Random Crackling Pop Generator - Art MP S. Preamp almost fully restored

[reinw33]

Well if you have noise on the emitter of Q1 then that is your problem.
Or at least the biggest of them.

I would forget everything else for now, until you fix that issue, the rest may follow.

The best is to remove noisy Q1 and see if it goes away.
Thank you NI.

Remove Q1 and scope the voltages on where its collector and base are connected.

Measure particularly the voltage and noise on the 47v Zener, including scoping its ground connection, in case you have some kind of scope reference issue.
You could have problems with the voltage doubler capacitors in this circuit, or indeed the Zener.

It is a 47v zener nominal.
I looked at some random datasheet I couldn't tell weather its a soft or hard one.
My scope is very sensitive it even picks up mains noise hum. Grounding (Decoupling, it says GND) its probes makes it go away. I'm still learning about the scopes capabilitites and shortcomings.
Thank you @NoisyIndividual your replies are much appreciated and a help to me.

 

[reinw33]

...

Measure particularly the voltage and noise on the 47v Zener, including scoping its ground connection, in case you have some kind of scope reference issue.
...

Yes.
But if you mean I don't find ground, that's not the case I assure you.

 

[reinw33]

Q1 is removed and the red LED flashing stopped immediately and permanently it seems.

Scoping BCE pads of Q1 w.r.t. ground.

Base of Q1:
(Note: There's mains static and buzz and hum being picked up on the probes by default. Still learning how to deal with it.)

Collector of Q1:

This pulse above is on top of the Vceo 87V of Q1.
Emitter of Q1:

The zener CR15 measured 47V on DMM. Seems to work fine.
The Q1 collector pad measured 87V to ground on DMM.

I remember the +21V showing +30V on DMM.

 

[NoisyIndividual]

Well there is some ripple at Q1 but not the noise.

I would put Q1 back, and see if noise appears on the 48v rail and/or 33v rail.
Perhaps the noise is from Q1 or Q2.

(lifting one end of R28 would separate them)

 

[reinw33]

Well there is some ripple at Q1 but not the noise.

I would put Q1 back, and see if noise appears on the 48v rail and/or 33v rail.
Perhaps the noise is from Q1 or Q2.

(lifting one end of R28 would separate them)

Yes, will try.

 

[reinw33]

Q1 back in and R28 removed from board

Q1 Base

Q1 Collector

Q1 Emitter




Now Q2

Q2 Base

Q2 Collector

Q2 Emitter

 

[NoisyIndividual]

I believe the first set of photos shows Q1 is noisy.
Though I didn't previously see that removing R28 means the only load on Q1 is the scope.
But it still shouldn't be noisy like that.

 

[reinw33]

I believe the first set of photos shows Q1 is noisy.
Though I didn't previously see that removing R28 means the only load on Q1 is the scope.
But it still shouldn't be noisy like that.

Agree, Q1 seems bad, the noise is back.
The noise of Q1 drives Q2 which supplies U1.
Q1 is 0,55€-1€ a piece + shipping.
Replace and report back to you.
Thanks for the help NoisyIndividual!
Have good and blessed weekend.

PS: Those flappy pcb wings on it are about to fall off. They annoy servicing so much and only get worse with time.
They could have used detachable molex connectors - No, it must be annoying to service as much as possible. -
But, as a cheap used gear to start learning as beginner, it's hard to beat.

 

[NoisyIndividual]

One thing that confused me - earlier you said that turning the gain control up did not affect the output noise level.

Is this still the case?