Marshall Reverb 75 (5275) - Hum issues

[decap]

What rear board? Rear jacks are only wired together, no pcb.

They were out and cleaned. Did not resoldered them yet.

 

[turk 182]

it's hard to troubleshoot something when you can't look at it...the pic's i've found (likely a different production version) show the di level control and jack, the fx loop jacks and headphone(i think) jacks all on a board...

 

[decap]

Sorry.

Uploading more photos.

 

[turk 182]

thanks muchly....


what's your take on the input jack and surrounding area, all good? if you can light that area and snap a pic i would appreciate it thanks! reason being that second control is Volume and the first is gain correct? i could be wrong but i think that replacement control may be faulty...or potentially miswired?

 

[decap]

The big push pull control (basically pot + simple on off switch) is GAIN. The wired small control (old fix from one of previous owners) is volume. I have measured the volume potentiometer and the value and type seems to be ok. It also works ok.

 

[turk 182]

but turning it all the way off (with no plug in the input) doesn't make the noise go away, correct?

 

[decap]

Nope, hum/buzz is not affected by the boost channel volume.

Maybe the prolongation wirec can catch some interference, I can try to unsolder it tomorrow.

 

[turk 182]

got clip leads?
if so put one on the middle( wiper ) of VR2 to chassis ground what happens?

 

[decap]

Yep.

No change if I connect the middle pin to the chassis.

 

[turk 182]

arrrgh...now i likely as frustrated as you are...

 

[turk 182]

what about pin 5 of IC4?

 

[decap]

I am tinkering with it actively more than 3 weeks. Lot of free time during COVID. :-/

 

[decap]

Pin 5? ... that is reverb signal path according to schematics. What should I do with it?

 

[turk 182]

ooops typo IC3 (i'm switching between two tabs and my memory is short due to life long consumption of scotch...)

 

[decap]

So pin 5 on IC3 ... what should I measure there?

 

[turk 182]

no measurement just looking to see if the noise goes away.


to be clear short pin5 to ground.

 

[decap]

Connecting it to chassis = shorting signal path?

 

[turk 182]

and?

 

[decap]

No change. Buzz is the same. That means the the issue is after IC3?

 

[turk 182]

yes.