Problems adding a DC jack to a Dynacomp

[BMF Effects]

I added a DC power jack to a '74 MXR Dyna Comp so I could use a One-Spot wall wart to power it up. When I run it off the battery all is well. When I run it off the One Spot or any wall wart for that matter the pedal picks up RF interference (radio stations). I've researched later Dyna Comp designs (ones with power jacks) and MXR added a 1uF cap between the positive supply and ground. I did this and it did help some. Any suggestions/ideas how to get rid of the RF interference? I do not have the problem with any other pedal I run off the One-Spot nor do I have the problem with newer versions of the Dyna Comp when running off the One-Spot. Thanks for the help.

[snoof]

possibly a small value cap to ground on the input; like 10pf-47pf or the like, after the resistor.  I've seen that done on some other designs.

[Mark Hammer]

A 220pf cap on the audio input (going to ground), and a 10uf or higher between V+ and ground.  Vintage schmintage!  Both of those should have been there in the first place.

[BMF Effects]

Thanks! I'll try it and report back.