If nothing is shorted on the (-) side, then something must be open on the (+) side.
You will need more precise measurements across components (one probe at each end) rather than to ground.
What is voltage across R198? R209,R210?
B to E of Q9,Q12,Q13?

A few things you haven't mentioned checking are Q9, D17, D20

You have pretty much -42V at both emitter and collector of Q20 and Q21 (and Q19)?
That would seem like a short.

What are e,b,c DC voltages for Q12 and Q13?

Does the schematic in the first post exactly match your unit?
From what I can see, there are 3 basic versions.
The last IC connects to a transistor which will be either Q4,Q5, or Q12.  Which is it in your unit?

The 2 lugs on the back of the pot are a switch.  If it is measuring like that it must be bad.
You can see it (switch) was wired in series with the fuse in your other thread here:
https://www.ssguitar.com/index.php?msg=41327

It should be pointed out that you may find voltage regulator circuits that use regular transistors as the pass transistor.
This is seen in older equipment from when the VR IC's were not yet available (ex. Yamaha G50-112II +/-15V supplies).

See this page, particularly 'differential voltmeter' and the explanation of the 5 knobs:
https://control.com/textbook/dc-electricity/null-balance-voltage-measurement/

You have to deal with SMPS to get anywhere with these kind of amps.
see attached

another one here:  https://docs.google.com/viewer?a=v&pid=sites&srcid=Y2Fyc3UuZWR1LnBofGpiaGFsaWJhc3xneDoyYjgyY2E5OTZmN2FjZWQ0

What is the symbol in the circle on the far left side?
Can you post the circuit from the textbook?

It looks to me like there is an AC source on the left side (circle with symbol), and a DC source on the right side.  That should not be done.  You can not get a p-p voltage from a DC supplied passive circuit like that.

Quote from: saturated on September 05, 2023, 01:30:30 PMthe yellow green went to an 8 pin chip...and back to the primary fuse holder....

If this is true, do not operate the unit.  The secondary and primary sides of transformer should never be connected.
The AC cord white and black should go to the primary (through the fuse and the power switch), and nowhere else.

If that fuse is on the primary side, the fuse holder should not connect to anything on the board.

It's worth a try to contact the company, you never know if they will supply schematic until you ask.
https://www.mcadamsinstruments.com/contact

Not sure what you mean about getting same reading on AC or DC.  On AC, the waveform is centered around the middle '0' line of the scope.  When you apply the DC offset of the generator, the whole waveform shifts upwards, approx. 2 lines on 2V/div setting, so 4 volts.  That is AC signal riding on DC offset.

Turn the generator 'amplitude' all the way down.  You should just get a flat line on the scope.  Adjust the 'DC offset'.  On AC coupling the line should not move, on DC it should.

Just above the probe connector is a switch for AC or DC coupling.  In the pic, you have it set to AC.  That is why it will not hold the DC offset, you need it to be set for DC coupling.
It is not that often that I use DC coupling on my scope, but useful for some things.  Generally I use my meter for DC measurement.

Yes, tracing through the preamp with scope should enable you to locate the fault.
You need to highlight the signal path on the schematic and check points along the way with the scope.  But you also need to have an idea of what to expect, as the signal level may normally go up and down along the way.

 They have range of DC voltage at Q101 source shown (.5 to 2V), I think that should follow the pot setting.
(copy/paste from MEF  )