Marshall 8080, missing components

nethanger1 · 12th December 2011, 05:53 AM

I have a Marshall 8080 that has a missing resistor (R76) and capacitor (C35). Would these components missing cause there to be zero signal coming out of the speaker? I couldn't seem to locate R76 but it appears C35 is a feedback capacitor which, without it, shouldn't prevent throughput.

Enzo · 12th December 2011, 11:16 PM

C35 would kill HF unwanted stuff, but note on the schematic next to it "N.F." That means "not fitted." In other words it was left off intentionally. I don;t spot R76 either, but I'd bel;ieve it was also an intentional omission.

Isolate the problem.

Is power supply OK? Both HV for the power amp and LV for the op amps? And is the tube lighting its heaters, and has some sort of B+ at its plates?

Does ANY sound come out the speaker? Background hum or hiss?

Is R120 OK? 0.33 ohm 5w.

Turn on and turn reverb up and shake the amp to crash the reverb springs. Does that come out?

PLug a signal into the input, now does that6 signal appear on the FX send jack? Does plugging a signal into the FX retirn make output?

IC8B drives the power amp, so touch pin 6 with something - a meter probe, a paper clip, whatever. Does that cause some sound?

Once you narrow dow what is and is not working in the amp, it should be easy to determine the exact cause.

12th December 2011, 05:53 AM	#1
nethanger1 diyAudio Member Join Date: Dec 2011	Marshall 8080, missing components I have a Marshall 8080 that has a missing resistor (R76) and capacitor (C35). Would these components missing cause there to be zero signal coming out of the speaker? I couldn't seem to locate R76 but it appears C35 is a feedback capacitor which, without it, shouldn't prevent throughput.

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12th December 2011, 11:16 PM	#2
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	C35 would kill HF unwanted stuff, but note on the schematic next to it "N.F." That means "not fitted." In other words it was left off intentionally. I don;t spot R76 either, but I'd bel;ieve it was also an intentional omission. Isolate the problem. Is power supply OK? Both HV for the power amp and LV for the op amps? And is the tube lighting its heaters, and has some sort of B+ at its plates? Does ANY sound come out the speaker? Background hum or hiss? Is R120 OK? 0.33 ohm 5w. Turn on and turn reverb up and shake the amp to crash the reverb springs. Does that come out? PLug a signal into the input, now does that6 signal appear on the FX send jack? Does plugging a signal into the FX retirn make output? IC8B drives the power amp, so touch pin 6 with something - a meter probe, a paper clip, whatever. Does that cause some sound? Once you narrow dow what is and is not working in the amp, it should be easy to determine the exact cause.