" When I unplug from the loop in the rear its very loud:

This statement is very confusing.

From what I can discern so far:
- The input jack on the front produces low sound
- The FX Return produces sound (as does the Aux In)
- The Ext Spkr jack works
(so the power amp is good)

What is not clear is what happens when you use the FX Send jack to another amplifier.
Also, have you run a patch cable from the FX Send to the FX Return jacks?

It appears that the power amplifier is good.
Until you prove out the FX Send, it is not clear whether or not you have a preamp issue or an FX jack issue.

I see C20 as the negative rail voltage cap for the ic's.
The schematic shows it as 100uf. (not 100p)

Those voltages will work.
The second test is to flip your meter to read Volts AC.
That will measure the 'ripple' of the supplies.
Ideally, it should read zero.

On the input to the first opamp/ pin 5, there are two back to back diodes.
These diodes are used to limit the input signal to the opamp. (to prevent damage to the IC)

Why D2 was removed and a cap was installed at D1 I do not know.

That should do it.

Have you attempted to contact Kustom?

Kustom Musical Amplification INC.
3015 Kustom Drive
Hebron, Ky 41048
Tel: 1-859-817-7164

" I  could not measure a stable AC voltage before or after the 2.7k resistors.  The readings continuously ranged from a few hundred volts down to less than 0.1V."

What kind of meter are you using?

If the Vac ripple is jumping around like that you either have a bad load downstream or the caps are bad.

That + Vdc supply is not working correctly.

I would suggest that you measure the voltage on both supplies .
+Ve & -Ve.
They should be more or less the same except for polarity.
Flip your meter to read Volts ac & measure them again.
You want a low amount of ripple.

If this test passes, measure after the 2.7K resistors.
Same thing.
Volts dc and then Volts ac.
The Volts ac ripple should ideally be zero.

The zener diodes will 'dump' the voltage that is higher than the zeners voltage rating to ground.

If you do not have enough voltage going in to the diode, it cannot make the difference up.

Pin #3 is an input pin.

It can't possibly have +14 Vdc on it as shown.

I vote that the ic is toast.

To prove pin #3, remove the ic and see that the pad on the pcb  reads 0 volts.

There is something seriously amiss with T3's voltages.

You have 44Vdc on one side of R53 (1K) and 43Vdc on the other.
That is to be expected.

But 43Vdc through R57 (100) seems amiss at 130Mvs.

I would check R57 and T3.

You have measured the U11 outputs but not the inputs?

Please post the input/ output voltages to U11.

In working mode & non working.

What was in the amp is a Bulgin IEC connector.

There may be something going on with the switch IC, CD4053.
http://www.ti.com/lit/ds/symlink/cd4053b.pdf

You will have to check the inputs and see if the outputs are responding.

This can be accomplished with a volt meter.

I don't have that schematic but have you tried contacting Blackstar?

They will not release the schematics but they will help with component identification.

The ic does seem hard to find.

Try this: https://www.questcomp.com/SearchResults.aspx