weird RF noise

[Ewo]

This one's got me scratching my bald head.

I've got a Lakland Skyline 55-01 with Nordy Big Singles and an Aggie OBP-3 pre in it. I love the instrument, but it gets a pretty irritating level of RF noise under some conditions.

In active mode, the RF is there even with volume pot all the way down. Passive there's no RF even at full volume.

There's one particular spot on the mid pot at which there's no RF at all--about halfway turned in the boost direction. The RF comes back at full boost, full cut, and flat.

Any suggestions? Other diagnostic things I can check?

[bassbenj]

Originally Posted by Ewo

This one's got me scratching my bald head.

I've got a Lakland Skyline 55-01 with Nordy Big Singles and an Aggie OBP-3 pre in it. I love the instrument, but it gets a pretty irritating level of RF noise under some conditions.

In active mode, the RF is there even with volume pot all the way down. Passive there's no RF even at full volume.

There's one particular spot on the mid pot at which there's no RF at all--about halfway turned in the boost direction. The RF comes back at full boost, full cut, and flat.

Any suggestions? Other diagnostic things I can check?

Are you sure it's RF? I mean like picking up radio stations etc.?

Assuming it is, there can be a couple of things. One would be to simply try to filter it out before it gets into the bass. What happens is that RF if strong can get rectified at op amp inputs and then that acts as a receiver. Usual fix involves experimenting with 50 or 100 pf capacitors on the op amp inputs or output jack or for that matter anyplace at all. Usually fixing RFI problems involves more than just a little bit of black magic!

The Tone pot thing is suggestive of parasitic oscillations in the preamp. My Alembic came with those. The circuit oscillates in the RF range for certain knob settings. You can't hear it but it acts as a local oscillator in a receiver and the circuit then picks up RFI. The fix in my case was new electronics from Alembic.

Good Luck!

[Ewo]

Thanks for the reply, good buddy! Appreciate the suggestions.

Yeah--it's RF. I can hear (faintly) a local music station. The tunes are nice, but I don't want 'em coming out of my bass...

I know about 60 cycle hum, and it ain't that.

Wondering if a complete copper foil shielding might help...The control cavity's got what looks like conductive paint, but copper works better, no?

[FunkMetalBass]

Not really. And from what I understand, that usually only helps with a 60-cycle issue.

I'm thinking the issue is either in your pickups or the amp, and my vote goes to the latter.

Have you tried your bass in a different environment and through a different amp?

[Ewo]

Again, thanks for the suggestions!

Well, the problem is unique to this bass, with the same amp in same place. My other basses don't have it, with everything else in the rig held constant (even time of day).

Copper foil's no good for RF? Dang...

[no_fret]

I'd double check the wiring since the preamp and pickups have been changed. Something may be wired backwards.

[lem8r]

May be wrong shielding/ground loop.Try different cable with one unshielded jack. (Like Planet Waves, etc..)

[zenrad]

Could be your cable - easy thing to change - try a different length cable also. Your house wiring+amp wiring+bass wiring+cable are combining in some way and acting as an (I'll guess AM radio) antenna.

RFI has all kinds of differnt sounds depending on its origin. As a Ham radio operator I have problems with local power line noise and I'm very familiar with differnt types of RFI, but what you're describing is something that's acting as an antenna.

[mongo2]

Originally Posted by Ewo

Copper foil's no good for RF? Dang...

A Farady cage made of copper will block RF.

[mech]

Mongo is correct. Copper shielding works for RFI but does basically nothing for electromagnetic interference (transformer hum). Copper or aluminum foil is better than conductive paint.

Make sure there is a GOOD ground from the output jack to the preamp and the shielding material. Make sure the shielding material IS conductive (check with ohmeter). Also make sure all the bodies of the pots/switches are grounded. They can be grounded to the shielding material. Any part of them that is outside the shielded control pocket and not grounded are antennas and will pick up and inject electrostatic noise (rf) into the preamp.

Check that the hot side of the pickups are going to the input of the pre and not the ground.

mech

If it doesn't pick up the rf in passive mode then the problem is not likely to be with the pickups themselves, cable or amp although I would try a different cable just for grins.

[zenrad]

If the OP is hearing a radio station he has an antenna. This is different than 60Hz hum or a lifted ground. A guitar cable, the power cord or the house wiring can all make a good antenna, alone or in tandem with a device's internal wiring. This includes the wires and circuitry inside the bass, pickups too. The combined length of wires may be resonant at a particular frequency. The problem is that the AM radio station could be recieved from the cable or the power cord - both also two very good antennas. Trying a longer or shorter power cord with the amp and that particular bass could also help. It's also very possible an issue with the amp's shielding. One at a time, try different length of cable then power cord to hunt down the wire that's acting as an antenna.

