Ampeg BA115HP buzz caused by DC on output

[stratfordade]

I'm trying to fix an Ampeg BA115HP which works in all respects apart from having a hum with no signal at input. The hum is caused by a 0.35V DC at the speaker.

The amp has 4 power mosfets and the unwanted DC is where all the drains are tied together. The bias readings are all correct (between 15-20mV across each 0.33ohm 10W between source and rail for each misfit).

Any suggestions for please for how to eliminate this DC? Is it a case of making sure all the mosfets are a matched quad?

Thanks for any help.

[g1]

Do you have any DC at pin 1 of U5 ? 
Schematic attached.

[stratfordade]

Thanks for posting schematic. I tried to post same schematic on first post but couldn't see the attachment button (now I can!).

There is barely any DC (6mV) on pin 1 of U5. But I have now got the amp much quieter with no signal — whisper quiet compared to how it was. You have to put your ear right up to the 15" speaker to hear any hum, and at higher gain/master settings the noise level is higher.

I found that the preamp board gets its ground via a multi-pin cable to the power/PA board, and the 'ground' on the preamp had about 2 ohm resistance to chassis ground! Turned out to be dirty header pins on the connector. Cleaning them up and both boards at same ground give or take 0.1 ohm which would be the linking cable.

Could still hear hum and had about 0.4VDC at speaker. Removed the heat sink carrying the power mosfets and on a tester there was a difference of a few tenths of a volt between the threshold voltage of the IRFP140 mosfets and also between the two IRFP9140. I had exactly two spares of both types and on my tester the 140s measured exactly the same for threshold as did the 9140s! Replaced the ones in the amp and the DC across speaker dropped to about 0.2V and that hum is practically inaudible at that level.

Listening to it now. Sounds great!