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I got myself a good ol' chunky linear power supply at a yard sale. The output is marked 13.8V/25-27A.

Picture of a DC regulated power supply named PS1225. It has one output marked 13.8V/25-27A.

Switching it on made a spectacular bang with lots of thick white smoke. No biggie, just a large cheap old dead capacitor to be replaced with a new one. Only one out of six had blown, but while I was in there I replaced all six for good measure. It now turns on and works like a charm. Not a ripple to be seen with my oscilloscope.

However. The voltage is much too high, coming it at around 25.6 volts. At first I thought it was because it was unloaded, but attaching an 8.2 ohm power resistor as a load still showed 21.6 volts across the resistor. Am I missing something obvious in my troubleshooting?

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    \$\begingroup\$ The only obvious answer is that the spectacular bang with lots of thick white smoke smoked more than the capacitors. A lot of these old regulated supplies used active circuits for the feedback so if any semiconductor burned, it would pass the input but with no regulation. \$\endgroup\$
    – Yiannis
    Commented Sep 1, 2021 at 23:32
  • \$\begingroup\$ @Yiannis I find that difficult to believe, as the capacitor was one of several wired in parallel, it was part of the output smoothing stage, and it failed open. \$\endgroup\$
    – konaya
    Commented Sep 1, 2021 at 23:41
  • \$\begingroup\$ The regulator must have failed shorted or driven full on. \$\endgroup\$
    – user80875
    Commented Sep 2, 2021 at 0:06
  • \$\begingroup\$ The regulator is broken. What else? It looks like PS1225 is ham radio PSU. \$\endgroup\$
    – jay
    Commented Sep 2, 2021 at 1:39
  • \$\begingroup\$ It's possible that the over-voltage is what caused the capacitor to fail, and it may be doing the same to the new ones you put in. Something else is broken in the regulator. It could be the main pass transistor is shorted or something else is broken. It's certainly possible to fix it, a schematic would be of great help to do so. \$\endgroup\$
    – 65Roadster
    Commented Sep 2, 2021 at 3:43

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The problem turned out to be one of the power transistors having failed short, which apparently is a common fault in linear power supplies. I found the problematic transistor by disconnecting them one by one until the output voltage became as rated.

While I could have just replaced the one transistor, in the end I opted to replace all of them with the much beefier MJ15003. They seem to be doing the job.

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