1970s Gibson GA60 Solid State Amp has DC on output

[telecaster]

Schematic attached (scan from J. Darr book)
GibsonGA60.pdf
Amp output has -7.7Vdc on power up, slowly sinking to -0.4Vdc steady state. Running on light limiter with no load.

Transistor voltage measurements attached for Q5,Q6,Q7,Q11,Q12,Q3 and Q4


All transistors and diodes check ok with multimeter diode check. All resistors in output section check ok.
Looking for help in how to isolate and find the problem

[The Dude]

Nothing looks RADICALLY off on your voltage readings. However, .9ish volts E-B on Q3 and Q4 looks suspect and higher than "normal". Also, those transistors need to be pretty well matched. I'd pull and check them out of circuit if you haven't. It might not be a bad idea to just replace them to see if it solves the problem.

[Helmholtz]

What is the DCV at the output with a 4R load resistor?

[telecaster]

The Dude - I'm ordering some 2N5210's for Q3 and Q4, I'll check the old ones when I make the swap.

Helmholtz - on the light limiter with a 4ohm resistive load, the output is -7.7 Vdc. Under these conditions I checked voltage across R61 (it was 0V) and R62 (it was 0.9V). I think this means Q11 is off and Q12 is conducting almost 2 amps (0.9V/0.47ohms=1.9A), correct?

[x-pro]

... Q12 is conducting almost 2 amps (0.9V/0.47ohms=1.9A), correct?

No, it's not. Part of the current is tapped to Q7 to drive Q12.
I suspect a break or change in rating in elements R19, R20 or/and cap (4/40) between them.
Therefore, the zero at the output takes a long time to set and the operation is unbalanced.

[Helmholtz]

No, it's not. Part of the current is tapped to Q7 to drive Q12.

Should be a small part.,

[x-pro]

Однако напряжение EB около 0,9 В в Q3 и Q4 выглядит подозрительно и выше «нормального».

Minus the base voltage it seems to be okay.
The question is, "Why is it different?" Either measurement error and different timing.

[telecaster]

x-pro - R19 and R20 measure correctly at 10K and 4.7K. The capacitor measures 6uF, I am looking around the shop for a suitable replacement to try.

I remeasured Q3 and Q4: Q3 base = -348mV, collector = 30.6V, Emitter = - 0.96V. Q4 base = -412mV (jumps around a few mV), collector = 31.3V, emitter = -0.96V
The junction of R19 and the 4/40 capacitor reads -21.62V

[x-pro]

No load on the output. What is the voltage directly between the bases of Q6 and Q7?
Put the plus side of the voltmeter on the base of Q6, the minus side on the base of Q7.

[x-pro]

Amp output has -7.7Vdc on power up, slowly sinking to -0.4Vdc steady state.

How long, in seconds, does this transition to an operational state take?

[telecaster]

x-pro: The transition from -7.7V power on to -0.4V steady state takes 5 minutes.
The measurement you asked for Q6 base (+ lead) to Q7 base (-lead) is 1.59Vdc