SoundTech QM4 PA Head

[aoresteen]

I have another project I'd like to get done.  Two years ago I picked up a used SoundTech QM4 4 channel PA amp head for my son-in-law to use for rehearsals.  The first time he used it it worked for 20 minutes and then went dead.  They tried it again a few days later and again after 20 minutes the sound wend dead.  They set it aside and used another amp.  It found it's way back to me  so I would like to fix it. Sounds like a cold solder joint or other heat related issue or perhaps a defective component.

All 4 channels cut out so I think the place to start is in the power amp stage.

I was able to track down the schematics - it is in three parts: preamp, preamp2, and power amp (attached).



While I do have a signal generator I do not have a scope to trace the signal through the circuit.

How would you approach finding the source of the problem?

Many thanks!

[Roly]

Agree it sounds like it's heat related.

I'd pull the chassis, feed in some music from a CD/MP3/FM radio and let it soak.  I'd have another amp ready to see if there was still signal at Line Out when the output failed, and if so that would point towards the power output stage, but the interconnects, CN2 etc, are also suspects.

It's rather unusual for a power stage to just pack it in after half an hour without any signs of distress, so you will need to be ready to probe the +/-15V rails and main supply rails B+/B- when the fault appears, as well as the voltage on the output half-rail.

Use your "digital thermometer" (finger tip) to check the output devices and power supply regulators for overheating.

The primary thing to do is to get the amp to fault, 'coz you can't fix a fault that isn't there.

[g1]

  As well as checking the line out as Roly mentioned, you can run a signal into the FX return jack when the problem is present, and see if the return is working.