Ampeg SVT III Pro - Distorted sound

[binito]

I have an Ampeg SVT III Pro. It sounds distorted and choppy, as if the MOSFETs were poorly polarized.

I have seen a difference with respect to the original scheme, the resistors R28, R29 and R30 are 1Kohm instead of 47ohm. But it seems like this has been going on since it left the factory and it worked fine.

The problem is that through the 0.47ohm 5W resistors R32, R33,R34,R35,R36,R37,R38 and R39 there is 0.0mV and even if I move the trimmer it is not able to go above 1mV.

I have removed the MOSFETs and checked with a semiconductor analyzer and they seem to be fine.

I don't understand where I can have the polarization problem. What could be happening?

[Jon Snell]

What is the voltage between Q10 and Q14 sources? Is it adjustable with RP1?

[g1]

Did you replace the resistor next to RP1, or is that from a previous repair?

[g1]

I have seen a difference with respect to the original scheme, the resistors R28, R29 and R30 are 1Kohm instead of 47ohm. But it seems like this has been going on since it left the factory and it worked fine.

I very much doubt it left the factory like that. There are 2 different styles of 1K there, not all the same. And R31 is the correct value 47R.
You may be dealing with errors from a previous repair. Maybe those wrong values are not directly causing the problem, but some other error.
Are you 100% sure it was working correctly before you received it?

[binito]

I very much doubt it left the factory like that. There are 2 different styles of 1K there, not all the same. And R31 is the correct value 47R.
You may be dealing with errors from a previous repair. Maybe those wrong values are not directly causing the problem, but some other error.
Are you 100% sure it was working correctly before you received it?

At the moment I have not changed any components. It is possible that the 1k resistors were changed in a previous repair, but the welds look like the factory ones.
I have only removed the mosfets to check them with a semiconductor analyzer and they seem to be fine. 0.47ohm resistors are also fine. however there is no current through them without sound.

[nevetslab]

At the moment I have not changed any components. It is possible that the 1k resistors were changed in a previous repair, but the welds look like the factory ones.
I have only removed the mosfets to check them with a semiconductor analyzer and they seem to be fine. 0.47ohm resistors are also fine. however there is no current through them without sound.

It looks like the gate drive buss voltage level is low. Typically they would be around +/- 3.6VDC to get the MosFET's to start conducting. Typically you'd want around 50mA thru the MosFET's Source Resistors (about 23.5mV across the 0.47 ohm resistors for 50mA Source current thru the MosFET's). If you're not getting them to start turning on with the range of the bias pot, you may have an issue with the bias xstr Q2, or perhaps the bias adj string. Looks like Q3 and Q4 are conducting, as you do have drive buss voltage (2.4V, -2.3V).

[x-pro]

The supply voltages of the output mosfets are higher than in the schematic. At the same time, the supply voltage of transistors Q3, Q4 is lower.
Check for the ratings of capacitors C16, C17, C31, C32.

[binito]

It looks like the gate drive buss voltage level is low. Typically they would be around +/- 3.6VDC to get the MosFET's to start conducting. Typically you'd want around 50mA thru the MosFET's Source Resistors (about 23.5mV across the 0.47 ohm resistors for 50mA Source current thru the MosFET's). If you're not getting them to start turning on with the range of the bias pot, you may have an issue with the bias xstr Q2, or perhaps the bias adj string. Looks like Q3 and Q4 are conducting, as you do have drive buss voltage (2.4V, -2.3V).

I changed Q2, Q5 and Q6 and the amplifier has started working fine.
I already have 3.8V on Gate of Mosfets.
In resistors 0.47R 5W there are R32=24mV, R34=24mV, R36=32mV, R38=11mV, R33=29mV, R35=29mV, R37=13mV,R39=13mV. I see that the mosfets are not well balanced?
I have checked Q2 (MPSA06) with a semiconductor analyzer and it comes out HFE=8. I think this was the problem.
I took the opportunity to replace R28, R29 and R30 for some reason that I don't know, they were 1k, I have installed 47R resistors as indicated in the diagram.

Thank you all for the help, today I learned something else :-)