Stereo power amp - odd fault

[JohnyLondon]

Hi - this is my first post and I found the forum just from google. Looks pretty good and as I sometimes dabble with circuitry could be just the very thing.
I have an oscilloscope, a dmm, and a soldering iron I have been into electronics since I was a kid.

On to my current problem...

I've got a stereo power amp - in fact it is a Marshall 8008 mosfet output stage thing for guitar. The fault is as follows...

With input A connected
Channel A level control sends output to B only, loud.
Channel B level control sends output to nothing.

With input B connected
Channel A level control sends output to B only, loud.
Channel B level control sends output to B only, even louder.

With input A & B connected
Channel A level control sends output to B only, loud.
Channel B level control sends output to B only, even louder.

BUT here is the weird thing ... as per the last scenario (both inputs A&B connected) you can get quite a bit of volume out of both sides IF you balance the level controls in just a certain way (roughly so that B is 30% higher than A).

So... dead channel A or what? I would have expected just to not get anything out of which ever side was dead, and also taking into account that either input A or B would expect to be able to use as a single channel input, outputting to both sides.

If anyone can tell me where to start, I'd be grateful - a visual inspection of the underside of the pcb didn't reveal anything except the fact that someone else has been over it for dry joints in the past... possibly in an attempt to repair the current fault.

thanks!

 

[dorke]

Not too odd...
Ch A and Ch B are not symmetrical.
What are you actually connecting to the inputs?
Mono plug Stereo plug?
Read about it here
http://www.drtube.com/schematics/marshall/8004-8008-hbk.pdf

 

[JohnyLondon]

Hi and thank you for the reply. I actually just connected the outputs from its matching pre-amp, using a mono jack to jack for each side. I have the pdf of the 8008 but I don't get what you mean about not being symmetrical. It sounds as though one of the inputs on the 8008 can be used to drive both outputs, I guess that's like a switched jack - are you suggesting that's where the problem lies?

 

[dorke]

Here is the diagram of the inputs(part of the service manual).
As you can see:
B input with a Mono plug connected (while A has nothing physically plugged into it) can drive both A and B channels
with separate A and B volume control.
Can you try that?

If you are getting sound from A it is obviously not totally dead,but can be weaker.

 

[JohnyLondon]

Hi Dorke,

I thought you might like to know that I found the problem. It wasn't until I connected up another stereo power amp and got the same fault that I realised it must be something in the wiring ins/outs. After inspecting the speaker jack leads, I found one that was s/c! Fortunately, both amps had survived the ordeal of being shorted - must have protection... but what a strange fault.

 

[dorke]

Nice to hear you solved the problem