Help repairing DBX 160x rack compressor

[focusbob]

Hi, I knowingly bought an "untested" DBX 160x compressor that happens to not work properly, and I would like to fix it. Have some minimal electronics building and repair experience but feel like I'm in way over my head, need help! Schematic attached.

Bypass, in/out metering, and output gain all seem to work properly, but in hard knee mode, the threshold and ratio knobs do nothing (no compression); in over easy mode, there are 2 leds of gain reduction on the meter irrespective of what I pass through, and turning the threshold and ratio knobs can cause more leds of gain reduction to illuminate (but only up to about 5, and in a non signal dependent manner). In either mode, the "below threshold" indicator is always illuminated... I obtained the schematic, changed the main two psu electrolytics, and now I'm stuck.

I am a hobbyist. I have built and repaired a few simple things (mostly vintage tube amp circuits). But, this is a complex (to me) circuit and I am out of my league. If folks read my problem description and it rings a bell, any advice would be appreciated. Not counting on that, I'm wondering if anyone can recommend resources on how to troubleshoot a rack compressor? I'm not sure what to measure with my voltmeter, in what order, etc. I've read some troubleshooting resources online and in print but they all seem to expect you have an intimate knowledge of the circuit function, which I do not. I also am lacking in general analog electronics troubleshooting procedural knowledge. Thanks in advance!

[The Dude]

Start with power rails. Make sure they are symmetrical and free of ripple.

[focusbob]

Start with power rails. Make sure they are symmetrical and free of ripple.

Thanks! I'm not sure if this is exactly what you were suggesting, but I measured voltages at the voltage regulators. Everything is within spec. Is that what you meant by the "power rails"? Also, I kinda know what ripple is but do not know how I would evaluate it. If it requires an oscilloscope, is there anything else I can do with my multimeter to troubleshoot?

[eschertron]

The good news is that this is all discrete components and they seem to be available online.
The bad news is there's a LOT of circuitry to go through if you don't have a plan.

What are you measuring at the regulators? I don't see a voltage printed on the schematic, so am unsure if it's +/-15vdc or +/-12vdc.
You may be able to check for ripple with your DMM set to AC volts (millivolts), depending on how good the meter is. An Oscope is quicker and more reliable.

If this were my pet project (and I thank God that it's not! ), after checking the power rail stability I'd trace all the power rail connections to each chip and confirm every device is getting the voltage it needs. Maybe make several printed copies of the schematic, tracing out power on one, signal on another, and control voltage on a third. I'd do that. But then again, I get easily confused and lose track of the circuit I'm tracing. Good luck!

[focusbob]

The good news is that this is all discrete components and they seem to be available online.
The bad news is there's a LOT of circuitry to go through if you don't have a plan.

What are you measuring at the regulators? I don't see a voltage printed on the schematic, so am unsure if it's +/-15vdc or +/-12vdc.
You may be able to check for ripple with your DMM set to AC volts (millivolts), depending on how good the meter is. An Oscope is quicker and more reliable.

Thank you for your input! Your message caused me to go back to the schematic I posted to realize that it is not the correct schematic for the dbx 160x, though the internet said it was so. Since then, Ive found 2 schematics for the closely related 160xt, neither of which have legible power section schematics. Poking around forums, it is unclear whether the power should be 15v or 12v. So, I appear to be SOL at the moment I'll have to think on this some more, amd maybe post a request in the schematic forum. Thanks, Jim

Edit: the dbx 160xt service manual with the illegible power section schematic states in the service notes that power at test points (which sadly don't exist on the dbx 160x) should be +/- 15vdc. Alternatively, on other forums, Jim Williams from audio upgrades states power on the dbx 160x is 12vdc, so

[g1]

How do you know that the schematic you posted is wrong? It shows the +/-12V at IC's. Is there other stuff that is different?
What are the part numbers on the voltage regulators?

[focusbob]

How do you know that the schematic you posted is wrong? It shows the +/-12V at IC's. Is there other stuff that is different?
What are the part numbers on the voltage regulators?

The reason I judged it to be wrong is that my unit has 1/2 the psu filter caps that the schematic shows. Also, the component numbering is way off. I will check the vr marking on Monday and report back (hopefully they are legible)!

[focusbob]

Back with the unit, I can confirm the voltage regulators are 12vdc (7812, 7912). Maybe I just have an early version of the circuit, before they added additional filter caps and re-numbered components (+?)...

Anyway, vr1 measured 26.4vdc in and 12.16vdc out, and vr2 measured 26.72vdc in and -14.27vdc out. Although -14.27vdc out sounded concerning to me at first (relative to the intended -12vdc), I next measured the voltage across the diodes following the regulators (what I'm presuming to be d130 and d131 on the schematic, though labeled cr30 and cr31 on the pcb), and found both to return very close to 12vdc (-11.9 & 12.17).

My conclusion would be that the psu is functioning adequately in terms of voltages (only), but I don't know much What would good next steps be? Thanks!

[focusbob]

Also, I tried measuring ripple via the suggested multimeter method. My EXTECH 430 DMM returned (approx.) 33v then 17v then 9v then 5v then .2v, then back to 33v and so on in a circular loop, with each cycle taking approx. 2 seconds. I won't pretend to know how to interpret this or whether it is interpretable in any way

[g1]

What do you measure at the 'ground' pin of VR2 ?

[focusbob]

What do you measure at the 'ground' pin of VR2 ?

Please pardon my ignorance, but I'm not sure what you are asking. To measure the ground pin, where would I place my other DMM probe? Chassis? Other VR2 pins? To make the above measurements, I have been placing my black probe at the ground pin of VR2 (i.e., pin 1). My "in" V was read with my red probe at VR2 pin 3 (the middle pin) and my "out" V with my red probe at VR2 pin 2. Thank you for your continued support!

[g1]

If you have a regulator labelled 7912 that reads 15V between pin 1 and 2, replace it.

Sometimes they will elevate pin1 off ground, but it will still read 12V from pin 1 to 2. I thought maybe they had done so (by 3 volts), but then most likely they would to it for both the + and - regulators.

[focusbob]

If you have a regulator labelled 7912 that reads 15V between pin 1 and 2, replace it.

Yeesh, I just repeated my measurements and this time I am getting -11.9v (pins 1&2 of 7912) and +12.16v (pins 3&2 of 7812) from the regulators. I have to assume the discrepancy from my previous measurements was user error... What would you recommend I do next given these readings?

Edit: To add, in case I haven't already, I replaced the psu electrolytic capacitors before starting this post.

[g1]

Maybe repeat the measurements from post #9. You should be able to get a stable reading of AC voltage close to zero. For both the + and - supplies.
It's possible you are just having measurement error, but it could also be some intermittent problem. If you have trouble getting consistent readings anywhere, you may need to sharpen your probes, or dig in harder.

[focusbob]

Maybe repeat the measurements from post #9. You should be able to get a stable reading of AC voltage close to zero. For both the + and - supplies.
It's possible you are just having measurement error, but it could also be some intermittent problem. If you have trouble getting consistent readings anywhere, you may need to sharpen your probes, or dig in harder.

Back from a work trip... I repeated the measurements as suggested and am still getting within 0.15 of (+/-)12v from each regulator. Also, I worked with setting the AC voltage range properly on my DMM and am getting a reading close to 0 for each regulator! What next steps would you recommend?