Ampeg SVT-6 Pro No Output

[g1]

Q3 C(+3.7v) B(+42v) E(-42.7v)
Q4 C(-0.5v) B(+42v) E(-42.7v)

I would say you wrote wrong polarity for Q3 E and Q4 B

Need all E,B,C for Q18 and 19

[ca7922303]

I would say you wrote wrong polarity for Q3 E and Q4 B

Need all E,B,C for Q18 and 19

Q3 E(+42.7v)
Q4 B(-42.7v)
Q18 C(-42.3v) B(-41.9v) E(-42.3v)
Q19 C(+42.9v) B(+42.3v) E(+42.9v)

Anything standing out to you as off? Any resistor, diode, or transistors voltages look wrong? Thanks.

[g1]

If that is correct readings for Q18 & 19, they both appear to be shorted from E to C. Check if they measure shorted. If not, double check those voltages.
Q3 and Q4 voltages look more reasonable.

[ca7922303]

If that is correct readings for Q18 & 19, they both appear to be shorted from E to C. Check if they measure shorted. If not, double check those voltages.
Q3 and Q4 voltages look more reasonable.

Measure across E to C on Q18/19?

[g1]

Measure across E to C on Q18/19?

Yes, power off, measure with resistance or on diode check range, one probe on E, other on C.
Compare with same readings at Q3 & Q4.

[ca7922303]

Yes, power off, measure with resistance or on diode check range, one probe on E, other on C.
Compare with same readings at Q3 & Q4.

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)

[g1]

Then I think you should recheck those voltages for Q18 and Q19 from post #258

[The Dude]

Only 256 posts to go.

[g1]

Only 256 posts to go.

If you think we're already at the half way mark, you must be of those 'glass half full' guys.

[The Dude]

...

[ca7922303]

Then I think you should recheck those voltages for Q18 and Q19 from post #258

Rechecked @55vac on variac, voltages are correct per post #258 What's your thinking on that.

[g1]

Something wrong there, collector voltages should less, more more like Q3 and Q4.
Check R53, R82, Q22, AP2 trim pot. Compare to equivalent components in Ch.1(+) half.

[ca7922303]

Something wrong there, collector voltages should less, more more like Q3 and Q4.
Check R53, R82, Q22, AP2 trim pot. Compare to equivalent components in Ch.1(+) half.

Variac @ 55vac
R53(10/10v)
R82(10/9v)
Q22(9/9/10v)
AP2(9/9/9v)

R10(32/32v)
R11(32/32v)
Q2(32/32/32v)
AP1(32/32/32v)

[ca7922303]

Something wrong there, collector voltages should less, more more like Q3 and Q4.
Check R53, R82, Q22, AP2 trim pot. Compare to equivalent components in Ch.1(+) half.

And these voltages are of course showing up on the mosfets and resistors. Ch 1+ (7-10v) everywhere and Ch1 - (2.5-5v) everywhere. Drains are all at about (+11/-11) for both boards. Turned variac down to 20vac for these numbers.

[ca7922303]

And these voltages are of course showing up on the mosfets and resistors. Ch 1+ (7-10v) everywhere and Ch1 - (2.5-5v) everywhere. Drains are all at about (+11/-11) for both boards. Turned variac down to 20vac for these numbers.

What components allow over 25mv to go to the source resistors? At this rate, source resistors will be getting 65v when I'm at 110vac.