LA610 Distorted Compression

[guitarrock04]

LA-610 Mk1. Unit has a faint but noticeable fuzz, only when the compressor is engaged. It comes out on the back end of a sound, as the gain reduction recovers. Not noticeable without signal present. Unit was perfectly fine til recently. I suspected something in the T4 was amiss and replaced the LDRs but to no avail. Tube swap changed nothing, but now without schematic, I'm not sure where next to look. Any thoughts or known issues on these I should check? Trying to help a friend but at my wit's end.

Any help appreciated

 

[Bo Deadly]

Maybe someone can confirm if that compressor circuit is the same as the LA2A. Or just look at the LA2A schem and see if the parts are accounted for.

If the level of fuzz is proportional to gain reduction as indicated on the meter, tat sounds like bleed from the side-chain.

Could indicate a grounding problem.

Did it start happening after moving it or after some kind of mechanical stress?

Look for bad solder joints using a light and magnifier.

I'm just guessing of course.

You need a scope.

 

[guitarrock04]

It's similar to la2a, but nowhere near verbatim. EL90 driver for the EL panel and no 12BH7 output stage.

The noise is constant, not proportional to gain reduction, but only present when the meter is off zero. Noise goes away when compressor is bypassed. But having the compressor engaged while no signal, there is no noise - if that makes sense.

 

[fluidartmedia]

I'm actually having the same problem - LA610 mkii that has a static kinda crackling noise when the compression is engaged. Preamp stages all produce no issue. The static is there both with compressor and limiter, and only during gain reduction - not when unaffected signal passes. Same thing with bypass.

Did you end up finding anything guitarrock04? I have some time coming up soon to poke around mine, maybe we can figure it out.

 

[saint gillis]

Try to see if the audio signal goes through certain tubes only when comp is engaged, then locate the anode resistors of these tubes and replace them.
If the symptom is intermittent little sparkles "skrkk chkrrrk" it could be this

 

[fluidartmedia]

This fellow has the same problem:
https://groupdiy.com/threads/help-repairing-la-610-se-that-is-distorting-when-compressing.67547/
The link he posted in the thread has an example of the sound.


Saint Gillis, is that the "skrrk chkrrrk" you're talking about? They also mentioned checking the T4 cell, so I'll report back after I try the handful of ideas here.

 

[saint gillis]

I'm not sure if the distortion is related with the bass sound or not, other people here may have useful advices.
I'll try to replace the anode resistors (I already had one of them causing an issue on this compressor), I'll do that before thinking of checking the expensive T4B