Neither the pass transistors nor the emitter resistors are that closely matched, so the resulting displayed value is only approximate.

That said, who really needs that accuracy in a low-priced test bench supply?   The Astron method of reading the voltage drop across the ballast resistor that carries 1/2, 1/4, 1/6 or 1/8 of the total current is "good enough" for any rough measurement.   If you need better, you are probably not using an Astron.   If you are, then there are two options:

• Buy (or make) a proper meter shunt and position it in series with the total power supply output, and rewire the Astron meter (it's a 1ma meter) across it, then adjust the calibration pot to set the meter to the correct value, or...
• Get a meterless Astron and cut holes, then mount digital meters in the front panel along with a properly rated current shunt positioned as mentioned above. You can get decent digital panel meters (DPMs) for under $20 from Circuit Specialists and several other sources.

More on power supply metering, from yet another email to repeater-builder:

There are plenty of 3-1/2 digit LED and LCD meters that can be purchased from electronics and surplus businesses.   For example, Marlin P. Jones & Associates (at www.mpja.com) has several in the $9-$12 range.   Make sure that the meter you buy will work with a common ground for its power input and meter input.   Not all meters can do that. In particular, those that require a 9V supply often state they can't measure their own power supply.   Get one that operates on 5V and use a LM78L05, LM78M05 (or similar) regulator to run the meter off the Astron's internal unregulated DC supply across the main filter capacitor.   The 5v regulator can be mounted almost anywhere.

You may also be content with a small analog meter.   There are plenty of 0-15vDC meters for under US$10 that will do quite nicely.   You can also use just about any meter you have and add an appropriate resistor in series to give you the scale you want; Astron uses 1mA DC meters in their supplies for both voltage and current.   See any of the power supply 'M' schematics for details.   Or use a 5 volt meter and "stack" it on top of a 10vDC reference, then calibrate it as 10v at the bottom end and 15v at the top.

For test purposes I disconnected one end of both of the two meters on an RS‑35M and connected a Fluke digital multimeter, a 10k resistor, and a 10v power supply all in series. Both meters went to just about full scale, and the DMM read 995uA, so these are definitely 1mADC full scale meters.

There's about 15k ohms in series with the voltmeter. This is done with a small pot soldered in series with one terminal on the back of the meter. I suspect the full-scale resistance is in the 20-25k ohm range.

There's about 360 ohms in series with the ammeter. This is also done with a small pot soldered in series with one terminal on the back of the meter. I suspect the resistance is in the 500-1k ohm range, but it will definitely vary depending on the current rating of the power supply.

I calibrated my supply by setting the voltage to 14.00 on an external meter and adjusting the Astron's voltmeter to 14 volts. I then hooked a pair of 1.0 ohm 250 W resistors in parallel across the output terminals, and adjusted the Astron's ammeter for 28 amps. I had to work fast, those resistors get hot in a hurry (with almost 400 watts being dissipated).

From another email to repeater-builder describing another failure mode, and the fix:

Most of the Astron designs have a single secondary with three taps on the power transformer for a total of 5 wires.   The secondary is one winding, with the center section being the heavy (high current) wire and the two outer sections use much thinner wire and is just for powering the voltage regulator board.   See the diagram below:

----------------- ----------------- Thin wire (to voltage regulator board) ) ( ) ( ) ( Thin wire section of the secondary winding ) ( Primary winding ) ( ) ----------------- Thick wire to main rectifiers ) ( ) ( ) ( Thick wire section of the secondary winding ) ( ) ( On the 120/240 ) ----------------- Center tap (thick wire) models the ) ( primary is in ) ( two sections ) ( Thick wire section of the secondary winding that are in ) ( parallel for ) ( 120vAC and in ) ----------------- Thick wire to main rectifiers series for ) ( 240v. ) ( ) ( Thin wire section of the secondary winding ) ( ) ( ----------------- ----------------- Thin wire (to voltage regulator board)

I've seen two supplies with one of the outer sections opened up.   There is no way to do a stock repair of it short of a new transformer.   The inexpensive method to salvage the supply is to abandon the thin-wire section and move the two thin wires from the regulator board to the secondary of an added small separate 24vAC transformer.   There is plenty of room inside the cabinet for it.

