I learned a lot about power supplies since I bought an Astron RM-35A. Rated for 35 amps intermittent and 25 continuous, this linear supply is heavy. They sell for over $300 and I got it at a bargain, $100. It’s very clean and the previous owner even added an analog current meter and digital voltmeter to the front panel.
I noticed it was running at 14.4 volts. Although the radios connected to it weren’t complaining, I wasn’t comfortable running them with a high voltage long-term. I opened up the case and found the trimmer I needed, a 1k pot, R5. I gave it a tiny turn in one direction and the supply immediately shut off, sort of. It was only putting out 1 volt. I moved it back and nothing. I called the previous owner who referred me to repeater-builder.com as they have some excellent, very thorough write-ups on these very power supplies. It turns out that I had gone over the voltage limit and the power supply’s crowbar circuit kicked in. Thank God the current-limiting circuit was good as my transistors were not fried.
I turned the pot back and it turned on but the voltage was very unstable and it shut off again. I turned the pot a few times back and forth and set it right about where I remembered it was to begin with. The supply came back to life and has been putting out a nice, steady 14.1 volts ever since. I can live with that.
I added an external fan to the heat sink that runs on the same 14.1 volt output and my transistors are nice and cool. I plan to drill a hole to attach a ground strap but after that, I’ll just leave this bad boy alone and let it power my radios.
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