Oh!

I forgot to say if the 2N3055's don't have individual
base drive resistors, if their bases are all wired
together, then you'd have to pull them and check the
base-to-emitter and base-to-collector junctions with
the VOM to know if one had failed. I've seen a smaller
supply built like that and one of two 2N3055 had failed.
It made 8A instead of 12A output before going out of
regulation.

If there are individual base resistors the 2N3055 with
the higher base voltage, because there's no current drawn
though its drive resistor, is the bad one.


73,
Grumpy


Grumpy The Mule wrote in
:

Load the supply until the voltage drops.
Then measure the base voltage at each 2N3055.
The bases should all be the same, a base to
emitter voltage drop higher than the output
voltage. One of the pass devices may have
failed or the drive current could be insufficient.