Troubleshooting 101 - Master Logic.

The console uses an X and Y logic buss to set up the signal path. The modes are INPUT, TAPE, and MIX.

Using the switching logic you can place the Mic Pre or the Tape In to the Chamnel (EQ, Phase, Cues, etc) or the Left Right Monitor Mix.

The Input Output Modules (IO) use a CD4011 quad CMOS Nand gate to define the logic states from the X&Y busses.

The output of the logic controls JFETS that are placed to either allow or block the signal path from the mic pre or tape inputs to the channels. When the JFET has a -15VDC on the gate it turns on the FET and either allows signal or shunts it to ground.

What really confuses me is that the circuit is using negative logic. In other words VSS is not +15vdc as in normal cases. VDD is -15V. So a "true" or "1" state is actually 0vdc ground. a "false" or "0" is -15vdc.

Here is a more in depth example below. The Tape Input flows thru the N-channle FET Q13 if S2C is at -15 logic state. And in this state Q11 shunts the input to pin 2 + input of the LM351 opamp. effectively to ground... "muting" that input to the Master LR 2 mix...

Now on to more troubleshooting....