Fusing:
Most of the Astron schematics show a fuse in the AC mains side of the transformer.   Some of them do not specify slow-blow or fast blow.   You will want to use a slow-blow fuse rated at the mains voltage (or greater) when powering a highly inductive load, like a big transformer or motor.   There are "32 volt" fuses made for low voltage circuits (i.e. automotive, etc).   Don't use them - you want one rated at 125 volts (mains voltage in the USA) or 250 volts (in localities where 220 or 240 volt circuits are used).   Linear supplies draw a lot of current during the first few cycles of the AC voltage after being switched on.   If there is really a short circuit, the excessive current draw will blow the slow-blow fuse rather quickly.

The fuse that is came in my stock RS‑35M supply (probably 15 years old) is a Littelfuse part number 326008.   That is an 8 amp, 250 volt, Slo-Blo fuse with a ceramic body, also known as type 3AB.   The markings on the back of the supply just says "8A".   Some lower rated supplies use a 5 amp or even smaller fuse.   If someone else is going to be servicing your repeater it wouldn't hurt to add a label reading "8A Slow Blow Fuses ONLY" (replace the "8A" with the appropriage amperage for your supply), and put a few 3AB style fuses into the on‑site toolbox.   Or put some fuses in a zip-lock bag along with a magnet and use the magnet to stick the bag to the front, top or side of the steel power suply housing.   Or replace the fuse with a resettable slow‑blow circuit breaker.

Linear Power Supply Design and Theory

• The Power Supply Page by Karl Shoemaker AK2O of the Spokane Repeater Group
  A good overview article (offsite link to the SRG web site)
• "Power Supply Analysis"   This is a PDF of an article from the December 2005 QST that explains how a linear power supply works, and uses an Astron RS‑35 as the "victim".
  Thanks go to the ARRL for permission to use their article, and to George Henry KA3HSW for scanning it and PDFing it.
• Jim Larsen, AL7FS (the ARRL Alaska Section Manager) took that QST article and the schematic and produced an annotated schematic that takes advantage of the annotation fuction of the PDF architecture. Use a recent version of the Adobe Acrobat reader and just mouse over the yellow symbols. Worth studying.
• "Understanding and Using the 723 Voltage Regualtor" By W.C. Cloninger, Jr., K3OF from Ham Radio Magazine, March 1989
• Regulated Linear Power Supply Construction, or What's inside your Astron™?, by David Metz WAØAUQ
• National Semi's Application Note #556 - An Introduction to Power Supplies - Very worth reading.
• Here's a copy of the actual National Semiconductor LM723 / LM723C data sheet.
• Here's a copy of the Texas Instruments µa723 data sheet.

• LM138 and LM338 5-Amp Adjustable Regulators Data Sheet
• National Semiconductor LM340-nn and LM78nn Three-Terminal Positive Voltage Regulator Data Sheet
• National Semiconductor LM79xx Three-Terminal Negative Voltage Regulator Data Sheet
• National Semi's Application Note #103 on the LM340-nn and LM78nn three-terminal regulator chips
• National Semiconductor LM117/LM317A/LM317 Three Terminal Adjustable Regulator Data Sheet
• National Semi's Application Note #181 on the LM117 and LM317 adjustable three terminal regulator chips
• National Semi's Application Note #182 - Improving Power Supply Reliability with IC Power Regulators
• National Semi's Application Note #1148 - Linear Regulators: Theory of Operation and Compensation

Miscelaneous Data Sheets:

• 1N1184A stud-mount diode   This is the diode used in the high current Astrons like the RS‑70. It's rated at 100v, 40A (note that the plain one, the 1N1184 is rated at 35A).
• S035J SCR   This is the 50 or 70-amp SCR that is used in the higher power crowbar circuits (the amperage depends on which manufacturers data sheet you read).
• 2N681 SCR   This is the 25 amps peak device used in the smaller supplies.
• 2N681A SCR   See above.

