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80's JMF Spectra 125 SS Combo Amp Screeches and Minimal Distorted Volume

Started by jbarrie88@sbcglobal.net, May 22, 2024, 05:24:20 PM

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Yes, that is where to check.
If no DC there, check DC volts at e,b,c of Q6,Q7,Q8,Q9.  No signal, no load.


Yep, I get 23.2 Volts DC at the speaker output.  Still on the path that the problem is somewhere within in the Power Amp board.  Suggestions on where to test next?  Thank you.


Unfortunately, this is one of those 'flying rails' designs which makes things a lot more confusing.  The output transistor emitters go to ground, and the positive terminal of the output jacks connects to the center point of the main filter caps (marked 'CC' on schem.)
So a bit of reverse logic as far as how the speaker fits in the circuit.
As long as you make your voltage measurements to ground, they should be ok.
At the input to the power amp, shown to the left of U1B, there are 2 diodes, D1 and D2.  You should measure 0VDC at D1 anode/D2 cathode.
Is U1 in a socket by chance?  If so, clean it.