Ampeg SVT 3 pro no bias voltage drop

[AJStrummer]

Hello folks.

Another SVT 3 issue. I received this amp after another guy had a look at it. There was a bag of components and it wasn't working. He had changed all power mosfets, and appeared to have removed and tested almost all components on the power amp board. Owner said amp was dropped then stopped working. I diagnosed the preamp to be passing a sine wave well. Got the power amp to work but only passes a distorted sine wave.

My issue is I have no voltage drop across the source resistors to be able to adjust bias, but I know why, I just can't figure out what is causing the lack of voltage to the mosfets. There is only 2.5V at the gate of the power mosfets and they need about 4V I believe. The supply rails are correct at +/-95V (should be 100V) and +/-65V. TP4 has 159V (should be 165V). I have searched many threads and this one seemed the most detailed

https://music-electronics-forum.com/...o-bias-problem

I have measured Vdrop across R14 - 1.722V & R19 - 1.790V. I can get the gate voltage to swing between 1.376V & 2.593V by adjusting the trim pot AP1. I have removed and tested Q'2,3,4,5,& 6 on a Peak Atlas DCA tester. I have then replaced Q's 2,3,4,5 & 6 and installed a multi-turn pot at AP1. All resistor values in this circuit were within tolerance. I'm out of ideas. Anyone got any tips of what I should be looking at?

Cheers
AJS

[nevetslab]

I checked thru my service notes on the two Ampeg SVT3-Pro amps we have in our rental inventory at CenterStaging, LLC in Burbank, CA. I didn't find any details on the power amp section, other than finding the bias pot was a bit touchy, and ranged from 6mV to 25mV with an AC Mains current draw of in the 800-850mA range @ 120VAC. I'm assuming this was the voltage across the 0.47 ohm/5W Source resistors. In looking at the voltage gain stage xstrs Q2 & Q4, calculating what current would be flowing thru them based on a 3.6V zener less the B-E drop, that would be 3V/2.2k or 1.23mA.

Check to see what you get for the current thru those. This output stage is similar to that of the SVT4-Pro, SVT5-Pro, which is more complex, but typically the gate drive buss is typically +3.6V/-3.6V.

[g1]

Post E,B, & C voltages for Q2, Q3, Q4.
That pic of the schematic is a bit blurry, so full schematic attached.