Reset circuits for Astron Linear Series Power Supplies

• An external reset circuit for the Astron Linear series     A reset circuit by Kevin K. Custer  W3KKC
• An internal reset circuit for the Astron Linear series     A reset circuit provided by The Astron Corporation
  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 on the piece of perfboard is for.

Modifications for Astron Linear Series Power Supplies

• 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.

Battery Back-up Modification for Astron Linear Series Power Supplies
If you inspect the circuit diagram you will see that the BB option is simple - it consists of two diodes that allow either the power supply or the battery to feed the load. In the low current supplies they use half of a bridge rectifier, in the high current supplies they use two separate diodes in parallel for each current path.   The charging circuit is not well designed as it is just a single resistor from the battery to the output of the supply (look for R103 in the schematic for the RS‑12‑BB)... and this simple charger circuit can boil your battery dry!   This circuit requires you to have the supply voltage set EXACTLY at the sum of the diode voltage drop plus the battery float voltage regardless of what the voltage your load wants to see.   Overly simple charging circuits ALWAYS leave a lot of room for improvement.   But it does give you a starting point to design a better one.   Or if you have an external charger (like a Deltron "Battery Tender") you can simply remove R103 (or it's equivalent in larger supplies).   Or mount a toggle switch on the front panel of the supply near the diodes, wire it in series with R103 and label it as "Internal Charger Enable / Disable".

• Schematic of the RS‑12A‑BB     Factory Battery Back-Up Schematic donated by John Lund
• Schematic of the RS/RM‑50M‑BB     Factory Battery Back-Up Schematic donated by Robert Burton KD4YDC

Schematic Diagrams of some popular Astron Linear Power Supplies

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, etc.   For example, the early supplies use discrete stud-mounted diodes instead of half of a bridge rectifier (switching to a 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 sending us a scan.

We have received emailed requests for schematics on the model 1212 and 1212-18 switching regulated converters and for the model SS10, SS18, SS25, SS25M supplies.

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 ID's and values.   Unfortunately this is important because if one chooses to buy replacement parts (from Astron) they (according to Astron) need only to supply the model and component ID's....  Fortunately everything but the filter caps, transformer, and sheet metal 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.

Notes:

• An "RS‑" prefix is a standard power supply. I suspect that RS stands for 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. Usually there is little (if not zero) 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 wiring changes shown in the schematic.
• SS-series are switching power supplies - their schematics are further down the page.
• SL-series - you'd think that the leading "S" would imply a switching supply, but these are low profile supplies.
• A trailing "M" indicates the metering option.
• A trailing "S" is the speaker version.
• A trailing "BB" indicates the battery backup option.

Astron builds supplies for several manufacturers - for example a Kenwood KPS-12 is based on the Astron RS‑12, the Motorola HPN1007A (photo) is an RS‑10 derivative, and the Motorola HPN9041 is an RS‑20 variant.   Astron also builds supplies for EF Johnson, GE, Icom, Midland, Uniden and Vertex.

Donations of additional schematics for the library below are always welcome !!
Send them to Mike WA6ILQ at: (callsign) -at- repeater-builder -dot- com - and thanks in advance!
(you will be credited unless you tell us to assign it to A. Nony Mous)

See the "Notes" section above for an explanation of the prefix and suffix letters

If you don't find the schematic for your Astron below then we were not given it.

• RM-60A, RM-60M   1.8 MB PDF, dated 08-16-1988 but counterstamped 03/20/96   Donated by Mike Morris WA6ILQ
  The RM series has a 60 amp model, the RS series does not.   At the moment this is the only RM-specific model in the library.
  
