I swapped new 16v zeners to old ones 1n4744 (15v) and other 12 volt rated that I cannot read. I got DC +15.5v / -13.5v.
R19/20 are kind of hot.
Now this lowers amp noise to a bearable point but nowhere ideal, silly high freq is gone.
Master Reverb produces less noise, but still does.
Ch1 makes no sound at all.
Ch2 works.
I think noise is coming from Ch 1.
R19/20 are kind of hot.
Now this lowers amp noise to a bearable point but nowhere ideal, silly high freq is gone.
Master Reverb produces less noise, but still does.
Ch1 makes no sound at all.
Ch2 works.
I think noise is coming from Ch 1.
Channel 1 has a long string of stuff including the "tone correction" module. Do you have the footswitch installed, or is there any plug at all connected to the footswitch socket?
Anyway, it looks like channel 1 sound comes out of pin-5 on the "tone correction" module. Audio goes into the module through a resistor at the output of opamp just after the treble tone control (R17?). What you need to do is figure out where in that string of circuitry the problem is. One one to track down a noise is to start disconnecting things. Another thing you can do is visually check all the circuitry in that part of amplifier. Make sure its using the correct opamps for one thing. Look for swollen, bulging, or leaking electrolytic caps, look for cracked solder joints, etc. If everything looks okay visually then you could take some voltage measurements at key places in the circuit, such as the opamp outputs. Measure for DC and for AC (since a noise would measure as AC). Other than that disconnecting the string of opamps snd "tone correction" module may give a clue where the noise is coming from. Also make sure the input jacks are shorted to ground to minimize any noise pickup there. Another thing you can try is flipping all the switches and turning the tone control knobs, etc., for channel 1 and see how many of them affect the noise. That may give a clue as to where the noise originates.
Anyway, it looks like channel 1 sound comes out of pin-5 on the "tone correction" module. Audio goes into the module through a resistor at the output of opamp just after the treble tone control (R17?). What you need to do is figure out where in that string of circuitry the problem is. One one to track down a noise is to start disconnecting things. Another thing you can do is visually check all the circuitry in that part of amplifier. Make sure its using the correct opamps for one thing. Look for swollen, bulging, or leaking electrolytic caps, look for cracked solder joints, etc. If everything looks okay visually then you could take some voltage measurements at key places in the circuit, such as the opamp outputs. Measure for DC and for AC (since a noise would measure as AC). Other than that disconnecting the string of opamps snd "tone correction" module may give a clue where the noise is coming from. Also make sure the input jacks are shorted to ground to minimize any noise pickup there. Another thing you can try is flipping all the switches and turning the tone control knobs, etc., for channel 1 and see how many of them affect the noise. That may give a clue as to where the noise originates.
That module was faulty, typical chronic problem with these amps, found the schematic online build it and managed to get channel working prior to poweramp blown out. Here it is :
S1A, S1b and S2 wired according to the new schematic I posted above and all functioned accordingly.
Except the wires on amp schematic nothing is connected to footswitch socket. I don't have footswitch but Valve Sound switch (S2) used to work as it is.
Pin-5 is on pcb and goes to footswitch, it's currently not connected to my new module, however pin-2 is source of p-jfet and goes straight to master volume, which I read conductivity. That is right audio goes into module's Opamp and Middle and Presence pots. through R17.
Good ideas, I will investigate further. What is your opinion for signal tracing? I have a spare amp I may be able to find out where things get unpleasant.
For channel 1 none of switches or control knobs make a difference noise wise. Might be another blown opamp, will see !
"What is your opinion for signal tracing?"
Its kind of an old fashioned technique but in the absence of a scope, this might be a good time to try it. The noise may be strong enough that you need an attenuator before the signal tracer to avoid overloading its input, is perhaps the main thing. Also, capacitor couple the input to the the signal tracer amp. You could put some clamp circuitry to ground after the cap (e.g. back to back zeners) to further protect the signal tracer input.
Its kind of an old fashioned technique but in the absence of a scope, this might be a good time to try it. The noise may be strong enough that you need an attenuator before the signal tracer to avoid overloading its input, is perhaps the main thing. Also, capacitor couple the input to the the signal tracer amp. You could put some clamp circuitry to ground after the cap (e.g. back to back zeners) to further protect the signal tracer input.
