Here's troubleshooting and mods I collected in this thread to post in one spot.
Issue: No makeup gain.
Check the voltage on wiper of makeup pot changes from 0 to about 8V; check R53, R59;
of course, at pin 6 IC7 there's 0V, it is inverting ground-referenced amp;
if compressor works, i.e. you get compression when changing threshold, the problem is not IC7B.
Gain reduction signal, or control voltage, comes to IC7B, thru R54.
Makeup voltage comes to IC7B thru R59.
Issue: Audio passes through in standby. Audio passes in 4000 and gssl mode.
I can use make up to increase level but no compression. I hear the relays when switching from bypass to 4k and gssl.
Meter is not moving. Can someone give me a starting point for troubleshooting?
First, let's check the voltage beween gnd and wiper of threshold pot,
it should change from approx. +6.5 to -6.5 Volt when rotating the THRESHOLD pot.
Then, let's check if audio passing thru sidechain: feed about 1V RMS sinus 1Khz to input, set 4000 mode,
sidechain filter OFF, check with scope at IC3d (14), IC3A (1), there should be sinus 0.5V RMS;
than, check if there's a signal on CTL pin 5 (10 pin IDC connector);
than, check the signal on IC11A (1), sinus, it's amplitude changes with position of threshold pot;
than, check if rectifier works-on IC10A, (1) should be rectified full-wave signal when threshold set to low (CCW);
than, check if there's a voltage CTL-10, should be saw-looking signal which changes depending on amount of compression and time constants;
than, check the voltage on IC7D (14), this is the control voltage (should be meter deflection), etc.....
Before all this,
Check jumpers;
Check if CTL and SC cables going to their channels respectively,
i.e., CTL1, SC1 to left and CTL2, SC2 to right channel;
Check all connections etc...
Issue: The meter is showing gain reduction as I turn the threshold knob.
However the audio doesn't seem to be effected at all, and the make up gain doesn't change the output either.
When turning MAKEUP and THRESHOLD pots, at the pin 7 and 8 of IC7b the voltage should change within range of couple of volts.
If yes, check resistors R60=100r, R75=49r next.
Than, go to audio board and check if there's voltage change when rotating the trimpots at CTL connector, pins 1 and 3
(or, check at R100, R45). Check R100=909R. Maybe, bad contact at pin 1 of CTL connector.
If this does not helps, check on VCA adaptor PCB: R101=732R, R102=100r, check if IC9 (NE5534) installed correct as well.
MODS
Regarding the release switch, you easy can have 6th position, just install
R||C, between A6 or C6 point of release switch to gnd...Try 220K and 0.1uf,
for very fast aggressive setting, or you can mix two RC for another auto mode.
Add a 1k trim pot before the VCA at R6 in order to match the gain on the left and right channels.
Meter calibration: R50 = 2k can be changed to get correct reading.
