Ampeg SVT-6 Pro No Output

[ca7922303]

Yes, all sense resistors are now reading (0.44k ohms) both amps. Bias pots are both on full CCW. Took variac up to 40vac and running normal. DC voltage readings are consistent on both boards.

At closer look, @40vac on variac, all source resistors on board (Ch 1+) are (4.0vdc), on board (Ch 1-) are (1.0vdc). All drains of all mosfets on both boards are reading (+4.0/-2.0vdc). Pin 4 both output relays reading(1.0vdc).

[ca7922303]

(Ch 1+) source and gate busses(4/4 vdc) N & P
(Ch 1 -) source and gate N (1/1vdc)
P source (1.0vdc) gate(0.5vdc)

[g1]

So was there an error in measurement in post #225, or did things change (more likely). Those are different enough readings that I doubt it was just a mistake.

[g1]

Pin 4 both output relays reading(1.0vdc).

I thought we had established that there was an error on the schematic and relay pin4's go to ground, pin5's are output buss.

[ca7922303]

So was there an error in measurement in post #225, or did things change (more likely). Those are different enough readings that I doubt it was just a mistake.

So, I was measuring all the drains voltage on both boards + and - and they are all the same. Then I read back where you stated to check voltage on source resistors and that's where I'm getting varying readings. Not sure what to make of the schematic about the relays. K1 relay pin #5, #4, #1(1.3vdc), pin #2, pin #3(5 vdc). K2 relay has same readings with variac @ 40vac.

[g1]

All drains are connected to the + or - 65V supply rails. So drain voltages won't tell you much about the Fets. Source resistor voltages should be measured across each resistor. So one probe at each end of resistor. This will give you a reading for each Fet. Voltage divided by resistance tells you the idle current through each Fet.
Now you can tell if some are way out of whack (bad Fet or circuit problem) or just a bit mis-matched.
25mV is probably where it should be set when everything is good and at full line voltage. For now as you bring it up at lower line voltage, I would expect any good Fets to be reading 10mV or lower.

[ca7922303]

All drains are connected to the + or - 65V supply rails. So drain voltages won't tell you much about the Fets. Source resistor voltages should be measured across each resistor. So one probe at each end of resistor. This will give you a reading for each Fet. Voltage divided by resistance tells you the idle current through each Fet.
Now you can tell if some are way out of whack (bad Fet or circuit problem) or just a bit mis-matched.
25mV is probably where it should be set when everything is good and at full line voltage. For now as you bring it up at lower line voltage, I would expect any good Fets to be reading 10mV or lower.

At 110vac on variac, getting mostly (1.4mv) on Ch 1+ board and mostly (0.3mv,) on Ch 1- board across source resistors.

[g1]

Are the other voltages in agreement for both 1+ and 1- amps?

[ca7922303]

Are the other voltages in agreement for both 1+ and 1- amps?

Busses getting (+9/-3vdc) on both boards. K1/K2(relays)pin #4(3vdc). All drains were only (9vdc). I think these readings are way low for full power aren't they?

[ca7922303]

Are the other voltages in agreement for both 1+ and 1- amps?

I also tried turning up the bias pots up to 10 o'clock and saw no increase in voltage?

[g1]

I'm not sure exactly what you mean when you mention busses, so you need to be more specific. If you are at 110VAC line voltage and you are not getting near +/-65V on the supply rails (drains) then something is very wrong.
The bias trimmers should not be moved from full CCW until everything else is right. And once you are there, the only place you will see a difference due to bias trimmer is across each source resistor.
Will be back tuesday.

[ca7922303]

I'm not sure exactly what you mean when you mention busses, so you need to be more specific. If you are at 110VAC line voltage and you are not getting near +/-65V on the supply rails (drains) then something is very wrong.
The bias trimmers should not be moved from full CCW until everything else is right. And once you are there, the only place you will see a difference due to bias trimmer is across each source resistor.
Will be back tuesday.

Yeah, definitely not getting +/-65V at the drains. Busses are what the sense and gate resistors connect to according to Nevetslab.

[g1]

Busses getting (+9/-3vdc) on both boards. K1/K2(relays)pin #4(3vdc). All drains were only (9vdc). I think these readings are way low for full power aren't they?

Sounds like either you are using a limiter lamp and it is lit up bright, or maybe your black probe is not really getting to circuit ground.

[ca7922303]

Sounds like either you are using a limiter lamp and it is lit up bright, or maybe your black probe is not really getting to circuit ground.

So, how about I disconnect the two amps(Ch 1+, Ch 1-) and try to figure out what keeps killing mosfets on Channel 1+ board. I've lost two(2) more mosfets and one(1) source resistors(5w cement). Everything else tests correct. I've gone over parts that I can see are associated with the mosfets but I'm obviously missing something. Seems to be affecting Ch 1+ board only. Even thought maybe drains could be grounding out to heat sink but checked all to ground and they were not grounded to heat sink. Insulator strips are correct I'm assuming.

[ca7922303]

Sounds like either you are using a limiter lamp and it is lit up bright, or maybe your black probe is not really getting to circuit ground.

I'm going to order some more insulator strips. The ones I put on it have been taken on and off so many times that I guess at higher voltages could be shorting out. They show correct with no voltage connected to heat sink. Maybe strips being heated up and compressed/decompressed numerous times causes it to fail?