MXR Distortion + pedal problem

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lojo

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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...
 
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