RFI Problem

[thomasdj]

Hi,

Anyone who knows how to solve RFI problems?

Just finished my spitfire clone and it sounds awesome!! - But picks up my mobile phone etc..

I googled the issue but can't find anything usefull:s

I don't think I am the only one with this problem...

Any suggestions?

Best regards

Thomas

[xFallen]

Although it could be many things, there are some easy things to check before digging into your new build. It can be any jack, plug, cable or anything else where metal touches metal from the pickup to the first soldered connection inside your amp. I'll leave those internal issues to those more qualified than I although sheilding the input wires can't hurt. I like RG-174 for that.

Dissimilar metals that come in contact with each other can act like diodes. These diodes can rectify all sorts of signals including strong EMI, RF signals such as cell phone transmission (unusual but possible) AM broadcast and so on. The same holds true for any contamination bewteen two metal on metal connections. If the jacks or plugs are dirty or even ill-fitting (loose) this can happen. Marginal soldered joints can do it too, as could a dirty pot.

The last guitar cable I fixed was because the internal connection underneath the shrink wrap on the plug had degraded somehow and was causing this very behavior. Took me a couple of days of messing about to clue into this possibility.

Sometimes simply orienting the amp, cable or guitar (or effect boxen) a certain way can induce or cure the problem too.


Barry

[loudthud]

Most GSM cell phones make quite a racket once an hour or so when they are polled by the network or there is an incomming call or text message. Not much can be done for this except keep the phone some distance from the amp or guitar. If you are playing guitar with a phone clipped to your belt, it's gonna get picked up. While shielding your guitar can help some, it won't get it all. Put the phone on the floor and it shouldn't be a problem.

If you are getting all kinds of radio stations (usually AM or CB), a grid stopper (a 68K resistor in series with the signal) on the first tube will ususally fix it. Your amp chassis should provide adequate shielding unless one side is completely open (as in a Fender black face). Some type of metal foil should be applied to the case to close it up. The philosophy is to block the signal while it is RF because once in the amp, it is converted to audio and can't be effectively removed.

[thomasdj]

Thanks for your reply

My input jack is not isolated from chassis (simply because I stargrounded the amp and it is quiet as a mouse!) maybe this could be a solution??

The amp is not picking up AM radio..only my cell phone! so i think the only solution is to turn the phone off or keep it at the floor

Thanks

Thomas

[loudthud]

My input jack is not isolated from chassis (simply because I stargrounded the amp and it is quiet as a mouse!) maybe this could be a solution??

Huh? NOT isolated? If it is isolated, try connecting a small cap, 100pF ceramic from ground terminal of the jack as close as possible to the chassis.

If you are hearing voices, check with your doc, two or more of your meds could be interacting My phone makes a ca-chucka-ca-chucka sound.

[Enzo]

Local AM radio stations would be a sign of poor RF grounding etc, but if the ONLY problem is the cell phone, keep it away from the amp. If you sat your beer on the grille while you cooked steaks, would you gripe about it hetting warm?

ANd an often overlooked detail. When you build an amp, the chassis has a large open side of course. You should cover that open side elecgtrically when in the cab. You could screw on a cover plate, or do what Fender does - glue aluminum foil up uner the roof of the cab (or wherever) so the bare chassis contacts it when mounted in place. or staple metal wondow screening up there. or staple some thin sheet metal in there.

[TD_Madden]

I like that aluminum a/c ductwork tape.