MXR Distortion + pedal problem

L

lojo

New member
Joined
Apr 13, 2026
Messages
6
Reaction score
4
Greetings, and thanks for letting me join.
Working on a very old MXR Distortion + stomp switch died & added 9v power supply jack. It works, & looks good on the 'scope - but severe squealing. Could it be one of those little 1nf tantalum caps?
 
Does it squeal when you just use the battery? Or just the power jack? the 741 op-amp may need to be replaced...

Any pics of the guts?
 
VictorB said:
Does it squeal when you just use the battery? Or just the power jack? the 741 op-amp may need to be replaced...

Any pics of the guts?
Click to expand...
Thanks for the interest Victor - I should have been more specific. Pedal works, but screams with battery or power supply. 741 seems fine. 1 nf tantalum hard to find...
 
Guts - took 2 pics but can't upload them - file too large...
Good schematic & more at Electrosmash.com - can't seem to link to it.
 
Last edited:
Reshot the pics with lower resolution...

IMG_1182.JPGIMG_1181.JPG

There we go, yes I know it's not supposed to be a 3PDT switch - but I had it in stock. Could that be my problem?
 
I'm not sure if its the switch, I would clean up the wiring a bit, and re-route these wires:

1776453435750.png
 
Struggling to understand your advice, why reroute wires? MXR must have known what they were doing?
It' not clear where the added 100nf cap goes - on the +9 supply? But I have the same problem running off battery, no filtering required.
What i have done is replace the 1nf cap after the input jack with a 1uf eletrolytic (smallest I had) no change...
 
I just suggested to clean up the wiring as a precaution...

It could be the 3PDT switch.
 
Victor i believe you may be right. The 3pdt switch is on/off, the original Carling spdt is on/on. I have ordered the original & we will see.
 
As best I can tell from the blurry images, looks to be a late 70's model. Looks very similar to my '78 Distortion+

It's tough to discern from the photo's, but it appears you have the brown and blue wires sodlered to the same lug on the footswitch.

If so, that's gonna be your problem.
 
Back
Top