Picking up FM radio on Randall RGT100

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I have a 1989 Randall RGT100 head that is picking up two FM radio stations. I can hear both stations coming through at the same time. One is a rock station at 98.5 and the other is a country station higher in frequency at 103 or 105. This happens with no guitar or guitar cable plugged in. Adjusting amplifier volume does not change the radio interference volume.

I've tried several things to check this problem:
- Multiple cables connecting head to cabinet
- Plugging into outlets in other rooms
- Plugged amp into power conditioner (Panamax M4300-PM)
- Checked outlets with outlet checker and they check out okay
- Disassembled amp and checked power cord connections which seem fine

I can change the radio reception by moving the power cord around. If I grab on to the cord in the back of the amp near where it connects, the rock station almost goes away but I can hear the country station better. I have even gone so far as to try shielding the power cord with grounded tin foil since it seems to be acting as an antenna. I recently had the amp in the shop for another issue and the technician said he couldn't reproduce the radio interference problem. I haven't been able to take the amp to someone elses house and try it there.

I'm at a loss. Seems like dirty power. This is a new house and I've had the problem since living here. The outlet checker says the outlets are grounded and wired properly. Seems that the power conditioner would clean up any RFI. I'd like to try a better power conditioner, but I don't want to spend a lot of money if it's not going to fix the problem.

Does anyone have any ideas?

Thanks, Steve
 
Usually it's an AM station that amps pick up. The most common, but hard to find cause is a cold solder joint. Without getting too technical, a cold solder joint acts a semi-conductor junction, and rectifies an AC signal. (Not talking about your ac line cord.) In radio speak, a detector. It produces a audio signal from rf (the radio station) and amplifies it along with the guitar. A quick check to isolate were in the amp it's coming from,
If you have separate gain, drive and master controls, first, turn the gain, or drive down, and the master up and see if you still get it. Then try the gain. Also, try the individual tone controls. It's odd that it's an FM (frequency modulation) station since simple rectification won't demodulate (turn into audio) an FM signal. You can install bypass caps on both sides of the AC line cord, have your tech do it, which are usually .005mfd @ 600V ceramic disk, which will pass RF to ground and stop your line cord from being an antenna.
 
Thanks for the quick reply kc2eeb.

I tried the master and gain controls on both channels and this has no effect on the radio interference. Also tried the tone, presence, and reverb controls with no effect. The RFI volume never changes except when I move the power cord causing a change in reception (or possibly a change in grounding?). So I guess it's safe to say the RFI is not going through the amplification circuitry.

Thanks for the advice on installing caps, but wouldn't the power conditioner be similar in that it's removing the RFI from the line before getting to the amp? Although, if the RFI is somehow being generated in the power cord, that's after the power conditioner. If that's the case, it seems that the shielding I tried on the cord would have done something, but it had no effect. Granted my tin foil shielding probably wasn't real technically sound.

Does anyone think installing a shielded power cable would help?

Could this be a grounding issue even though the power cord connections and chassis ground checked okay with a meter?

Thanks, Steve
 
No, it's not a grounding issue, it is an RFI issue.
First, you notice the "reception" changes when you move the "antenna."
Definitely, an RFI problem.
Second, when you wrapped the cord in foil you made a basic magnetic field antenna, which responds to the magnetic component of an RF signal.
Third, a bad ground connection usually produces a multiple of 60 Hz, not RF.
Fourth,
Since grabbing the cord where it enters the amp cuts down the "reception"
go to Radio Shack and get snap on magnetic toroids they sell to eliminate computer noise. Wrap as many turns of the ac cable as you can, through and around, and as close to the amp as possible.
You are creating an rf choke.
It should at least lower the interference.
Fifth,
See if you have any cordless devices, remotes, electronic air cleaners, wireless modems, cordless alarms or doorbells, etc. in the area.
Hope this helps! Pat
 
Thanks for the advice Pat.

I tried the RF choke from Radio Shack. This one:
http://www.radioshack.com/product/index.jsp?productId=2103222

It had no effect at all.

There are no cordless/wireless devices in the area.

I've also noticed that it's not just moving the power cable that changes the FM reception. I can also touch the ground points of the amp and weaken the signal. So if I touch the speaker cable jack or the front inputs, that has an effect on the FM signal.

Any other ideas?

Thanks, Steve
 
Update...

I have found that by removing the reverb unit, the FM reception goes away. If I grab on to the reverb output cable, the rock radio station at 98.5 FM comes in very clear.

The easy workaround here is to just leave the reverb disconnected and remove the reverb output cable (or route it in such a way that no RF is encountered). If I want reverb, I could use an external effects unit. I would, however, like to use the amplifiers built in reverb.

I spoke to a tech here that said the amp is working as it should and that I'm just in an unfortunate spot that has strong reception for that radio station. He said that a ferrite bead choke on the reverb output cable might help.

Wondering if anyone else has any ideas?

Thanks, Steve
 
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