Ampeg SVT-3 PRO - no bias voltage - low and distorted output

[spunko]

Hello, this SVT-3 pro has low and distorted output. Wanted to check the bias and there is no volatge across any of the mosfets resistors (R32 to R39).
Checked Q2, Q5 and Q6 and they all seem fine. AP1 trim pot is working, tested with my meter. Q5 and Q6 had been replaced in the past.

If there is no voltage across those resistors, does it mean that the output mosfets aren't working at all?
What could cause this problem? what shoudl I check next?

Thank you very much.

[Latole]

Sorry I can't help you ;  Schematic ;

[tubeswell]

IRFP240 is an enhancement mode MOSFET and requires gate voltage to pass current. So check to see why you aren’t getting gate voltage.

[spunko]

I got the flu so I was K.O. for a while.

I got some readings from the amp against the schematic and there are some low voltages on V1 and V2 from the power amp (TP7, TP2, TP3, TP4)
Could it be that Q1 is bad.

what do you measure to ground on top end of R10 and at Q2 emitter?
And end to end? (notvrelative to ground).
So 3 voltage values.

I *expect* around +3.5V, -3.5V and +7V (or thereabouts) but YOU tell us.

I got asked this on another forum. I got 2.6V / -2.7V and 5.2V (purple)

[spunko]

I got another SVT-3 PRO which has its own problem, but it sounds loud and clear.

I checked the same test points and voltages are exactly the same as the amp in question, except for Q2 (purple). The good amp has +3.5V, -3.5V and +7V   

[tubeswell]

Also suspect the zeners

[spunko]

I have tested all voltages again against the schematic, and also the other SVT-3 pro I have here.

All voltages are the same in both amps, all test points, all pins from V1 and V2.

The only voltages that aren't the same, are those in purple from Q2.
I already replaced Q2 with the same results.

The "good" amp gives +3.5V, -3.5V and +7V​
The "bad" amp gives +2.6V, -2.7V and +5.2V

The zeners arround Q5 and Q6 (D4 to D7) all have the same voltages, on the good amp +/- 3.5V, and on the bad amp +/- 2.6V

[spunko]

I cleaned thoroughly the trimpot, turned it back and forth a million times and now I have voltage across R32 to R39 from the output mosfets as it should.

Volatges on purple (Q2) went up, but they fluctuate between +/- 3.57v to 3.67v. The other amp voltages don't fluctuate and stay always in +/- 3.5v. Should I replace the trimpot?

Also voltages across R32 to R39 are irregular, only one gets up to 25mV and the rest stays between 14mV to 16mv with the trimpot turned all the way!!!

On the other SVT-3 PRO (the good one as I call it) these volatges are even lower and more irregular. Some resistors read 9mV, some read 15mV and one of them reads 2mV.
Like the "bad amp", only one resistor reads 24mV. In this "good amp" I can dial the trimpot a bit higher (like a quarter), but I don't want to go up from 24mV on that particular mosfet.


Summary:
- Now the "bad amp" is sounding clear but it should be louder IMO. I have irregular voltages across R32 to R39, but the trimpot is all the way up.
- Now the problem on both amps is irregular voltages across those resistors, and only one beeing at 25mV, the rest from 2mV to 16mV. I'm afraid to go higher on the "Good amp" because one is already at 25mV. The "bad amp" can't go any higher tho.

- The owner of the "good amp" says it used to be louder, and he thinks it lacks of power now. The same I feel with the "bad amp" now that it sounds clear, I think it should be louder.

Does this have relation to the irregluar volatges scross R32 to R39?

[glass54]

Hi Spunko,
Well done 
For Quiescent Current pots, I only use Cermet or Conductive Plastic pots (eg Bournes) as carbon type get degraded/noisy and cause random type responces. Oxidation of the carbon track?? You were quite right to do multi turns of trimpot!
Personally, I would upgrade.
For R32-R39, they are used DC feedback loop in each half of output stage. In each half they assist in current sharing of the parallel Power Mosfets (ie assist with Idss of each device) originally to stop thermal runaway 
I would expect a small variation in mV between the mosfets at quiescent conditions ie at idle (This is based on rebuilding many large Power Mosfet amps eg Australian Monitor AM1200, AM1600 etc).
As for "loudness" my only useful suggestion is check output power into a dummy load with 1kHz tone. Yes Dummy load to save your ears.   
Assuming min of 35Vrms output at 1kHz, based on 100Vp-p (min) into Load, I would expect well in excess of 150W into 8R or greater than 300W into 4R load. I would expect to see a clean sine wave. (based on a good Power supply at approx +/-60V DC (maybe a little higher at quiescent)
My thoughts and trust this helps.
Regards
Mirek