vintage MXR Distortion+ scratcy noise while not moving pots

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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vintagecharlie
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Post by vintagecharlie »

Hello folks,

i purchased a vintage MXR Distortion+, which sounds as it should, but it has a constant noise that is similar to that of a scratchy pot noise, but lower in frequency. It is quite loud and it renders the pedal unusable. Here is a clip of that (sorry for the large file - this is how my camera spits them out) https://we.tl/t-ySgK5Vtwla
No matter how the pots are set, i always get this noise. Also turning down guitar volume does not make it go away. There are no electrolytics in the circuit and i do not see anything obviously wrong with the PCB. Have you had some similar issue with this or similar pedals and what might be the initial best guesses from this description.

Thanks in advance for any input!

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plush
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Post by plush »

Hi.

It's either scratchy pots that require cleaning/replacement
or leaky capacitors (even not-electrolythic ones)

I'd start with cleaning pots and replacing caps

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Intripped
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Post by Intripped »

What Plush said,
and since we're talking of a vintage pedal, look also for cracked solder joints.
Use a magnifier and good strong light, they're difficult to see
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ppluis0
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Post by ppluis0 »

Hi Charlie,

Perhaps the chip is not completely burned but partially damaged due static electricity and this is the origin of the noise.

Try to remove the 741 and solder an 8 pin IC socket observing the pin 1 orientation.

Image

This way you can try several op amps (TL071, TL081, NE5534, LF351, LF441, LM308 or any single op amp) and see what one suits you.

Also you can replace the four ceramic condensers (the yellow disks in the above image) with poliester type and the two tantalum (the blue ones) with a fresh electrolytics of the same value.

In order to upgrade the remaining components, replace all the resistors with metal film type.

Cheers,
Jose

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