Please help trouble-shoot radio frequency interference in my pedal build!

[Steel Curtain]

Hello all...

          I've been working on a Rangemaster type pedal, but with a 2N5088 transistor and the volume pot on the end of the circuit. When it was on the breadboard, there was some very loud radio frequencies (Local FM stuff, nothing interesting). I would only hear this with the guitar plugged in and the volume control on my guitar all the way off. It happened with all my guitars. I also notice that when I remove the 47uf cap that's parallel with the emitter resistor, the interference goes away. So, I went ahead and built the pedal and hoped that it wouldn't be there when I clicked it on, but it is. I'm still getting the radio interference just the same..

        I tried putting a 100pf cap from input to ground to shunt those frequencies as I read after doing a search here, but the RF is still there.

And I've only run this pedal off a 9V battery so far. Both on the breadboard and in the pedal.

Any ideas guys? Any and all help is greatly appreciated!

-Rob G-

[gritz]

FM? That's interesting (although not fun for you).

Ummm... the Rangemaster originally used a germanium transistor, so dropping a silicon one straight in the circuit might not give optimum biassing and may have turned the circuit into a RF demodulator.. Can you post the schematic that you used and the voltages you're getting on the transistor's emitter, collector and base relative to ground?

Disconnecting the emitter cap will reduce the RF interference because it reduces the overall gain of the circuit, but it's still there. Don't worry - I've built stuff hoping that problems in the breadboard stage will fix themselves when it all goes in a "proper" box and have got bitten too!

[Steel Curtain]

I guess it's more like a modded LPB now that I'm looking at it. The rangemaster gave me this idea with the emitter cap.

I don't have the voltages as I'm not so equipped yet, but it's extremely high gain and loud! I was originally breadboarding it on my digital modeling amp and I fried the input. Amp is garbage now.

So yeah, it's loud and very high gain..


Oh, the input cap is .1uf as well. Forgot to put that in the schematic there...

[Steel Curtain]

And I'm not sure if it is FM or AM or whatever... Mostly talking. 

[ch1naski]

Have you shielded the input/output wires?

[Steel Curtain]

I wired everything up with plain old 24 gauge braided hookup wire I got from Smallbear. So, I guess not..

Is this some sort of noob mistake I made?

But if I disconnect the guitar, the RF stops. Would that happen if the problem was internal shielding?

I'm currently researching ferrite beads as a possible fix. I put small value caps and resistors all over the breadboard trying to fix this and nothing's worked so far. ( I currently have this circuit in a pedal and on the breadboard.) I once opened up my EX LPB reissue and found ferrite beads in there. Maybe this is why.

All suggestions are welcome!

But no, I didn't use any shielded wire in there..

[pakrat]

Someone else recently posted on this topic with a possible cure: http://www.diystompboxes.com/smfforum/index.php?topic=103504.0

[Steel Curtain]

Thanks Pakrat..

          Read that one. The fix suggested in that thread didn't make any difference at all in my circuit.

[R.G.]

Put a 0.01uF ceramic cap from +9 to ground as close to the collector and emitter resistors as you can.

Put a 47 ohm in series between the bypass switching and the input 1M. Put a 47pF in parallel with the 1M input pulldown resistor.

Put it in a metal case, and ensure that the ground/sleeve lug of the jack is connected to signal ground on the circuit board.

[Steel Curtain]

       "Put a 0.01uF ceramic cap from +9 to ground as close to the collector and emitter resistors as you can."

I don't have a ceramic cap that big. I tried a film box .01uf just now and it didn't help.

       "Put a 47 ohm in series between the bypass switching and the input 1M. Put a 47pF in parallel with the 1M input pulldown resistor."

I put the 47pf parallel to the 1M. It lowered the RF volume a little, but barely. But it did have some small effect. I tried a 270pf as well. It had had slightly more effect but again, very little. Plus, it killed the guitar tone.. I did all this to the breadboarded circuit.

       "Put it in a metal case, and ensure that the ground/sleeve lug of the jack is connected to signal ground on the circuit board."

I triple+ checked all grounding in the pedal and on the breadboard. they both appear to be correct and having exactly the same RF problem.


Thanks for the suggestions R.G. (Those are my initials as well, R.G.)

When I plugged it in this time, I'm pretty sure I heard a Japanese radio station! And then some Bill Withers. Cheared me up a little.  I'm starting to doubt if I can get the noise out of this pedal while keeping the circuitry intact though.

[mistahead]

Can you post a photo (housed, semi housed, whatever)?

Its for my own edification and might not help at all...

[R.G.]

I don't have a ceramic cap that big. I tried a film box .01uf just now and it didn't help.

It needs to be ceramic, because ceramic in general has lower parasitic inductance and capacitance.

       "Put a 47 ohm in series between the bypass switching and the input 1M. Put a 47pF in parallel with the 1M input pulldown resistor."
I put the 47pf parallel to the 1M. It lowered the RF volume a little, but barely. But it did have some small effect. I tried a 270pf as well. It had had slightly more effect but again, very little. Plus, it killed the guitar tone.. I did all this to the breadboarded circuit.

The series resistance is critical. The incoming cords - including the shield braid! - can and do act like antennas at RF, and are mildly inductive. A capacitor to ground may tune in radio stations, making them no better, or sometimes stronger. The resistance spoils the Q of the antenna/capacitor resonance. For really bad situations, raising that resistor to as much as 1K can be helpful. Again, you want ceramic.