Partially working Symetrix 501 - help!

[focusbob]

I obtained a partially working symetrix 501 compressor limiter. Specifically, the limiter side of the unit works, as does the makeup/output gain, but the compressor part of the unit does not work (i.e., attack, release, ratio knobs do nothing), though there are some working bits within that circuit (i.e., the threshold led triggers and its behavior is responsive to the threshold knob and incoming audio level, but only when the compressor "in" switch is engaged, which is how it should be). Because it was easy enough to do, I tried a replacement vca in the existing vca socket, to no avail.

I have attached the only schematic I could find for this model, though it details a later version than the one I have (mine is the alison/valley ppl vca, toggle switch version). I have also attached a picture of the "big black box" that appears in my version but not the schematic; the internet has suggested that "the rms detector" lies within.

I have confirmed that the voltage regulators are outputting the proper +/-15v (within 1% tolerance) and the resovoir filter capacitors have acceptably low ESR. I am hoping that all of the above information will provide you with enough detail to guide me as to what I should do next to evaluate and repair this unit. Thanks!

[g1]

Have you verified the switch that is supposed to do the changeover from comp to limiter?

[focusbob]

Have you verified the switch that is supposed to do the changeover from comp to limiter?

Yes! There are 2 separate switches for the comp (in/out) and the limiter (in/out). The comp switch appears to work because, with it off, the threshold light doesn't trigger, but with it on (and a suitable audio signal applied), it does.

[The Dude]

What input level are you using for testing purposes? Perhaps it's too low to achieve compression.

[focusbob]

What input level are you using for testing purposes? Perhaps it's too low to achieve compression.

Line level. It is high enough to achieve limiting (on the other working side of the unit).

[g1]

Yes! There are 2 separate switches for the comp (in/out) and the limiter (in/out). The comp switch appears to work because, with it off, the threshold light doesn't trigger, but with it on (and a suitable audio signal applied), it does.

Without the correct schematic it's pretty tough. Is the compressor switch a SPDT? If it's more than that, (DPDT or more), it's possible only part of the switch is working.
All the push switches in your picture would do more than just switch the threshold light circuit.

[focusbob]

Without the correct schematic it's pretty tough. Is the compressor switch a SPDT? If it's more than that, (DPDT or more), it's possible only part of the switch is working.
All the push switches in your picture would do more than just switch the threshold light circuit.

It sure is tough without the correct schematic! I believe the in/out switches are both dpdt (6 lugs, 2 parallel columns of 3 lugs)... Is it possible to test the switches in circuit or would I have to remove them to test them?

[g1]

Yes you can test them in circuit with your meter set to measure resistance.

[nevetslab]

I checked my Symetrix folder, as I have the 501 Comp/Limiter that DOES match the multiple revision release Schematics like you published. I don't have the one you show in the photo though. I wonder of THAT Corp has any documents on that Allison/Valley People Compressor module? They came into being around that time last century.

I did find in my folder the VCA's Investigated Part 2 from Studio Sound Mag July 1989 (Part one is June 1989) that covers a wide range of VCA modules used in dBx and others' technology. Perhaps this could offer some help.

Valley People TA-101 + VCAs_Ben_Duncan.pdf

[nevetslab]

I found a website that has most of the Studio Sound Magazine scanned. I downloaded June 1989 and extracted Part One of this VCA article, and have attached it here.

Valley People TA-101 + VCA's Part 1.pdf

[focusbob]

Yes you can test them in circuit with your meter set to measure resistance.

Thanks, will do and report back tomorrow! Really appreciate your help

[focusbob]

I found a website that has most of the Studio Sound Magazine scanned. I downloaded June 1989 and extracted Part One of this VCA article, and have attached it here.

[ATTACH]n974144[/ATTACH]

Thanks so much for posting!

[focusbob]

Yes you can test them in circuit with your meter set to measure resistance.

Well, I'm starting to think you are either a Wizard or from the future, because I think you may have done it again! Attached are results from testing all of the dpdt switches (pins inside a rectangle have continuity for a given state). Precisely one half of the compressor "in" switch appears to be not functioning correctly (see bottom left)!

[g1]

Now I would desolder the questionable pins and recheck with them disconnected from the circuit. There may still be a short somewhere that switch connects to.

[focusbob]

Now I would desolder the questionable pins and recheck with them disconnected from the circuit. There may still be a short somewhere that switch connects to.

Will do!
Related question: what do you call this type of dpdt switch (in order to identify a suitable replacement)?