Mesa Rectifier Recording preamp - no sound!

[spy]

I'm trying to repair this preamp without the schematic. Does anyone have it?

There's no sound at the output and I think it's a problem from the mute circuit. I'm not sure but while I was tracing the input signal I discovered that it disappears at the output of both master volumes (center taps). Then I lifted the wires from the center tap of the pots and voila, there it is the whole signal.

Is there any common part at the mute circuit that fails? I can't understand how it works and with no schem I don't know what to do. There are some transistors I see on the other schematics of Mesa circuits like 6426 and a lot J175 fets. By the way, standby switch is also part of the mute circuit.

I don't know if it's the same circuit or close but at the attached file you can see the mute circuit of the Lonestar.

Mesa Boogie Lonestar schematic.pdf

[52 Bill]

Mesa uses jfets to shunt the signal to ground at various points of the circuit at different times, like at startup and during channel switching, etc.

I have fixed a few with bad fets that would kept the signal grounded. Follow the wires to the master volumes and try and find the fet that is connected to it.

[spy]

Yes, you're right! I found that fet and I took it off the circuit to test it. It looks good with my DMM but I switched on the amp without it just to see what will happen to the signal at the master volumes and now it's not grounded at both ends and the preamp works just fine except the standby switch that doesn't mute.

I'll replace it with a new one, hope the standby switch will work again and don't need to find the fault somewhere else. Thanks.

[52 Bill]

I'll replace it with a new one, hope the standby switch will work again and don't need to find the fault somewhere else.

Measure the dc voltage at the gate of the fet (or where it would be) and see if it changes when the standby switch is thrown.

If the voltage changes with the standby switch, the circuit is probably okay and the fet is suspect. If the voltage does not change then the standby circuit is suspect.

[Pedro Vecino]

If not remember bad outputs of both chanels are connected to a relay. Behind him there is a J175 fet to ground used to mute the signal temporarily during the switching (probably marked as "m", "mu" or "mute") just before the contour circuit. The output of this circuit is connected to the effects loop send (a good reference point for testing).
The stereo master and Solo function at the end also have another fet J175 in each side that performs the same mute function.

[spy]

Thanks Pedro for the info! I have checked the relay, also I replaced it with another one to be sure. I was talking about that fet behind the relay, it's the one I took it off to test it. Today I'm planning to replace it with a new one.

Hope it's one of those J175s, easy to replace them, too.

Did some measurements, today (without the fet on the pcb). With the standby off there are 0mv at the gate of the J175 and with the standby on there are 60mV. It seems the circuit works just fine.

[spy]

I found only J174s, are they suitable?

Edit:
Not enough lucky. I replaced it with a J174 and it doesn't work so the problem is somewhere else unless the J174 is not suitable.

At the other side of the pcb, I found the mute circuit and there's a 2N6426 marked as M. I did some measurements,

Standby off
E = 10mV
B = 440mV
C = 0V

Standby on
E = 10mV
B = 440mV
C = 60mV

Aren't these too low?

[g1]

Standard gate voltage to turn off mute fet's is usually in the tens of volts, not millivolts.
But a bad fet can pull down the control voltage, so it can be a chicken/egg type scenario. Ideally, you would want to lift all the fets connected to the mute control line, then check the voltage.

[spy]

g1, you're the man! I did the "ideally" case and found the little bugger.

There are two more J175s before the record and live outputs, one of them was the cause.

Thanks all for your help!