Ampeg BA115HP Output and crackling/static

[ca7922303]

What is the voltage on the bottom pin of R31?
I ask because for IRFP140 and IRFP9140 the D-S voltage is 100V.
If the lower supply is also more than 50V, it is very bad for these transistors. The voltage is too high.

R31(-61.2vdc/-61.2vdc)both pins.

[ca7922303]

multimeter in diode mode. Red probe on b, black probe on c, then on e. The values should be similar, in the range of 560-700mv. B-e should be higher.
There should be no short between e-c.

b-c(643)
b-e(648)
e-c(OL)

[x-pro]

why is the supply voltage 20% higher ?

[ca7922303]

What component on schematic regulates the voltage?

[x-pro]

power supply

[ca7922303]

power supply

What can be checked with dmm?

[g1]

Are you using variac? If so, what is variac output VAC (measured) while amp running and reading +/-60VDC at main supply rails?

[ca7922303]

Are you using variac? If so, what is variac output VAC (measured) while amp running and reading +/-60VDC at main supply rails?

Not using Variac, but will use it and see at what vac R31 goes beyond -/+50vdc.

[ca7922303]

Are you using variac? If so, what is variac output VAC (measured) while amp running and reading +/-60VDC at main supply rails?

At (102.1vac)R31 is @ (-50.0vdc).
(120vac Variac)R31(-59.5vdc)
(123.5vac Variac)R31(-61.2vdc)

[x-pro]

Let's go back a little bit.
What is the DC voltage at pin 5 of U5?

[x-pro]

all further measurements at this point:

At (102.1vac)R31 is @ (-50.0vdc).

and measure the DC voltages again: b-e Q4, b-e Q1, TP10, TP11

[x-pro]

I would check AP1 for a dirty wiper, set the quiescent current, with no signal or load, to 15mA.
.

I can of course do a proportional recalculation, but what if something gets soldered and restored to the values of message 97.

[g1]

for IRFP140 and IRFP9140 the D-S voltage is 100V.
If the lower supply is also more than 50V, it is very bad for these transistors. The voltage is too high.

Maybe they use the limiter at OC1 to prevent over-voltage?

[x-pro]

It's possible.
Because the limiter levels do not converge to +/-50V

[ca7922303]

Let's go back a little bit.
What is the DC voltage at pin 5 of U5?

(101.5vac)R31(-50vdc)
U5-pin 5(-12.2mvdc)