I have a Pyramid PS-52KX (yes, monster, 40lb) DC 12-15V 50A power supply.

Was working fine when last turned on - probably 12 years ago. Was sitting in a box since.

Now - flip the switch, it turns on. The fan hums. The power light is on.

The voltage output (has to be variable, 12 to 15) is 23.8 and does not vary as I turn the knob. Is there a way to fix this or should I junk it. Hate to junk a big 40lb thing that probably just needs a small part to work...

 

There is a couple of youtube videos you may be able to glean something from.

 

Check the connections on the back of the supply! Most power supplies of that size have connections for external sensing the voltage, and must have the sense lines connected to provide any adjustable voltage output, or any regulation. so without that connection the regulator part is not controlled.

 

I found a back view of the power supply, and there are no external connections, so it would appear something inside failed.
The video posted by Max may help.

 

If the output is at a fixed 23V, then the odds are one or more of the pass transistors are shot (shorted). Pull each of the final output transistors and test them. This is similar to some Astron linear supplies, when the output is high (same V as voltage on filter capacitors), then at least one of the pass transistors is shorted between collector and emitter and passing full filtered capacitor voltage to the output.
Some of the pass transistors may still be good, all it takes is one bad one. However, if indeed you find a bad pass transistor, it is best to replace them all. If one of the old ones was stressed enough to blow, there is a chance other old pass transistors may not be performing at proper specifications any longer....

 

I found a back view of the power supply, and there are no external connections, so it would appear something inside failed.
The video posted by Max may help.

OK, so the external sense terminals are missing!! That is still what I would guess. BUT a shorted pass transistor or afault in the driver circuit could possibly be the problem. AND THAT might be why it sat in a box so many years, until the failure was forgotten.

 

Schematic/manual for PS-52KX are at repeater-builder. It's your usual 723 and a bunch (8?) of 2N3773 pass transistors. I would say check them for short-circuits.
It's a good, big, heavy linear power supply that is in demand for car stereo displays due to the low noise and EMI.
Careful the 10,000uF filter cap will hold its charge if there is no output, which can spark when replacing the transistors. Check for a live (collector) voltage and discharge that if needed.

 

Thank you gentlemen I will see if I can do that myself. If not, I will just sell it on ebay for parts or for someone else to repair.

 

Briefly looking at the schematic posted above, there is a SCR protection circuit that should have fired when the voltage went well over 16V. I would examine the SCR/Zener circuit as well, it might not be working as intended.

 

I suggest also checking that big capacitor, C6, for leakage. That circuit is a bit strange, as I see it.

Quite a few components can affect the regulation.