Studer 962 preamp anomaly

[AudioIngenia]

Hi everyone. I am restoring a Studer 962 console. I have often encountered the same anomaly in other channels. Jfet's network does not allow to pass the microphone signal. Sometimes, if I touch the legs of the transistors, it opens and the signal flows .....
I have repaired many other channels with similar anomaly. But I have replaced many components. The point is that I don't know exactly which component is to blame, or maybe there are several responsible

 

[Bo Deadly]

Could just be a dirty board. When the gates are held high, there's nothing but the reverse leakage of the diode to hold the JFET open and let signal through. Try just washing the board top and bottom around those JFETs and diodes. Use isopropyl and a toothbrush and for the top around components maybe a lens wipe gripped by long narrow tweezers to get around the legs of parts.

 

[AudioIngenia]

First impression: Clean? But does it already look clean?
Well, I'm going to clean with isopropyl alcohol.
I do test ..... Wow, now there is a tone in all positions ....
Maybe remnants of alcohol.
I apply pressurized air with a gun, to dry ....
Surprise !!! All microphone inputs work correctly .....
But I feel worried. Too sensitive to problems ....
Will the problem return?
Anyway, thank you very much for the advice ...

 

[Bo Deadly]

Yeah, definitely make sure it's super dry before you power it up!

How long it lasts probably depends on how well you clean it. It's very easy to think you're cleaning it and you're actually just smearing the goo around. The best way might be to use a squirt bottle with 99% isopropyl and squirt a little on, agitate with a clean toothbrush but then squirt a lot over that to flush out the residue. If you clean it really well, it should last a long time.

 

[gyraf]

Nice trick, I didn't think of this possibility of failure..

Thanks!

/Jakob E.

 

[AudioIngenia]

I have another channel strip with the same problem. Only one position of the microphone selector works. I have replaced all the Jfet and diodes in the area. Now 3 positions work, there is one that does not work, only if I touch it with the finger or the tip of the multimeter.
Always the same, I suspect that something makes very sensitive to that area .... A problem too common in this preamp ....

 

[Bo Deadly]

I'm not sure I understand. So did cleaning it fix it or not?

Did you clean the one that you changed parts on? Flux conducts. Not much but that area is going to be 10M+ ohms impedance. Even a tiny spec if left over flux will provide a path for such a tiny current.

 

[AudioIngenia]

I mean another channel.
I have cleaned several times with isopropyl alcohol and Deoxit, but it continues to fail.
But then I cleaned with distilled water, applied air pressure and dried in a hot air heater. And magic, everything works now correctly.
Once again, thank  for putting me in the way of the solution

 

[Bo Deadly]

... then I cleaned with distilled water, applied air pressure and dried in a hot air heater. And magic, everything works now correctly.

I would not recommend using any of those 3 methods minus a gentle "air duster". It sounds like you just didn't wait long enough for it to dry.

 

[AudioIngenia]

Hello everyone again. I hope you all are well.
I still find these problems in Studer preamps with J112 fet switches.
I'm testing grounding the Drain pin with 10M resistors. It seems to fix the thing. What do you think?

 

[zamproject]

Hello

Hello everyone again. I hope you all are well.
I still find these problems in Studer preamps with J112 fet switches.
I'm testing grounding the Drain pin with 10M resistors. It seems to fix the thing. What do you think?

Input switching use remote switch via logic IC
If fet are switching with "manual" actuations, I'll have a look at rotary selector contact and CD40xx IC around (5-6-7) as signal potentially coming from outside remote (mute, remix, etc...) which can select input depending of global status.

Best
Zam

 

[AudioIngenia]

Hello

Input switching use remote switch via logic IC
If fet are switching with "manual" actuations, I'll have a look at rotary selector contact and CD40xx IC around (5-6-7) as signal potentially coming from outside remote (mute, remix, etc...) which can select input depending of global status.

Best
Zam

I think those logic ICs shouldn't affect the J112 switching system.
Maybe they only act on the signaling options?

 

[AudioIngenia]

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