Here is a photo of there opamp in question. Here it is used in circuit. It's different than the ones on the I/O board. I have worked out the following so far.
1.PSU unregulated is +/-50V, regulated it is +/-30V and further discrete regulation to provide +/-15V for the chips. PSU behaves the same in both units.
2.I confirmed both main boards are identical and measure the same as far as voltage test points.
3.I have the pinout for the discrete opamps on the I/O board which are different then this opamp. so I was able to work out which one has signal for the in and which ones have signal for the output to LF, MF, and HF. I confirmed if I hit the mute switches for the respective output I lose signal.
I confirmed the I/O board is working without issue in a second unit. I can swap I/O boards and they behave the same in a working unit.
4. I confirmed both filter boards work in a working unit.
There is a globe safe that mutes all outputs should conditions change.
When I install these things it's when the safe mode kicks in. So it has me thinking it has to be these things.
There are 8 in total in a unit.
I confirmed from signal probing which ones are for which and I thought I had a pinout from probing but I may be mistaken. The first two that are not installed at the moment are for the LF, the 4 that are there are input from the I/O board and then output of the MF and the last two that are not installed are HF.
As both main boards measure the same, it stands to reason that the fault is with the opamps I pulled. It also stands to reason that they are the culprits as when installed it trips the protection circuit(Global mute).
I thought I had the complete pinout of these as I can measure where I get VDC, probe where I get audio before it hits these and probe where I should have audio coming out. Sadly there is no documentation to go off of so it's been a slow process.
2 out of the 4 trip the