Mesa DC3 nightmare

[unparalleled]

Hi all,
I have not visited here for many years.
Have a problem with a Mesa DC3, very low output unless signal is fed direct into the power amp section via the loop return.
Seems to be something in the muting circuits which are not on the schematics.
Does anyone have a specific mute circuits schematic?
Is there a simple way to disable the mute circuits?

Cheers,
Alan.

[Pedro Vecino]

Schematic here:
http://www.schematicheaven.net/boogi...aliber_dc3.pdf

[g1]

Pedro the muting circuits are not shown on the schematics.

Are you sure it's the mutes? Have you tried a patch cord between send & return? No master or graphic faults?
Generally muting circuits use jfets between signal line and ground. So the fet shorts the signal to ground until it receives it's control (gate) voltage and turns off. When configured like this the easiest way to disable is to remove them.

[Jazz P Bass]

There is no mute function on these amps.
LDR's & a relay.

Troubleshoot it.

Do both channels exhibit the symptom.

Does the graphic eq affect it.
You can turn it on & off with a foot switch.

[unparalleled]

Found the problem. it was the 1uF coupling cap feeding the send jack which had turned into a 20 meg resistor.
Was not the jack switching, I had to scope the signal all the way through that awful mesa PCB to track the fault down.
Thanks for your replies gentlemen.

[Jazz P Bass]

Mesa amps sure are a layout nightmare, that's for sure.

[Enzo]

Yeah, I won't take them for service anymore. Oh wait. I don't take anything in anymore...