Can change in EQ be an LDR issue? [Mesa Quad Preamp]

[Greenback25]

I recently encountered a problem with my Mesa Quad and it happened overnight. I'm using the Quad on the first lead channel and a 750 Hz scoop on max on the graphic EQ and, without changing my signal path and settings one bit, the Quad started to sound much more mid-focussed and not as tight as it did before.

I went to a tech to check the tubes, filter caps, and clean the potentiometers and graphic EQ, but everything seems to be fine.

Then I was told that it could be an LDR problem, not that they're fully broken yet just not at full capacity anymore. The only problem is that there are 18 LDR components in the Quad and I'm not sure if all of them are active on first channel.

Would you say that this could indeed be an LDR issue?

I was fortunate enough to record a clip just the day before it went awry, I recorded the Quad through Mesa 2:90 into a reactive load, so there's no cab/IR on top so that you can hear the full range.

Here's a clip from before it changed its character:
https://drive.google.com/file/d/1dRFgqf ... ENPNp/view

And here's after the character change:
https://drive.google.com/file/d/1U_ukHG ... MjhXJ/view

The difference is not overwhelming but it is there. The mids are much more present in the after clip and there's less tight bass / top-end. Can this really be down to not bad but not good either LDRs/Octocouplers?

[maxkracht]

I'm not so familiar with this model, so grain of salt, but I think the vactrol/ldr is only turning the eq section on or off. Should be just the one vactrol per eq section, so not too hard to find, especially if you have a schematic. When the vactrol's LED is on, you should get low resistance, off high resistance. You could also make another drastic eq change then wait for it to act up to see if the whole eq section turns on or off, or if it is just a change in mids, indicating another problem.

[LOUDthud]

According to the schematic I found : https://el34world.com/charts/Schematics ... preamp.pdf

LDR 23 and 24 enable the LC networks in the Graphic EQ by connecting them to ground.