This week's weird problem: My rack is acting as an antenna

[midwayfair]

This might possibly be the weirdest thing I've encountered:

I've got a 4-pre box in a desktop rack, and while I was rearranging things to separate some things from a piece whose power transformer's hum was getting picked up by another transformer, I found out one of the channels that I very rarely use was beeping. I took it out of the rack and it stopped beeping. Put it back in. Beep beep. Might have been Morse code. Get this though -- it's not even one of the outside channels! It's one of the middle channels!

Okay, so I can't troubleshoot it outside of the rack. Poke around and I can't find anything wrong with the wiring -- all the lead dress is twisted pairs for the power, shielded cable for the audio, except the 1" leads from the OT to the output jack. Resoldered the ground bus for the box just in case.

But then I found that if I don't tighten ONE of the screws all the way down that it stops beeping. Start wiggling the thing around in the rack and I can change which screw pisses it off.

So now I've basically rigged something with plastic washers to isolate one thing from the rack itself, even though it's earthed otherwise.

It's not surprising to me that a couple big towers of metal would act as an antenna. But ... the rack itself is earthed. The chassis of everything it is grounded, then earthed through their 3-wire plugs. So the fact that I can connect a grounded thing to a grounded thing and get radio interference is nuts to me.

Sometimes I just don't know, man.

 

[abbey road d enfer]

So the fact that I can connect a grounded thing to a grounded thing and get radio interference is nuts to me.

A big piece of metal has low impedance at LF only. When it comes to MHz, particularly if its length approaches a 1/2 wave, it's like it wasn't there.
You have to find why this preamp is so RFI sensitive. Is it SS or tube?

 

[midwayfair]

You have to find why this preamp is so RFI sensitive. Is it SS or tube?

It's solid state. 1:4 Input transformer, germanium transistor, gain control on the emitter, DC coupled silicon transistor, silicon output stage like the Hamptone, 200:600 output transformer. There's three other preamps that are almost identical to it in the same box -- there's one that's identical except the gain control is a volume control, and another one that's identical except that the input transistor is silicon, and the transformers are all the same. Star ground to the chassis. No power transformer (it uses an external DC psu). I'd be less mystified if more than one of them picked up the radio signal.

What's a reasonable thing to try here? I don't think I can't give it a different ground, so should I try giving the PCB a tinfoil hat?

A big piece of metal has low impedance at LF only. When it comes to MHz, particularly if its length approaches a 1/2 wave, it's like it wasn't there.

I feel like I should have a better handle on this stuff, my job literally involves radios (software, though...).

 

[emrr]

I'd certainly be checking the chassis and ground connections in the preamp. 

 

[Gold]

I'd certainly be checking the chassis and ground connections in the preamp.

I'd also make sure the audio common isn't connected to chassis through the mounting holes on this PCB but not the other PCB's.

 

[midwayfair]

I'd certainly be checking the chassis and ground connections in the preamp.

I'm using metal output jacks (1/4" and XLR"). I originally had the 1/4" jacks just grounding the chassis, but I ran a wire from them to the ground bus now, and the XLRs have always gone to the same star ground that the PSU input jack went to (on the floor of the box). The jacks on the front panel aren't metal, though, because there wasn't a metal-bodied combo jack. So the ground pin for those goes to the control panel board, which is grounded to the PCB and to star ground. The PCB is grounded to the star ground, and the OT is grounded to star ground. All four boards are grounded the same. I resoldered each of these joints the other night when I had it out of the rack, and I'm really not sure what else I can do in that regard.

I do have a little more information after poking around some more:

I plugged my headphones right into the pre (1/4" jack) with only a microphone plugged in and there was no noise.

Then I ran one of the XLR cables and split the output to the interface, and the beeping was there. It doesn't seem to matter which output I plug into as well. Swapped out the cable with a few different things, plugged the headphones into the interface and used a 1/4" cable instead -- beep beep. So then I grabbed an alligator clip and ran it from the chassis (really any unpainted surface) to any of the grounded points on the interface and there's the beeping.

So I just started pulling every cable from the interface one at a time, and the noise sort of goes away if nothing from my stereo compressor is plugged in, or if the compressor is taken out of the rack, but if I listen carefully, I think it's still there, just quieter. I went through and reflowed the star grounding/ground bus in there as well. I also swapped every PSU and IEC cable.

The last thing I did was that I noticed that the beep increased with gain, so it was definitely coming from the input. I grounded the input and the noise went away; then I took the input transformer out of its little clamp and the beeping got MUCH louder, but moving the wires doesn't change anything, and the transformer is held to the metal chassis by a metal clamp, and putting a wire directly from the casing to the star ground doesn't seem to make a difference. I guess it's at least somewhat possible that there's some sort of flaw in the input transformer, but the preamp sounds exactly like it's supposed to so that seems unlikely. And I've noticed that even if I get things to where the input is silent with an XLR, if I plug in an unbalanced cable to ANY of the four preamps, I get the beeping again.

I decided to try to isolate the pre from the rack, but unfortunately I don't really have the materials to do it properly. I made some cardboard spacers to go between the screwhead/faceplate/rack, but I think what I'm going to do is try to find some plastic screws that are the right size, either that or I need some sort of hard plastic tube I can cut to isolate the screw shaft from the faceplate.

Edit:

Looks like there's something for exactly my problem:
https://www.sweetwater.com/store/detail/IsoTabs--humfrees-rack-isolation-tabs

 

[midwayfair]

Got some shoulder washers: Turned out not to be deep enough on the shoulders. Got some of the "Humfrees" clips, the screws were too large from my rack, so off to the hardware store. Isolated all the things:

It was still beeping! Albeit a little quieter.

Then I realized I had played with all the mic cables, but my keyboard was plugged into the interface. I unplugged it and the beeping went away, plugged it into the "bad" pre and started hearing the beeping again.

Swapped out the cable, then the PSU for the keyboard and it finally stopped. Which is funny because it's the original -- and probably decent -- PSU for the Roland, but at least I've tracked down the source of the noise. I'm using some dinky little wall wart, which has some hum (maybe can't supply the current needed), and I can use a laptop supply for direct recording but the speaker won't work with the higher (19V) voltage for whatever reason, but neither of them result in beeping.

 

[midwayfair]

Update on this in case anyone else ends up with the same issues:

the shoulder washers didn't work for me -- they just weren't deep enough to keep the screws off the frontplate.

The Humfrees clip thingies did work for me, though. They go all the way through and completely isolate all the metal on the faceplate from the screw and the rack.

Things did get a little complicated, though, since the screws in my rack were too big. I had to get new bolts and nuts (mine were too big) and get clever with the nut holder clips until I can replace those.