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Marshall Valvestate 8080 power Amp repair.


 I recently started repairing Marshall Valvestate 8080. First symptoms were vere quiet sound and a hum. So first a checked for bad connections, broken solder joints itc. Because it is the most popular issue with these amps. I noticed that when turning on, the speaker was making a loud knock. So I disconnected it and checked the voltage on the speaker terminals and it read -7.4V. Power supply works fine, there is 15V on each Zener diode. I found a similar post on this forum but it wasn't resolved so I decided to try my luck here.  ::) The older post said that the voltage on the speaker says something about which semiconductor is broken. Could somebody help me figure it out?

Jazz P Bass:
-7Vdc on the speaker out connection indicates a failed output section.
The lower minus part is turned on when it should not be.
Do not hook up the speaker until this is resolved.

I could be way off base, so consider that.  But you have both 15v rails at the zeners, but do you get both rails right at the IC, IC8b?

Just thinking, but normally that IC sits between the two 15v rails and centers between them.  Well, the center between plus and minus 15 is zero.  And whatever appears coming out of that IC is what is going to output the amp.  At least at idle.

SO what if one of the power rails doesn't make it to the IC?  Then the poor thing tries to center between 15v and zero, which is 7.5v, and that is what is on the output.  Obscure, yes, but possible.


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