LA500A opto comp MAY 2021 BUILD THREAD 500/51X

[rob]

So. For the lack of a schematic:
- I've swapped the OPAs. They're fine.
- Measured the 78L05. Perfect.

With OPAs removed and 1.23 VAC 1 kHz signal on the input XLRs, I've measured different +In B voltages on pin 5 of the OPA sockets:
Good unit: 0.607 VAC
Bad unit: 1.34 VAC

Do NSL32s die suddenly? Can't think of any other part on that PCB causing this. Can I check them in-circuit?

[Joe Malone]

Hi,

I'm trying to re-calibrate two 2017 units. No issues back then. Now I'm having an issue with one of them early on:

When feeding them with the same 1.23 VAC 1 kHz signal @ HPF=flat, 3:1, both pots fully CCW:
- The "bad" LA has the output jump to +4dB (meter) = 2.74 VAC on the output XLR when switching from "OFF" to "VU".
- The "good" LA slightly decreases to 1.19 VAC. I guess that's perfect.

I've unplugged the BIO500:
- The BIO loopback test shows perfect unity gain on both units.
- The pots measure 87k4 (thres) and 9k69 (makeup) on the bad unit, the good one has 10k0 makeup. Should be fine.

How can I narrow down the issue with the bad unit amplifying a signal that it shouldn't?

The 1.23vac should be measured on the output of the LA500A. If one output is a bit low just turn up the makeup pot slightly until the outputs are the same to do the +4dBu = 0VU and rest of the cal. If you want a stereo gain match with no makeup applied then adding a large resistor value across the makeup pot pads that has the lower output will match them.