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Restoring / Refreshing an Astron Linear Power Supply By Robert W. Meister WA1MIK

Someone recently wrote me and inquired:

I have an Astron RS-35M power supply that still works fine but is about 20 years old. I thought it would be prudent to upgrade and restore the unit so I don't run into problems down the road. Do you restore them or offer an exchange?

I only repair them when they break. I'm not a business. I've got a VS-35M that I bought in 1988 that's still working just fine, yet an RS-35M purchased in 2000 failed in less than two years with a shorted diode. For the most part these supplies just keep going and going and going (like the battery bunny) unless a power surge comes in and shorts the diode bridge, which itself is fairly easy to replace, or a nearby lightning strike gets into the supply.

While you could just go buy a new RS-35M, today's switching supplies are much lighter and smaller but they do have a fan inside that might bother you if your operating environment is otherwise quiet. Switching supplies are much better filtered to suppress RF noise than they were 20 years ago. Samlex makes an SEC-1235M and Astron makes an SS-30M; both have slightly lower current capacities. I'd stay away from MFJ just on general principles. Some of these are available as rack-mount units.

So what sort of maintenance can you really perform on an old supply? Here are some things you can do. First, unplug the unit, disconnect any loads, and remove the cover, then...

• Blow the dust out of the supply and any fans that may be present.
• Tighten all the hardware you can find: the screws on the transistors on the heat sink(s), the screws on the main filter capacitor (if it has them) and its mounting clamp, the mounting nuts on large diodes, ground wires/lugs, the big screws holding the transformer to the chassis, the meter clamp screws.
• Replace missing adhesive rubber feet or tighten the mounting screws if present.
• Remove, clean, and replace the threaded output terminals. Make sure the shoulder washers are properly installed.
• Solder all the crimped connection lugs inside the unit.
• Remove the series-pass transistors from the heat sink, clean the heat sink compound from everything, test the transistors (one or more may not be working and you might not know it), apply new compound on the mica insulators (or use silicone pads that don't require compound), and reinstall the parts.
• Install a new AC line fuse. These can weaken with age and heat. If you don't see a round fuse holder, the fuse is located inside the IEC power input connector and can be popped out with a small screwdriver.
• Replace the AC power switch if the neon bulb inside has extinguished.
• Remove the regulator board and inspect for damaged, bulging, or burnt components.
• Reseat the regulator IC in its socket. In the older supplies, this IC was soldered in.
• Rotate the internal voltage adjust pot (underneath the regulator board) back and forth several times to clean the wiper contact. Reset it to the desired output voltage, or 15.0V on a variable supply with the front panel pot set fully clockwise (32V on a VLS supply).
• Replace the electrolytic capacitors inside, as they may have leaked or dried up if the supply was operating in a very hot environment. Always replace them if they're bulging.
• Recalibrate the front-panel meters, if you have the ability to accurately measure the output voltage and current.

If you don't trust the supply, you can buy a complete new regulator board directly from Astron for about $25. This will have new capacitors on that board and it may require some rewiring to fit a new style board into an old style supply. You might as well buy a new main filter capacitor from Astron, but screw-terminal caps are getting expensive, and they may supply a different style that requires a different clamp to hold it in the chassis.

There are plenty of articles on the web where the guts of an Astron supply are replaced with completely new components and use a different regulator design. You could go that route as well.

Contact Information:

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.

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This page originally composed on 04-Mar-2019

Article text, layout, and hand-coded HTML © Copyright 2019 and date of last update by Robert W. Meister.

This web page, 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.