Pro Audio Repair Question - troubleshooting compressor/de-esser

This is a very esoteric question, but I’ve not had much luck with Google, so I turn again to the SDMB. I’m thinking maybe fishbicycle has dealt with this, and if not, QED knows all about, well, everything.

I’m doing a PA rebuild/re-install for a club here in town, and I’m troubleshooting all the rack gear prior to recommending a ‘new equipment needed to buy’ list for the owner.

Everything seems to be okay except for an older Rane compressor/de-esser that doesn’t seem to be working as efficiently or accurately as it should be.

Is there a way to specifically test the compression ratio, or should I just give up and recommend the owner get a newer model? This an earlier model, incorporating both anolog and digital compression schemes, and would probably be worth repairing if the cost is under $200 or so.

So far, I’ve run signal through the problem compressor in a sidechain on one channel of the board, and run a reference signal through a known good unit in another, and gone back and forth trying to detect discrepancies.

It seems like the unit’s ratio knob is off kilter, and (for example) a 4:1 ratio on the good unit sounds quite different from the same 4:1 on the problem unit. However, the discrepancy varies depending on the compression level. Obviously, there’s a good chance that this is due to differences in the compression algorithm and there’s nothing to be done but deal with it or dump the unit.

What I guess I’m looking for is a way to empirically demonstrate to the owner that something is off with the unit, rather than saying, ‘Well, can’t you hear the difference?’ He’s not the type of guy to just say ‘oh okay, here’s $300 bucks, get me another’; he’s gonna want see the results of some test that I can run in front of him.

Also, the owner doesn’t have good ears - he couldn’t hear the difference between the problem unit and the good one at all when I tested it, while it was night & day to me and my assistant.

(Yes, I am well aware that there are plenty of ‘good enough’ compressors for under $150 - this club records every show and the owner wants high end gear that works well and maintains some value on paper as it gets older.)

I’m fascinated to hear the answer. And, one would think if the guy is really interested in quality over value that the mere fact that it seems unreliable (or at least inconsistant) would be enough for him to want to replace it.

Out of my area of expertise, I’m afraid. I understand the basics of audio compression/decompression, but I don’t have much hands-on experience with them either in use or on the bench. However, I might suggest that if you have or can borrow an oscilloscope, you might be able to use to to at least show the owner the difference that he can’t hear, if not actually measure the extent of the problem or diagnose it.

Thanks for thinking of me, but I’m sorry, I can’t tell you anything useful about your situation. I use compressors all the time, but I have no idea what’s inside one, and if it went wrong I wouldn’t know where to look. There used to be a time when you could take pretty much any audio device to a tech shop and ask them to recalibrate it. Not anymore. Considering that the unit is fairly old, you might ask your client to weigh the options of having it repaired and then working until such time as it fails again - or buying a new dbx unit. I would try to convince him to get a dbx. You know, the difference between $$$ now + $$$ later, or just $$$ now.