Ampeg SVT-6 Pro No Output

[g1]

They (source resistors) will all be blown long before that. Over 25mV at source resistors means the fets are conducting too hard. Gate voltage determines how hard they conduct.
You say Q18 and Q19 collectors are -42 and +43V. Those are the gate busses. Each gate is connected to the gate buss by a 47 ohm resistor. If those voltages aren't showing up at the gates, those 47 ohm resistors must be blown.
In post #268, I meant check resistance and diode check transistor, not check voltages. Compare to other amp side.

[ca7922303]

They (source resistors) will all be blown long before that. Over 25mV at source resistors means the fets are conducting too hard. Gate voltage determines how hard they conduct.
You say Q18 and Q19 collectors are -42 and +43V. Those are the gate busses. Each gate is connected to the gate buss by a 47 ohm resistor. If those voltages aren't showing up at the gates, those 47 ohm resistors must be blown.
In post #268, I meant check resistance and diode check transistor, not check voltages. Compare to other amp side.

Gate voltages are the same as Q18/19, and all the gate resistors read 47 ohms in circuit.

R53(3.1k)
R82(14.6k)
AP2(9.4k)
Q22 B-C(0.6v) B-E(1.8v) C-E(2.8v) diode test

R10(3.1k)
R11(14.6k)
AP1(8.4k)
Q2 B-C(0.6v) B-E(1.9v) C-E(2.7v)

[ca7922303]

They (source resistors) will all be blown long before that. Over 25mV at source resistors means the fets are conducting too hard. Gate voltage determines how hard they conduct.
You say Q18 and Q19 collectors are -42 and +43V. Those are the gate busses. Each gate is connected to the gate buss by a 47 ohm resistor. If those voltages aren't showing up at the gates, those 47 ohm resistors must be blown.
In post #268, I meant check resistance and diode check transistor, not check voltages. Compare to other amp side.

R11 and R82;are (22k) out of circuit, per schematic. Any thoughts on the last readings given? Thanks.

[g1]

No new ideas. Have you gone through the fets (diode check) again and compared between the 2 amps?
And just to double check my understanding, Q18/19 collectors are giving much different voltage readings from Q3/4 ?

[ca7922303]

No new ideas. Have you gone through the fets (diode check) again and compared between the 2 amps?
And just to double check my understanding, Q18/19 collectors are giving much different voltage readings from Q3/4 ?

Yes, Q18/19 collectors are much higher than Q3/4. How about I replace Q18/19?. Will check mosfets again, diode test this evening. Thanks.

[g1]

Voltage readings imply that Q18 & Q19 are shorted. Diode check says they are not. Might as well replace them to be sure.

[ca7922303]

No new ideas. Have you gone through the fets (diode check) again and compared between the 2 amps?
And just to double check my understanding, Q18/19 collectors are giving much different voltage readings from Q3/4 ?

Ch 1+ P Ch mosfets consistent, same with N Ch.
Ch 1 - P Ch consistent, N Ch also consistant, but Ch 1+ and Ch 1- are not reading exactly the same, but fairly close. P Channels(.040v), N Channels(.23v) diode test.

[ca7922303]

Voltage readings imply that Q18 & Q19 are shorted. Diode check says they are not. Might as well replace them to be sure.

Checked voltages again on mosfets boards @ 20vac variac.
Ch 1+ P Ch Source (+10.7v) Drain(-11v) Gate(+10v)
Ch 1+N Ch S(+10.7v) D(+11v) G(+13.7v)
Ch 1- P Ch S(+2.8v) D(-11v) G(+3.8v)
Ch 1- N Ch S(+2.8) D(+11v) G(+5.5v)

Do any of the (+)s, (-)s look wrong. Also, even if Q18/19 are replaced and come down to where Q3/4 are at, won't the source resistors still be too high @ (2.8v). What could be causing (2.8v) instead of the (25mv) @ source resistors? Maybe change Q3/4 also?

[g1]

Do any of the (+)s, (-)s look wrong. Also, even if Q18/19 are replaced and come down to where Q3/4 are at, won't the source resistors still be too high @ (2.8v). What could be causing (2.8v) instead of the (25mv) @ source resistors? Maybe change Q3/4 also?

If you are measuring to ground, the source voltages don't mean that much.
Measure the DC voltage across the source resistors. One probe at each end. If any of them really had 10.7V or even 2.8V across them, they would be blown for sure. Verify that they also all measure no more than 0.47R

[ca7922303]

All 20 source resistors (.47R). Measuring DC voltage across source resistors, Ch 1+ (highest about 10mv @ 55vac), Ch 1-(all about 0.3 mv@55vac).

Found a bad R13(Ch 1+ board) is the wrong value. Schematic calls for 2.2M and it is a 22k. R87(Ch 1- board) is correct @ 2.2M. Could this be causing the difference in voltage on Ch 1+ to Ch 1-? Will order 2.2M with 2N3440s/2N5415s

[ca7922303]

Took all these measurements to compare. Each line is the corresponding component per schematic. I pulled one leg to measure out of circuit R13(wrong value) R15(good), D9(good) D36(diode test in circuit tests correct). Anything else on list look wrong?

[ca7922303]

Better photo

[ca7922303]

Better photo

Voltages were taken at 20vac on variac.

[ca7922303]

If you are measuring to ground, the source voltages don't mean that much.
Measure the DC voltage across the source resistors. One probe at each end. If any of them really had 10.7V or even 2.8V across them, they would be blown for sure. Verify that they also all measure no more than 0.47R

Installed Q18/19. Readings @ 20vac now are
Q18 C(-3v) B(-12v) E(-12v)
Q19 C(+0.9v) B(+13v) E(+13v)
Q3 C(+13v). B(+13v).E(+13v)
Q4. C(+9v). B(-12v). E(-13v)

The collectors on Q4 and Q18 aren't the same and the voltages are different on Q3 and Q19 collectors.

[ca7922303]

Diode test Black probe C, Red probe E, reversed probes 2nd #.
Q3(1.2/1.8v)
Q4(1.8/1.2v)
Q18(1.8v/OL)
Q19(OL/1.8v)

Now after new install:
Q18(1.8v/1.4v)
Q19(1.3v/1.8v)

Not seeing any change in the big voltage difference between Ch 1+/Ch 1-. Have all mosfets installed and all associated resistors are reading correct resistance, but still trying to find why the voltage differences before I put normal ac voltage on it. All drains on both boards are the same.