  
• RS-3A and RS-4A   364 KB, dated 12-89   Donated by Oscar Ramsey NV3G
  (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 10-94   Donated by Mike Morris WA6ILQ
  
  
• RS-10A, RS-10S   105 KB, dated 9-81   Donated by Bill Netzlof KL7IGB
  
  
• RS-10A, RS-10S   368 KB, dated 9-88 (but rubber stamped Sept 22 2003 on the drawing)   Donated by George Franklin WØAV
  
  
• RS-12A   199 KB, dated 11-83   Donated by Richard Reese WA8DBW
• RS-12A, RS-12M   125 KB, dated 6-88   A. Nony Mous found this on a Russian web site and forwarded it.
  
  
• RS-20A   71 KB, dated 11-12-1978   Donated by Gary Eldridge KC8UD
• RS-20A, RS-20S   79 KB, dated 9-23-88   Donated by Kevin Custer W3KKC
• RS-20A, RS-20S   185 KB, dated 9-24-88   Donated by A. Nony Mous
  
  
• RS-35A   135 KB, dated 1-11-1978   Donated by Joe McIntyre W4DEX
• RS-35M   46 KB, dated 4-87   Donated by Kevin Custer W3KKC
• RS-35A, RS-35M   216 KB, dated 4-87 but different   Donated by Mike Morris WA6ILQ
• RS-35M   47 KB, dated 4-87, again different   Donated by Mike Morris WA6ILQ
  Thanks to Ed Lambert K1ZOK who pointed out that there was an important error in this schematic file (the repeater-builder staff edited the image file and corrected it). There was an extra "line" drawn between the collector of the TIP29 and the bases of the pass transistors. This, effectively, shorted out the regulator driver Q2.   If you downloaded this schematic in the past you may want to get a fresh copy and replace your incorrect copy.
• RS-35A, RS-35M   132 KB, dated 5-91   Donated by Mike Morris WA6ILQ
• RS-35A, RS-35M   87 KB PDF, dated 5-95   Donated by Mike Morris WA6ILQ
• RS-35M   498 KB JPG, dated Jan-2000   Donated by A. Nony Mouse
  
  
• RS-50M   182 KB, dated dated Jan-2000   Donated by Kevin Custer W3KKC
• RS-50A-BB, RS-50M-BB   127 KB, dated 7-95   Donated by Robert Burton KD4YDC   Factory Battery Back-Up Schematic
  
  
• RS-70A, RS-70M   87 KB, dated 4-87   Donated by Henry Clark KC4KZT
• RS-70A, RS-70M   73 KB, unreadable date (probably 8-88)   Donated by Mike Morris WA6ILQ
• RS-70A, RS-70M   227 KB, dated 6-97   Donated by Avent Lane
  
  
• VS-20M   1.02 MB, dated 11-12-78   Donated by A. Nony Mous
• VS-35M   322 KB, dated 1-87   Donated by Steve Duncan, WA4ITA
• VS-70M   132 KB, dated 6-30-95   Donated by Brian Palmersheim KBØETC

Schematic Diagrams of some other popular Astron Switching Power Supplies

• SL-15 low profile supply   135 KB   Dated 4-15-1990   Donated by Ed Lambert K1ZOK
• SS-12 switching supply   55 KB,   Dated 8-22-1996   Donated by Eric Lemmon WB6FLY
  No parts values other than what Eric noted.
• SS-30 switching supply   70 KB,   Dated 9-25-2000   Donated by Eric Lemmon WB6FLY
  No parts values other than what Eric noted.
• A mod for the Astron SS-25 and SS-30 power supplies from NU4G: 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, 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 2watt 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 756PII on receive was enough to turn on the fan every 5 minutes or so. With the mod I've only had the fan come on while operating RTTY for an extended time on a very warm day.
  This mod may or may not apply to other models of Astron switching power supplies.
• 2412 Switching Regulated Converter   140 KB,     Donated by Ed Lambert K1ZOK
  This unit allows you to use a +12v device (like a two-way radio) on a +24 volt battery system. The negative side is common. If you need a ground inverting system there are transformer-based units from other companies that allow a negative source voltage.