FWIW, I regularly receive and talk to stations from Europe, and South Africa to Australia from my NJ home using a (very long) wire hanging 45ft off the ground transmitting with no more power than the average light bulb (100 watts). AM radio stations are VERY easy to receive, I can use a single wire just 2 feet long in my basement and receive AM stations. The single wire is called a random wire antenna.

Ewo, if it's not just radio you're hearing and you're hearing other RFI you can try ferrite beads on the cable or power cord. I have to use these in my house to keep the RFI off of my PC speakers when I transmit.

Here's a good starting point for RFI info
http://www.arrl.org/news/rfi/2001/1214.html

If you want to hear what differnt types of RFI sound like in the HF bands
http://www.arrl.org/tis/info/HTML/rfi-noise/

[Ewo]

Many thanks for the info, y'all.

Worked late yesterday, but did try a couple different instrument cables when I got home. A short cable gives the least RF. With a 20 footer, the RF is position-sensitive.

Yeah, the antenna hypothesis seems supported.

Please advise: it's still worth it to line the control cavity with copper foil, right?

[251]

Originally Posted by Ewo

Many thanks for the info, y'all.

Worked late yesterday, but did try a couple different instrument cables when I got home. A short cable gives the least RF. With a 20 footer, the RF is position-sensitive.

Yeah, the antenna hypothesis seems supported.

Please advise: it's still worth it to line the control cavity with copper foil, right?

Definitely, line the cavity. Remount the pots over the copper so they make the ground connection. Remember to check the liner to ground circuit from each piece of copper you use.

It sounds like your circuit is a tunable antenna. I hope you didn't pay extra for that. 8-)

[zenrad]

Originally Posted by Ewo

Please advise: it's still worth it to line the control cavity with copper foil, right?

Can't hurt at all. That may provide the little extra shielding you need especially if the RFI is getting in through the cavity.

When you change the cable length you're changing the resonant frequency of your "antenna". You may very well be receiving a different station now Try a longer/shorter power cable and also try using a power strip with filtering (if you have one), and/or a different outlet.

It can take a while to hunt these things down but with time and patience you can find it.

[Ewo]

Many thanks to y'all for the help!

I experimented some more with it, last night. I'm thinking all this stuff is further evidence my bass is moonlighting as a transistor radio:

1. Tried a couple other basses (MTD 535 and Modulus G5) with everything else in the rig the same. No RF on either of them, and what little noise there was in the signal went to zero when I turned the volume pot on the bass down. (On the Lakland, the RF stays the same level with the volume pot turned off in active mode, and disappears entirely in passive mode.)

2. Swapping out the instrument cable changes the RF with the Lakland. The shortest cable had the least. (That would be true of random wire antennas, right?)

3. The amp by itself (nothing plugged in) is dead quiet with the gain cranked way past normal level. There are no pedals in my signal path, anyway.

4. The RF with the Lakland varies with the position of the instrument cable. I held the middle of it up in the air, and could vary the RF by moving it around.

Again, many thanks for the help! Any other advice is greatly appreciated--like tips on lining the control cavity with foil. (Never had need to do it before.)

[zenrad]

Glad that you're tracking it down!

Here's a link to shielding info get you started, but there are countless others
http://www.stewmac.com/shop/Electron...ctions#details
I'm sure many here can point you to good instructions on doing that as well.

Just the way a guitar string has harmonic points and will vibrate sympathetically at certain pitches (frequencies), wires can be divided into wavelength sections that resonate at certain radio frequencies.

That short wire you're using is basically a bad antenna (or half of a bad antenna) so it doesn't receive too well. Not too convenient for on stage use though. Don't forget that ALL of the wires are part of your antenna, so your house wiring+power cable can be part of that antenna too. Moving your guitar cable around could be like moving half of an old TV set "rabbit ear" antenna - you move one part but the whole thing is in use. A different power cord (longer or shorter) could "retune" your antenna out of AM range.

So try the simple stuff first, like you're doing. I'd swap power outlets next (try a different room and breaker), or at the very least a different power cord of a different length and an RFI suppressor power strip. In fact, that power strip could be all you need. Home Depot has some inexpensive ones.

Keep us posted, I'm curious to hear the solution on this one.

[Ewo]

I'm back with a report on the RF adventure; figured it might be useful to other folks with a similar problem. It's a story with a happy ending: there's still a bit of the radio station signal, but it's greatly reduced. I'm gonna call good for now...

