Studer 961 Mono Input Distortion

[timtom]

I'm repairing/recapping a Studer 961 Mixing console. 7 out of 8 Channel are working fine. One Channel is distorting.
I think I narrowed down the source of the distortion.
When I follow the signal path the distortion shows up the output of IC 9. When I check the input pins 2/3 the signal is distortion free.

When I use the insert return the signal is clean as well.

I assume the fault is between IC 9 and and the insert return.

IC 9 also gets hot.

I switched IC 9 and 10 still distortion. I removed C55, then the output of IC 9 is clean.

Any Ideas?

 

[zamproject]

Hi

No clear idea yet, maybe check that there is no mess with DO -option 2- setup ?
Like if -wire 12- feedback/loop signal from fader buffer output or the 33Ω in circuit, wile you have a standard setup with sym I/O insert ?

 

[timtom]

Hi

No clear idea yet, maybe check that there is no mess with DO -option 2- setup ?
Like if -wire 12- feedback/loop signal from fader buffer output or the 33Ω in circuit, wile you have a standard setup with sym I/O insert ?

There everything looks how it should.

 

[Walrus]

What does the output of IC9 look like if you plug an unwired balanced jack into the Insert In jack?

[Edit] Sorry said Out but meant In.

 

[timtom]

What does the output of IC9 look like if you plug an unwired balanced jack into the Insert In jack?

[Edit] Sorry said Out but meant In.

It is still disorting.

When I take the output of IC9, disconnect C55 and connect it to the Insert In I get a clean signal. It’s phase reversed and ~4db lower but clean.

Taking out IC 10 it still passes a very low and distorted signal to the fader.

 

[Walrus]

For clarity, is Link W12 and resistor R208 fitted , or are they both open circuit?

 

[zamproject]

When I take the output of IC9, disconnect C55 and connect it to the Insert In I get a clean signal. It’s phase reversed and ~4db lower but clean.

Seem normal as unbal insert mod need gain change (R136/137) and polarity flip at IC11

Can you check or replace trim R39 (200Ω) ? I'm not that sure, but faulty trim may impact IC9 load, as IC10 cross-coupled behaviour ?

Make sure bantam plugs are clean, and try an external wire O/I loop to bypass the normalling, bad contact ca mess with the cross-coupled

Also you may try the -option 2- mod, to see what happen then for IC12 (fader buffer out) loaded with IC 10 ?
(no need to changes R136/137 for this test)

No more ideas yet...

Cheers
Zam

 

[timtom]

For clarity, is Link W12 and resistor R208 fitted , or are they both open circuit?

They are open circuit.

When turning the R39 it starts to oscillate like crazy.

 

[Walrus]

Sounds like R39 is dirty and goes open circuit when moved, this mean the op-amp has no feedback so goes maximum gain!
If IC9 is getting hot, then it is either oscillating at very high frequency, or it has to low a load. I suspect something happened to the tracks when C55 was changed....

 

[timtom]

Also you may try the -option 2- mod, to see what happen then for IC12 (fader buffer out) loaded with IC 10 ?
(no need to changes R136/137 for this test)

Distortion at the Direct Out and the Fader.

Leaving W12 out and R208 in sound is clear with the level loss.

I suspect something happened to the tracks when C55 was changed....

Most likely, I will try to use some wire to check.

 

[zamproject]

Distortion at the Direct Out and the Fader.

fine... this narrow the issue around the IC10 cross coupled circuit ?

You said you recap the desk,
-double check C63 and C64
-clean and check all connection around IC10 and the recapped lythic, solder blob lost / trace short
-change R39 which is suspect so far (use 2x100Ω if you don't have trim yet)
- I hade issue in the past with double sided pcb from 80' Studer equipment, it's easy to mess trace/pad and TH plating when desoldering, so check top and bottom trace continuity around C63/C64 pads

 

[timtom]

Good cleaning around IC 10 and bridging some traces fixed the problem!

Thanks for the support.

 

[zamproject]

What was the faulty trace ?

Cheers
Zam

 

[timtom]

I don't know exactly as I did several things before testing. I bridged C64 to R123 and I bridged both sides of C 55.
I also had to bridge W17 as I did cut the trace.

Another possibility could be the bantam sockets as I heavily used them during testing.
The console was in bad cosmetic condition. Lots of breadcrumbs, hair, dust and dirt.