sick '70 power supply?

[paul@PERSEPHONE.aegean-sea.dialnet.symbolics.com (Paul Cross)]

I've got a 3670 that upon power up does not turn off its fault light.  I
noticed that the main -12v light on the back back of the machine was not
lit, though all others were lit.  I don't know if the bulb is burnt out
or if indeed there isn't -12v.  On checking my powertec power supply,
model 6JA5-BBF-17, all the channels were normal except channel 3, which
was putting out 1v instead of its labeled voltage of 12.   The
machine was fine this morning.  I had to power it down to do a board
swap.

My questions.

What is the minimum board set one could put in an OBS machine and
have the fault light go out upon power up?

Can I adjust the voltage of channel 3? Or, is there a fuse or breaker
that could have tripped?  Or, do I need a replacement power supply?

Thanks for your help, fellow sluggers.