intermittent radio interference... argh

[runmikeyrun]

Hey guys,

I built an MXR Microamp for our guitarist.  Tested thoroughly in the basement, worked fine.  Took it to practice and plugged it into his amp, and got a radio station coming through his amp.  It's in a typical 1590 box so i don't think shielding is an issue.  He doesn't usually use any other pedals and has never had the problem with his amp before.  I used to have a similar problem doing builds at my old house, if a project wasn't in a box i picked up an AM station, but the station's 50,000 w transmitter was only 2 miles away.  Foolishly i didn't try the pedal on my amp because i was rushing out the door.  I'm just wondering what possible issues could cause this- bad cable?  wiring inside the pedal?  Can ICs pick up RF?  It only did it when the pedal was active, when bypassed it was fine.  I do know there's a small low power high school radio station in the next town over... do you think this could be the problem?

[R.G.]

I built an MXR Microamp for our guitarist.  Tested thoroughly in the basement, worked fine.  Took it to practice and plugged it into his amp, and got a radio station coming through his amp.  It's in a typical 1590 box so i don't think shielding is an issue.  He doesn't usually use any other pedals and has never had the problem with his amp before.  I used to have a similar problem doing builds at my old house, if a project wasn't in a box i picked up an AM station, but the station's 50,000 w transmitter was only 2 miles away.  Foolishly i didn't try the pedal on my amp because i was rushing out the door.  I'm just wondering what possible issues could cause this- bad cable?  wiring inside the pedal?  Can ICs pick up RF?  It only did it when the pedal was active, when bypassed it was fine.  I do know there's a small low power high school radio station in the next town over... do you think this could be the problem?

Classic story. The cables pick up RF acting like antennas. They channel it into the first stage of the circuit, where the nonlinearities of the semiconductor junctions make demodulated audio out of it.

Try a 47-100pF *ceramic* cap from signal to sleeve on the input jack and a 1K in series with the signal going into the circuit.

[John Lyons]

It won't make that much difference but if you put the pf cap on the jack you're
going to affecting the bypassed signal as well. Better to put the series resistor and cap on the
circuit board input or switch lug.

[electrosonic]

On my friends pedal board he has an MXR distortion+ and a Sparkledrive separated by a couple of other pedals. If they are both on you can hear a radio station, but they are fine by themselves. These are not clones, so he would rather not jump straight into modding them. How about a custom patch cable with the resistor and cap built in? It might be a handy thing to throw in the gig bag

Andrew