Studer 169 preamp anomaly

[AudioIngenia]

There is an anomaly here that I am trying to solve.
Switch in Line position
When the Gain is minimum, a white noise appears that accompanies the signal. As soon as I turn the Gain switch or fine adjustment a little, the noise disappears.
Any suggestion?

 

[Bo Deadly]

My guess is a leaky electrolytic capacitor like C14 in particular. When you move the controls it causes a brief shift in DC but after a while it settles and goes back to leaky mode. But this is a wild guess. Are the electrolytic still the old Frakos? If yes, you are long overdue to recap anyway.

 

[AudioIngenia]

Sorry, I forgot to say that all electrolytic and tantalum capacitors and have been replaced

 

[Bo Deadly]

Then double check electrolyitcs polarity. If something went in backwards (it happens to the best of us) that could be it.

But it could be a lot of things.

Look carefully for bad solder joints. Maybe just touch them with the iron and a little flux.

Is the board clean or is there flux residue?

Could be a faulty pot. Try jumping wire pins 112 to 102 or pot terminals.

Could be corroded switch contacts. Try jumping T1 pin 2 to the downstream side of L2. Try jumping S1B common to wire pin 112 or pot terminal.

Describe more accurately how the noise develops. You say it goes away when you move the controls, but then how does it start? Does the noise occur suddenly or does it increase slowly over time? Yes, it matters.

Does it occur only in a specific switch position? So it does not occur at all in the second position?

 

[AudioIngenia]

I'll go over all the solder points.
The noise appears when the switch and the fine adjustment pot are in the minimum gain position, the first zone of 1/4 turn, when I move the potentiometer and increase a few dB the white noise disappears instantly.
They do not occur in other positions of the rotary switch.

 

[Bo Deadly]

Sounds like a dirty pot or bad solder joint or corroded switch contact. Have you tried a little deoxit in the switch?

Does it only occur in the presence of signal or does it also occur with equal intensity with silence? If it only occurs with signal that would suggest that the higher level signal is breaking through the oxidation of a bad solder joint or corroded switch contact whereas the low signal level doesn't and the high impedance contact reduces feedback in the amp causing gain to go way up along with noise.

Try jumping things like I said. Particularly S1B and the pot.

 

[AudioIngenia]

Solved !!
I have lift some components for measurement. I have disconnected cables 102 and 112 and reconnected. I have reviewed welding points. I have cleaned everything with Deoxit and a brush.
Now everything works properly
Thanks squarewave

 

[Bo Deadly]

Good times.