D&R Compressor issue

[snk]

Hello,
I hope this is not an out of topic discussion (as this is about a studio rack unit, not a stompbox...).
I recently got a D&R compressor from the 80s. It is a stereo unit, and only one channel is working as expected.

The channel 2 only produce white noise 
The dry audio passes through when i put the unit in bypass mode, the led shows the compression action, and, when active, I can hear the processed audio a tiny, tiny bit at very low volume behind a constant white noise stream.

I opened the unit, and D&R kindly sent me the schematic and service manual.
It seems to be a rather simple design, with 3 chips and 3 transistors.
The chips (5534 and TL074 opamp, and a TFK267BG) are even mounted on sockets.
The transistors are 2x 2N5458 and 1x BC5468.

Would you have any idea about what could be causing a white noise stream instead of processed signal ? A dead transistor?

Here is a (crappy) picture of the unit :


And here, the service manual with the schematic :

Inside, it looks like that (it's not my unit, it's a picture I found on the web) :

[Rob Strand]

It depends if the white noise is very loud or very small, ie. small like the other channel except the audio doesn't get through.  The first points to a problem with the audio path. The second points to a problem with the side chain, eg. the compressor is on full all the time.


You could check the DC voltages on the opamps.   Since one channel is working you already have a reference for what the voltages should be.    Just make sure you set the controls on each channel to the same position.

If you have spare opamps it would be very quick just to try plugging in new ones to see if it's a dead opamp.     Another possibility is the Output Level pot P2 has gone open.

[snk]

Hello, Thank you for your answer Rob.

It depends if the white noise is very loud or very small, ie. small like the other channel except the audio doesn't get through.

It's not a sonic blast (it's below unity gain if compared to the bypassed sound), but it is not background noise either.

The second points to a problem with the side chain, eg. the compressor is on full all the time.

I don't think so : according to the led/vumeter, the compressor works as expected.

You could check the DC voltages on the opamps.   Since one channel is working you already have a reference for what the voltages should be.

Yes, it should be easy. The black pin of my DMM to the ground, and the red pin on the opamp, right?

If you have spare opamps it would be very quick just to try plugging in new ones to see if it's a dead opamp.

Yes, I have spare opamps, I will check that, too.
You're suspecting the opamps, not the transistors?

Another possibility is the Output Level pot P2 has gone open.

I hope not, as it isn't the regular kind of pot 

One thing which i noticed is that when I turn the Output pot, the noise doesn't change at all (it is constant). But when i turn the Output pot anti-clockwise, I can hear the compressed signal at very low volume (when Output pot is at max, i hear only white noise, when Ouptut pot is at min, I hear the same level of white noise + a little bit of the compressed sound).

[DIY Bass]

The one time I was getting nothing but white noise out of a pedal it was a dead capacitor.  Old capacitors can be a bit of a problem.  Do the checks that Rob suggested, as they are easy.  If that doesn't tell you anything then make an audio probe and probe through the circuit.  You will probably hear easily enough where the noise is being generated and that will give some clues about what is likely to be wrong.

[snk]

It was a dead op-amp : oh, how I love factory socketed chips, it's so easy to pull up 
Thank you both for your help, now the compressor works as expected!

[rankot]

There are a lots of schematics of their obsolete products on their web site, but not this one?

What is TFK267BG?

[snk]

Yes, it's a bit obscure indeed. But that's great to put every user manual, sevice manual and schematic on their webpage.
I think it was a mono compressor which got "adapted" to dual mono and housed in a rack (in fact, they just put 2 units into a single 1U rack).

What is TFK267BG?

I don't know 

[11-90-an]

What is TFK267BG?

Seems to be an LED driver... from the schem,

[Rob Strand]

Seems to be an LED driver... from the schem,

Definitely.   Not much info but it's probably in dB's   (possibly similar to uA267).

You could use any LED driver, leave it out, or make a simpler version with a one or more LEDs.

[11-90-an]

(possibly similar to uA267)

it is!  see photo:

[rankot]

They have two compressor schematics on the site, one is for mono compressor, using JFET as a control device, but those schematics are hand drawn and there are few different variations of a design in that file; another one is THAT 4301 based. The one you mention is not on the site.

[Rob Strand]

They have two compressor schematics on the site, one is for mono compressor, using JFET as a control device, but those schematics are hand drawn and there are few different variations of a design in that file; another one is THAT 4301 based. The one you mention is not on the site.

The model in this thread is from 1985.   The schematic link is under the first pic in the first post.

Yes, the two on the D&R website are different: older 1973 to 1982, and newer 1997 (based on THAT device).

[rankot]

Thanks! I've missed that.

Why does it use two parallel JFETs as control devices? Never seen similar thing before.

[Rob Strand]

Why does it use two parallel JFETs as control devices? Never seen similar thing before.

In theory, it halves the JFET resistance and that increases the maximum compression possible from the compressor.