Maestro PS1A Phaser repair

Hey all.

I've got hold of a Maestro PS1-a and it's very cool old phaser.

My small issue is that it's passing sound just fine, and works as it should.. just that their is a low level "Pop Corn" type crackle going on.

Is this the symptom of a dying op-amp or transistor?

How would I go about finding where the fault would be? I do have a 'scope.

Any help would be much appreciated.
Regards
Mike

(Schematics attached)

Some of the first things I do when repairing devices like old effects are
Smell test, did something burn or leak?
Visual test, look at, every solder connection, all the wires for breaks or corrosion,  breaks in the PCB and/or traces
Sometimes I check every resistor and transistor etc with a meter set to ohms.

Then I start to troubleshoot first with a DMM and then if needed check the power supply for ripple and signal trace with a oscilloscope

Often with older effect your problem can be as simple as a broken wire or bad solder connection.

dunno what you mean by 'pop corn type crackle'... Is it dependent on phaser speed?

Axel

Sorry,

I should have elaborated. (Thats what happens when I write a question in the small hours of the morning)

I've inspected the board visually, and it looks quiet good etc.. I cleaned it with a cotton bud and iso-alcohol.

As for the popcorn / crackling sound.. no, it doesn't change with the phaser rate.. it's more like a rumble / crackle... a random noise that consists of low rumble with a bit of random high freq clicks / bursts etc..

As the crackling does not seem effected by the phasing, I guess I can assume it's happening near the output? Maybe I should just replace the 2 x 2n3638A's and the 2x 2N43002's ?

Does anyone know is this sort of sound commonly caused by a dying transistor / op-amp ?

Kind regards
Mike

Before you go randomly changing things, troubleshoot.  Divide up the circuit to find if the problem is in a section or it is the whole circuit related

Turn down the osc trim pot, this removes the drive to the phase shift FETs.  Does the problem stay?

Do you have the problem with the unit powered on and in bypass?  The unit uses FET switching that divides up the circuit some more.

Scope the power supply rails look for ripple and noise.

It can be an opamp or FET BUT it can be other parts.  Crackling can be a bad wiper to trim pot carbon trace connection, it can be a bad switch contact.  Does it crackle more if you tap/lightly hit the unit?  You sometime need to measure the solder connections(connect at the top at say a pin of a IC and then follow the PCB trace and connect at the end of the trace) or use an magnifier to find bad connections.

Gus said:

Do you have the problem with the unit powered on and in bypass?  The unit uses FET switching that divides up the circuit some more.

Click to expand...

I just realised something.. on mine, I can't bypass the signal, even with all switches off, it still reverts to a 'slow' phase.
I really should address this me thinks.. Could it possibly be manifesting the noise?

I'm pretty sure the 'crackle / rumble' is not a physical thing.. tapping it etc doesn't change the noise.

Thanks for helping out a newby.

Kind regards
Mike