dbx 160XT distorting under compression

[pvision]

I have a dbx 160XT which distorts when it's compressing. It's not horrible, but it sounds pretty bad compared with a good 160XT

I've tested it with tone and watched the output on a scope program on my Mac. Both outputs (balanced and unbalanced) clip at the same point and both sound clean even with fairly large amounts of gain reduction showing on the meter. But that's with tone, not audio. PSU rails are spot-on and there's no signs of damage or previous work on the PCB

As it passes audio cleanly with no GR I am wondering if it has a bad VCA chip, but I imagine it could equally be a sidechain problem

Any suggestions on what to try next gratefully considered!

Edit: I just found a dbx 165 thread and will see if anything in it is relevant to the 160XT
http://groupdiy.com/index.php?topic=10769.0

See my other post re dbx parts wanted
http://groupdiy.com/index.php?topic=62343.0

Nick Froome

 

[moamps]

IMO, this type of the failure is very hard to find. You will need a scope to visualize what happens in a dynamic conditions, and it helps if you have an another unit which works properly so you can compare outputs and easily chase the problem.  Don't be mislead with correct response on the single tone input, sometimes distortions can't be noticed this way. 
Anyway,  you can do recapping (two of caps should be bipolar high quality  types, IIRC), it will not hurt, and also you can replace VCAs and RMS detectors. If you use sockets, you can easily swap between more ICs.

I have some used original dbx  VCAs, RMS detectors and pots from 166  so I can send it to you for free.  I got very cheap some parts for another project  from member barclaycon so I need to return something to the community  .  Please PM me if you are interested.
Also, you can buy some new parts  for 160xt here (pots, knobs)
https://www.studioelectronics.biz/sunshop/index.php?l=product_list&c=57
but they are very expensive.