Here's what I did since my last post, one thing at a time, in sequence. And many thanks to all the good folks on this thread who suggested trying these things.

1. Put a surge protector strip with RFI filtering between the amp's power cord and the wall.

Result: no change in the RFI.

2. Ran it from a different outlet. The amp had been plugged into an outlet on the original house wiring; the old work is BX, so the armor is the shield. I've heard that ain't the best grounding, so I plugged it into a new outlet, a homerun with Romex straight to the breaker panel. (I updated the panel a couple years ago, and it has a very good earth ground.)

Result: no change in RFI.

3. Substituted another amp, with everything else the same.

Result: no change in RFI.

4. Figured I'd pretty much tried everything outside the bass, so took the cover off the control cavity and checked continuity. There was a strip of copper foil, stuck to the side of the cavity, being used as a grounding tab. The pup grounds and pot grounds were soldered to this tab. Checked continuity for all the pot cases (to each other and to the grounding tab) and it was all good. Verified that the shield of the output jack had continuity to the copper tab, as did the bridge.

The weird thing was that what looked like grounding paint on the entire surface of the control cavity did NOT have continuity to the copper foil or any of the pot cases. I measured the resistance of the paint itself over about an inch distance, and it read about 180 ohms. I dug the VOM probes into it enough to scratch the paint.

Result: I ordered some copper foil from StewMac.

5. Unscrewed the pots and lifted them up so I could get the foil to sit under them. Lined the control cavity with the foil, checking continuity to the original grounding tab as I added each piece. (Kinda like roof flashing, I guess--except this foil sure gives ya paper cuts on your finger...). Finished up with a short piece lapping onto the routed edge where the plastic cover sits--the entire perimeter. Rechecked continuity for all the pot cases, the bridge, and point-to-point in the new foil. Checked that the aluminum foil on back of the plastic cover in fact was conductive, and screwed the cover down onto the new foil.

Result: dramatic reduction in the RFI. There's still a little bit in the bass's output, which I can hear with the amp cranked up--but the S/N is good enough for me, now.


So again many thanks to y'all for the helpful suggestions! Much appreciated; I wouldn't have thought of all of them, on my own.

Hey, just a couple more questions somebody might be able to help me with.

1. I lined the interior of the control cavity with copper foil, but not the hole through which the jack mounts. That worth doing?

2. The Nordy Big Singles have three wires: white, black, and grey. The white runs to the master pot, and the black and grey go to the ground tab. Is that correct?

3. I've got plenty of copper foil left. Is there any benefit from peeling the aluminum foil from the underside of the control cover and replacing it with copper?

[mech]

The ground of the output jack is the most important of all. This is where all grounds for shielding the circuits in the guitar originate. Be aware that the ground for the bass is supplied from the amp by the shield of the guitar cord.

Make sure the BODIES of the pots and any switches have a good connection to the output jack ground. Any metal parts that go from the control cavity to the outside world are antennas if they are not grounded. I think this has been the prob all along. Seen it too many times.

Replacing the aluminum foil with copper will probably make no difference.

180 ohms per inch is reasonable for conductive paint but it must have a good connection to the output jack ground.

mech

[archer121]

Does this bass have the stock wiring? I'm thinking it might be something in the bass itself causing it. Like maybe improper wiring or a bad solder joint or bad ground. Maybe even a problem with the pre. Just shooting at random here.

[bassbenj]

Originally Posted by archer121

Does this bass have the stock wiring? I'm thinking it might be something in the bass itself causing it. Like maybe improper wiring or a bad solder joint or bad ground. Maybe even a problem with the pre. Just shooting at random here.

Radio pickup is always a big crapshoot. What appears to be happening is RFI is getting into the bass and is being detected by the active preamp which then comes out as the bass signal.

Standard fixes include trying to shield the ENTIRE preamp (put it in a copper box or use copper inside the cavity). Another fix is to put 100 picofarad capacitors around. (using as short wires as possible!) Put one across the bass jack and one across the input wire to the preamp for sure! These are supposed to short all RF to ground while allowing audio frequencies to pass. From then you you'll just get more and more desperate! Some things like looping your bass cable through a ferrite core sometimes can work. Oh, I forgot, 100 pf from battery leads to ground too! You just have to try to block it everywhere! Bad news is that often these efforts don't succeed and then only fix is to buy a new (often different brand) preamp. Like I said, when I had this problem with my Alembic, I simply whined until they got tired of listening to me and finally sent me a new preamp without the problem.