70's Peavey Bass 400 module problem

[Nyqusit5]

Hi all,

I have a 1979 Peavey Bass amplifier in for repair (black faceplate, with 4 action footswitch and "harmonics" distortion effect on one channel). There is -12V on the speaker outputs (the amp emits no sound except a faint low frequency signal). I removed all output transistors thinking at least one of them was shorted but even if they are out of the amp, there is still a high negative voltage on the output.
Can you point at a common culprit?

Thanks in advance!

[Nyqusit5]

Forgot to mention: the supply is a steady +50/-50V.

[nashvillebill]

Is this the 400BH power module? (The schematic is posted in this thread http://music-electronics-forum.com/t17699/)

Referring to that schematic, I'd guess a shorted driver transistor Q12, or maybe Q7. If yours is a different version, I'd still guess whatever driver transistor on the negative rail side.

[Nyqusit5]

I don't believe it is that one as mine doesn't have op-amp on the board. I think mine is an issue "B"
Thanks for your suggestion anyway: I am going to check the drivers again.

[nashvillebill]

The Series 400 B version I've got only runs off of 40V rails, so that's why I thought yours was the BH. But yours might be the same as the LTD/Session 400 power amp module which uses a discrete LTP and runs off 50 volt rails.

With the output transistors out, measure the voltages on each leg of the driver transistors in-circuit. Look for one that has the same voltage on two legs, that usually means its shorted.

[Jazz P Bass]

Even if you do not have the BH, there is an incredible amount of information on the schematic.
Such as the expected idle voltages.
I would suggest that you monitor the base voltages of the drivers & the predrivers.
Any abnormal readings should point to the offending device.

[Nyqusit5]

Replacing the drivers cured the 12V on the output and I do have a signal coming out now but one side of the sine wave is badly clipped. I guess it could be a faulty op transistor even though they all tested ok out of the amp.

Here is the actual schematic of the amp (I just got it)
Peavey Bass 400 A Series-1.pdf

[nashvillebill]

Since you say one side of the sine wave is clipped, I assume you have a scope?

Have you gone through the various gain stages of the amp to find out exactly where the sine wave starts getting clipped? Might not be the output transistors...

[Nyqusit5]

Well it's not the op transistors as the signal driving them (when they are taken out of the circuit) is clipped in the same manner.

[Nyqusit5]

I have a scope and I have gone through the satges. But given the feedback loop it's hard to tell where the clipping is occuring. It could be the 3638. I am going to check that.

[Jazz P Bass]

Well it's not the op transistors as the signal driving them (when they are taken out of the circuit) is clipped in the same manner.

To me, it never felt right, running a SS amp without the output transistors.
I have found it best to insert a 1K/ 1W resistor at the base/ emitter junction of one of the high side & one of the low side devices, when all are removed.

[Nyqusit5]

To give you some background, I had the same clipped signal with the transistors installed and I removed them to check if they were the reason of this clipping.

[nashvillebill]

I have a scope and I have gone through the satges. But given the feedback loop it's hard to tell where the clipping is occuring. It could be the 3638. I am going to check that.

Are you injecting your sine wave into the power amp's input (bypassing the preamp)? Just want to make sure it's not in the preamp.

How about lifting the lead on the .001 coupling cap in the NFB loop where it ties into the front end of the LTP?

[Jazz P Bass]

I take it the lower part of the wave is clipped.

[Enzo]

Every transistor in the thing has resistors connected. Burnt transistors often take out resistors. Always check them for opens.

The output clips? Are you driving a load? Is the output put still clipped with NO load?