More from Skipp May WV6F:

I have for sale an exact drop-in replacement for the Astron regulator board.   This is a much improved circuit design... it addresses all the known problems, i.e. it has additional RFI and noise bypassing , overshoot control, improved regulation, fixes the dreaded crowbar circuit....   I test each board for proper operation, I've never had one fail, nor the crowbar circuit fire, even at high-level RF sites.   There is an option available for a front panel variable dc voltage control.   It's a complete redesign, much better than the original board supplied with your supply.   It comes fully assembled and tested.

Upon initial install, the user with the new regulator board retrofit tests the crowbar circuit.   Indeed no crowbar protection (function) has ever fired inappropriately in units with the new board installed.   This classic gremlin has been properly killed.

I have applications where power supplies simply cannot fail.   I came up with the retrofit regulator board project to keep the sanity of some very high-end customers and myself.   Most all of the 30 plus boards I have "out there" have been retrofit by me for customers as a part of a complete supply upgrade package.

Yes, they are pricey at near US$50 each, but well worth a retrofit into the 75, 50 and 30 amp supplies.   Commercial customers with life safety power supply failsafe requirements pay a considerably higher price for the same circuit board.

Installation is simple: You simply unscrew and unsolder your original regulator board after noting (and writing down) the original wire connection points.   The replacement board drops right in and you solder the corresponding original wires to the same locations.   The board connection points appear almost exact (but the circuit definitely is not) because I made an effort to lay out the board that way.   If your power supply was working before the retrofit, you simply power up, test and go.   Each regulator board is hand tested before they are sent out.

If your power supply had previously failed, you should first test the pass & driver transistors, emitter ballast resistors and a few other small items before you re-apply power to the supply.

Note that the regulator board must be ordered per the size / type of Astron supply that you have.   They do not interchange from one supply size (amps) to a different supply size.

If you are interested contact Skipp at Skipp025 -at- yahoo -dot- com      And that's skipp(zero)(two)(five), not skipp(oh)(two)(five).   And there are two "p"s in Skipp.

And an email from someone who bought Astrons' revised (newer) regulator board as a replacement part:

From Mike Perryman K5JMP 
Subject Re: Astron's own update package
Date Mon, 2 May 2005 

The package with Astron's replacement regulator board showed up this morning...

What a mess!  I did as instructed, and snail-mailed an order including a check... 
like "pre-paid"... ya know...   Package arrived $44.63 due COD?   Of 
course UPS wouldn't release it until I stroked another check.   So I
called Astron, and the sales guy blamed the mix-up on the shipping guy (not
surprising!!!).   Says they will return the last check, as the first one 
has most likely already been deposited.   I should have seen the 
"flake-factor" when they wouldn't accept a credit card.

The Astron sales guy said there is no documentation as it isn't required to 
change the regulator board.   I asked him to fax over the info, as I 
have one of the really old units, and the TIP-29 and SCR are mounted to the 
chassis...   this board bears zero resemblance to the one I have, and 
also must be modded for use with a variable voltage supply.   The 
Astron sales guy allowed that he would fax over the detailed information 
for the mod.

I never received any faxed documentation from Astron.

Following further harassment, the sales rep said I could call back and talk
to the tech when I got home, that the tech would be there until 5:00PM 
PST / 8:00PM EST.   With his assistance I managed to muddle 
through the modification to the board for a variable supply.

If you are familiar with the Astron linear supply and can do without 
documentation...   the Astron "fix" worked just fine.   But, 
if you need docs to get through the re-fit... Well, Skipp includes 
full documentation with his kit.

Next time I will buy Skipp's board, and avoid the flake-factor.

Mike K5JMP

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