Strange problem with Compex F 760 compressor

[samgraysound]

Hey,

I've got a compressor on my bench with a problem I've never seen before.

It's a Compex F 760 compressor. The owner says that with certain settings and input and outputs it will show gain reduction with no signal and show signal on the mixer but no sound (he's thinking super-sonic oscillation).

It does not do this when there is nothing plugged but will if:

unterminated xlrs --> inputs, outputs --> unterminated xlr

unterminated xlrs --> inputs, outputs --> studer mixer

certain outboard gear (but not others) --> inputs, outputs --> studer mixer.

The stereo input and output gains must be pretty high as well. He also reports that it is worse when run through the patchbay then when the unit is out of the rack and plugged in directly.

I'm a bit bewildered. Seems like it could be sort of grounding issue? What do I even look for?

Sam

 

[abbey road d enfer]

Have you checked inside with an oscilloscope?
Most of the stages in ADR gear have MHz bandwidth.

 

[David Kulka]

Look out for bad rotary and toggle switches on Compex stuff. They are often faulty and can cause all sorts of strange problems. It's best to check each of the possible "on" positions with an ohm meter set to a low range, perhaps while wiggling or switching the control between positions to catch intermittent problems.

 

[samgraysound]

Have you checked inside with an oscilloscope?
Most of the stages in ADR gear have MHz bandwidth.

With a scope there is a high frequency oscillation all over the place, including on the positive terminal of the power supply filter cap (pre-regulator) which I thought was odd.

 

[samgraysound]

Look out for bad rotary and toggle switches on Compex stuff. They are often faulty and can cause all sorts of strange problems. It's best to check each of the possible "on" positions with an ohm meter set to a low range, perhaps while wiggling or switching the control between positions to catch intermittent problems.

I can cause the problem on either channel. I cleaned the controls thoroughly on one side and it still did it. Do you think it is worth doing the ohms test on the switches still?

Sam

 

[abbey road d enfer]

With a scope there is a high frequency oscillation all over the place, including on the positive terminal of the power supply filter cap (pre-regulator) which I thought was odd.

Where do you connect the ground clip?

 

[David Kulka]

I can cause the problem on either channel. I cleaned the controls thoroughly on one side and it still did it. Do you think it is worth doing the ohms test on the switches still?

Sam

In that case, maybe not.

Could it possibly be a cabling issue? Most of those units were built pin 3 hot unbalanced, which can cause confusions. If transformers have been added, maybe something was mis-wired?

If you're still seeing HF oscillation all over the place, is it still there when you power down? If so, it's a measurement problem. If not, try to at least determine whether it's originating closer to the input, the output, or the power supply.

 

[radardoug]

I seem to remember that the ground in these units floats, because it was unbalanced. so you need to identify the internal ground and make sure it is grounded.

 

[samgraysound]

Where do you connect the ground clip?

Chassis

 

[samgraysound]

In that case, maybe not.

Could it possibly be a cabling issue? Most of those units were built pin 3 hot unbalanced, which can cause confusions. If transformers have been added, maybe something was mis-wired?

If you're still seeing HF oscillation all over the place, is it still there when you power down? If so, it's a measurement problem. If not, try to at least determine whether it's originating closer to the input, the output, or the power supply.

No transformers on input or output. Oscillation is not there when powered down, I can cause it to occur and disappear by bringing up the gain. I'll try to isolate it better.

 

[samgraysound]

I seem to remember that the ground in these units floats, because it was unbalanced. so you need to identify the internal ground and make sure it is grounded.

thanks I will look into this.

 

[abbey road d enfer]

Chassis

You should connect it to the audio ground.

 

[RuudNL]

Could it be that the input 'sees' the output?
Maybe some form of capacitive coupling, wires close together in the patch field?
I have had such a situation before.

 

[mutterd]

Agree that this sounds, at least partially, like an interconnection, and thusly a measurement issue.

I have 2 of these and both have the chassis referenced to earth/safety ground, with pin 1’s tied directly to chassis, but the chassis not referenced to power supply 0V.

Units came from different places,
bought at different times, one came with transformers, the other didn’t
but the grounding scheme was the
same in both.

These units are single ended in/out so that needs to be taken into account - so power supply 0V is audio (-).

I ended up putting transformers in the unit that didn’t come with them, and putting binding posts on the back of both my units with a wire referencing the power supply 0V, which has come in handy once or twice.

Good luck! These are worth the work
T.

 

[Rob Flinn]

Agree with radar that the ground floats in some of these A&D units, since I have a very early F760RS that is like this.  I also seem to remember them being pin 3 hot as well.  Maybe my memory fails me but worth checking especially since you don't have the balancing transformers fitted.

 

[samgraysound]

Agree that this sounds, at least partially, like an interconnection, and thusly a measurement issue.

I have 2 of these and both have the chassis referenced to earth/safety ground, with pin 1’s tied directly to chassis, but the chassis not referenced to power supply 0V.

Units came from different places,
bought at different times, one came with transformers, the other didn’t
but the grounding scheme was the
same in both.

These units are single ended in/out so that needs to be taken into account - so power supply 0V is audio (-).

I ended up putting transformers in the unit that didn’t come with them, and putting binding posts on the back of both my units with a wire referencing the power supply 0V, which has come in handy once or twice.

Good luck! These are worth the work
T.

Huh. So as soon as hooked the scope ground to the power supply ground, the oscillation went away. Same if I hook the audio ground to chassis ground at the jacks. I thought I had already checked this but oh well.

Looks like this has been converted to pin 2 hot btw.

So I guess I'll just hardwire that. Or put a switch in for it. Or put some screw terminals on the back for it? What seems best to y'all.

Sam

 

[mutterd]

I brought PSU 0V/Audio (-) out to a 5 way binding post on the chassis and that has worked for me.

Transformers would be a good idea in this instance.

Glad it’s looking up!
T.

 

[mutterd]

From binding post to leg of PSU filter cap:

 

[abbey road d enfer]

From binding post to leg of PSU filter cap:

I would suggest you move that to the 0v output of the regulated voltage, which is much cleaner.