Fender Blender grabbing AM stations

[gutsofgold]

I got this original 'Blender finally working, however it picks up AM radio LOUD and CLEAR. There are two points on the circuit board where you can hover your finger over or touch components there and the radio gets even louder. The two points are right around the first two gain stages. The most specific point is the input capacitor a 0.1u ceramic cap. I replaced this cap with a newer one of the same value and type and it still acted as a volume boost/antenna for the station coming in. I have a theory that by touching these early points in the circuit, you are feeding the radio station back in and its only getting even more amplified.

I traced the circuit all the way through and it seems like the radio is being picked up right after the first two diodes (D1, D2)... the ones that create the octave. And then following gain stage (Q4) is where the station first gets really audible/loud in my amp's speakers.

I am using a battery and have tried moving to different rooms in the house with no luck. Are the diodes at fault? Or is that their inherent nature? It's a problem enough that the pedal is definitely NOT giggable.

[NPrescott]

A standard practice to reduce radio frequencies in pedals and amps is to use a low-pass filter at the front end. I can't help you with anything specific to the Fender Blender, but this thread was useful to me:
http://www.diystompboxes.com/smfforum/index.php?topic=81738.0

Specifically this:

What guitar-amps historically used was 34K and 100pFd.

The 100pFd is the input of a 12AX7.

Fender 2-jack inputs had two 68K but for either input alone the effective resistance is 34K.

This is a 49KHz low-pass, giving >20dB attenuation in the AM band, yet negligible loss in the audio band.

Higher than 34K or so (maybe 100K) will add hiss even on tube or FET inputs.

Lower than 34K won't give much loss for high-power AM broadcasts, unless you raise the capacitance enough to start loading-down the pickup's treble.

edit: by hovering your fingers around the cap, I think your are altering the capacitance with that of your body's - sort of like a theremin. I'm not certain though, also I think you are finding a stronger signal around points of amplification/gain in the circuit. I don't have any reference in front of me for this though so take it with a giant grain of salt.