Possible solder bridge causing short in Mic Pre

[RoadrunnerOZ]

Do you have a cable you can use to connect insert send to return?

 

[telehumbucker]

Yes. A send from another channel shows up on the problem channel using the problem channel's return..... so it can receive a signal that way....just not at the mic/line input jacks. I sent a signal into the problem channel line in jack and then tried to send a signal from the problem channel's send to another channel and nothing happened......

 

[JohnRoberts]

continue to zero in on the location of the problem..

JR

 

[Khron]

Well, you COULD use the continuity indicator on your multimeter to make sure the wiring's in one piece, between the mic XLR in and the transformer, check the resistance of the transformer's primary and secondary windings, then check the wiring between the latter and the input amplifier (IC1, TDA1034N).

Triple-checking that all the opamps DO receive the power they're supposed to (on pins 4 & 7, as per the "power rails" section at the bottom-middle of the schematic, viewed in landscape), would also help.

 

[zonkers31]

If you have a multi meter, set it for ac volts, and look for signal at pin 6 of IC1 and 2. You should at least see a few hundred millivolts of signal at the output of the opamps. If not, one of those 47uf/4v caps could be open. If you can see some voltage then the issue is after that.

 

[RoadrunnerOZ]

There are a few IC’s between the transformer and the insert send/return and also some tantalum capacitors in the signal chain. The whole EQ stage can be bypassed with the EQ in/out switch. You really need to trace through with a signal injected - if you use a low frequency signal from a software based oscillator pretty well any multimeter (unless you have a scope) with AC will show AC on the output of each IC. Check the top of the sensitivity pot for signal and work forward (or backward) from there.