DBX 162 Stereo Compressor/Limiter No Signal

[fenderguy81]

I am working on a DBX 162 that does not pass signal on either channel. It powers up and passes signal when bypassed, but does not seem to be working on either channel. If I push the output I can very faintly hear the signal coming through, but that's only at really high volume. I'm going to order some caps to replace all the electrolytics as I'm sure they are overdue for replacement. Any ideas as to what would affect both channels in this way? I attached a copy of the schematic below for reference. Any help would be greatly appreciated, thank you!

 

[pvision]

Power supply. Check the ±24 and ± 15 volt rails

 

[pucho812]

I fixed a 162 ions ago. In the one I fixed one side worked one side did not.
Was in uncharted territory without a schematic but was able to find the issue and resolve.
They squish audio nicely.
If the problem is in both channels start looking and what is common to both channels. The PSU.
See if you are getting the correct voltages as mentioned above.

 

[fenderguy81]

Thank you! The power supply voltage was reading low and my parts finally came in. I'm recapping the unit and then will see how it's performing. I ran into the 4.7uf caps on the main board that are bipolar/n.p. but look like electrolytic caps. I've read that bipolar electrolytic caps aren't used as often today and am wondering if I should bother replacing them. If replacing is recommended, will any non polarized cap be fine or should I order electrolytic non polarized caps specifically?

 

[pucho812]

Thank you! The power supply voltage was reading low and my parts finally came in. I'm recapping the unit and then will see how it's performing. I ran into the 4.7uf caps on the main board that are bipolar/n.p. but look like electrolytic caps. I've read that bipolar electrolytic caps aren't used as often today and am wondering if I should bother replacing them. If replacing is recommended, will any non polarized cap be fine or should I order electrolytic non polarized caps specifically?

hard to say without measuring them.

I am working on some eq's at the moment that use BP electrolytics. The reason for the BP lyrics is they are often smaller in physical size for the value required. I find that I often can't go with anything else due to size requirements. A film cap of the same values would easily be much larger. In my case here I am finding that the caps measure correctly in terms of value but are a high ESR rating making them ineffective.

 

[fenderguy81]

After the recap I'm still having the same issue with no output when the DBX is engaged. I'm just using a SM58 on the input and output is sent to a little guitar amp. Again, it is passing signal in bypass mode. Same issue on both channels.

I checked the ESR on the bipolar caps I did not change (1.2, 1.5) so I doubt that they are an issue. I checked voltages and found the appropriate +/-24v and +/-12v on the power supply diodes and pin 7 & 8 on the daughter boards. Any recommendations as to where to check next would be greatly appreciated. Thank you!

 

[JohnRoberts]

You need to follow the signal from input to VCA and to output. If you don't have a scope you could make a sniffer, effectively an audio probe where you cap couple (in series with say a 2k resistor to not load down where you are probing), feeding into a small headphone amp or whatever and listen for where the audio signal stops.

JR

 

[pucho812]

Service manual

https://elektrotanya.com/dbx_162_sch.pdf/download.html

 

[fenderguy81]

You need to follow the signal from input to VCA and to output. If you don't have a scope you could make a sniffer, effectively an audio probe where you cap couple (in series with say a 2k resistor to not load down where you are probing), feeding into a small headphone amp or whatever and listen for where the audio signal stops.

JR

Just a note, I am getting audio through the B.P. cap so with the ESR test I am assuming I can rule that out. I have not touched any of the tantalum caps yet either.

I started signal tracing to the best of my ability but the schematic is a bit hard to read (board does not have corresponding schematic numbers ex. C19, R23, etc.) and I’m still a bit of a novice. All the TL08 op amps are passing audio through pin 6. I believe the schematic shows 301 chips? I tried testing the CA3046 DIP (5 transistors chip) and can not hear audio on any of the base or collector pins…

 

[pucho812]

What about the 301 chip at the output.
I find that if I pick an end and start working my way from there I will eventually find where the signal stops. Then I can work out why.
Can you trace audio up to the vca input?

 

[fenderguy81]

What about the 301 chip at the output.
I find that if I pick an end and start working my way from there I will eventually find where the signal stops. Then I can work out why.
Can you trace audio up to the vca input?

I'm trying to signal trace from the input. The schematic has different op amps, LM301 (schematic) vs TL08 (actual). I can get audio traced on the output of each TL08 (pin 6; these are the op amps used on the 230 output daughter cards as well for this particular unit), but I can't get any audio on the VCA 210 daughter card (input pin 3 correct?), but do have the +/-15v on pin 7 & 8. I guess I need to start looking closer at the VCA 210 daughter cards. Anyone have a schematic for these? Or anything that would affect both VCA's I should check? Thanks!

 

[JohnRoberts]

When troubleshooting without a formal schematic you can generally look at data sheets for VCAs and common op amps to figure out outputs and power pins.

JR

 

[pucho812]

I'm trying to signal trace from the input. The schematic has different op amps, LM301 (schematic) vs TL08 (actual). I can get audio traced on the output of each TL08 (pin 6; these are the op amps used on the 230 output daughter cards as well for this particular unit), but I can't get any audio on the VCA 210 daughter card (input pin 3 correct?), but do have the +/-15v on pin 7 & 8. I guess I need to start looking closer at the VCA 210 daughter cards. Anyone have a schematic for these? Or anything that would affect both VCA's I should check? Thanks!

The vca cards are discrete. It would be unusual to have both channels fail but then again anything is possible. On the one I fixed. Someone else put transistors in backwards on one channel. Once corrected. It worked.

 

[fenderguy81]

The audio signal stops at the other side of the 100k resistor that connects to pin 3 of the VCA on both channels (see attached highlighted schematic).

 

[Brian Roth]

The audio signal stops at the other side of the 100k resistor that connects to pin 3 of the VCA on both channels (see attached highlighted schematic).

That MIGHT be "normal" depending upon the design of the VCA. Do you see any signal at pin 6 of what the schematic calls OA6?

Bri

 

[fenderguy81]

That MIGHT be "normal" depending upon the design of the VCA. Do you see any signal at pin 6 of what the schematic calls OA6?

Bri

Yes, I am getting signal at pin 6 of the OA6 (TL08).

 

[Brian Roth]

Yes, I am getting signal at pin 6 of the OA6 (TL08).

Interesting. At "PC4 DA 239" (next step in the path) do you measure +24VDC at pin 7 and -24 VDC at pin 8?

I also see that cluster of four FETs used for muting. I recall those being a possible trouble source.

Bri

 

[fenderguy81]

Well after touching up some soldering joints it seems like it’s actually working now. It did blow a fuse so maybe not completely fixed yet. I’ll have to do some more thorough testing once I have it setup in my recording space.

 

[pucho812]

Interesting.
Is it still blowing fuses?

 

[cyrano]

hard to say without measuring them.

I am working on some eq's at the moment that use BP electrolytics. The reason for the BP lyrics is they are often smaller in physical size for the value required. I find that I often can't go with anything else due to size requirements. A film cap of the same values would easily be much larger. In my case here I am finding that the caps measure correctly in terms of value but are a high ESR rating making them ineffective.

I had no idea bipolar Cs were smaller...

Why didn't anyone ever told me?

Anyho, Pucho, thanks for this speckle of light in a world of darkness.