Mesa Mark III silence

[AtomicMassUnit]

I'm working quite unsuccessfully on a Mesa Mark III. Different one than last time, thanks Enzo! I've narrowed down the problem somewhat, but can't quite pin it down.

There is a problem somewhere between the effects loop and the input of the phase inverter. The preamp up to at least v2a works since I can come out of the effects send and everything works as normal, the 1st vol control and the tone controls, and the lead channel and lead channel volume. I can plug into the effects return, and I get nothing. However, the phase inverter and power tubes are working because when I check voltages on the plates and grids they both make noise and pass the old pop test.

The schematic says the preamp output after v2b goes "to graphic EQ or v5a driver". V5 is the phase inverter and the input of that is working, so it would seem that if I have the graphic eq turned off, then v2b should have a coupling cap off the plate resistor then the master vol to ground, then a coupling cap into the PI. However, the amp is not the same as the schematic. The coupling cap is connected to the plate side of the plate resistor to a 22k resistor and the other side of that is connected to the master volume. The master is also connected to a .1 cap. This cap is going to one of the transistors in the eq section. It seems with such a localized problem I'd be able to find it, but I haven't been able to figure it out.

One other clue. I am getting a hum out of the amp, not very loud, but it is modulated by the graphic eq, only when the eq is on.

[stokes]

This is going to sound simple but it is a common problem with Mesa amps.Try bridging the effects send and return with a gutar cable.If you get sound out of the amp you need to change the shorting jack.

[Amp Kat]

Loop jacks could be the problem as stated above and the eq switch could be faulty are a ground connection not making contact. See what you get across the plate resistor and across the PI cathode resistor from ground to cathode. If the signal stops there it's either a bad connection from ground which would explain hum in the eq or a high impedance souce after dragging it down

[Don Symes]

I'm going to suggest looking at the switching around the EQ. Since there is a hum from the EQ and modulated by it, lets assume the EQ would work if signal were getting in (but it seems the EQ output switch/relay is working).

My schematic library is down right now, so I don't know if this is a relay or LDR thing you're looking for, but right now it smells like a dead series LDR or a shorted shunt LDR.

Hope this helps.