Is a dirty pot a noisy pot when NOT being turned?

[slowmotion]

So I have an old desk. It's a live desk, it's analogue, it's not transformer input or anything, just opamps, simple simple. It's from 1978-80, made by Jands in Australia. A pub rock desk.

The thing is, it's a bit noisy. Everything works perfectly (well...). I know live desks will always be noisier than a "proper" recording desk, that's not my issue. It's extra noisy, to the point where I wouldn't use it live either.

I've replaced the power caps, but not all the caps (yet). I figured the old power caps would be the biggest cause of noise, but after swapping them I only noticed a slight drop in noise - and I mean tiny, I couldn't hear it but I could measure it.

None of the caps are visibly leaking (I know that's not an indicator of much) and I will eventually swap them all out, but couldn't help thinking about all the other parts. Particularly, all the pots and the sliders, figuring at least a little beer has gotten into them at some stage over the last 30 years.

So - if I am NOT turning or moving a pot or slider - can it still be a cause of noise? I am quite familiar with scratchiness and noise when moving them, but if they are still, can they still be a problem? I can't see how, but I ask.

Am I better off looking at things like the opamps or transistors? Can they "go" bad like caps can?

I *know* I could get a much quieter, much more useful, smaller and better desk for a few hundred bucks. But I want to use it to learn this kind of thing on. I can solder and I understand the risks of poking around in power sections etc etc.

Anyone?

 

[sweetbeats]

Absolutely...

I was having a conversation with evm1024 the other day...a pot is a resistor right?

He suggested that a lot of bad cap noise is going to be snaps and crackles, and that bad resistors will usually be a cause of hissing...I'm dealing with this in my M-520 made by Tascam right now. He suggested to look especially at resistors in the feedback loops of the opamps. Also, is the noise global, or isolated to certain channels/busses?

And regarding the pots specifically, I've got bad pots that will pass no audio until I sweep them around, and then I can find spots to park the knob where it will pass audio but it will be terribly distorted, or more clean but noisy hissing. Another thing that has been a noticeable cause of noise has been switches, either lever-type select switches or latching push switches. Applying DeoxIT to those and exercising vigorously has helped. Take the time to find out how to apply the cleaner so that it really gets into the switch. This may be a non-issue on your mixer, but the M-520 latching switches pretty much require me to pull the channel card out to squirt at the switch body...can't do it from the top at the actuator. Pots too...the resistive element in them...sometimes there are multiples like in pan pots or stereo pots for instance and of course in stacked pots. I've found it to be important to be able to get to the body of the pot rather than trying to force cleaner down the shaft from which you risk getting grease in the body...the pots on my Tascam board have a hole in the side of the body...I can tip the pot so that when I spray the cleaner in it will drain away from the element(s), hopefully taking impurities with it rather than loading cleaner in there only to have the whole mess sit on the element.

Some pictures and in-kind description here.

Hope that helps some, and good luck. Put up some pics of your board if you can.

Never heard of it.

 

[slowmotion]

Absolutely...

I was having a conversation with evm1024 the other day...a pot is a resistor right?

He suggested that a lot of bad cap noise is going to be snaps and crackles, and that bad resistors will usually be a cause of hissing...I'm dealing with this in my M-520 made by Tascam right now. He suggested to look especially at resistors in the feedback loops of the opamps. Also, is the noise global, or isolated to certain channels/busses?

Yeah I mean pot as in potentiometer. The noise is surely global. With everything turned down, except the master, it's hissy. Turning up any particular channel doesn't significantly add noise like I would imagine it would. Maybe that says my problem lies in the main output section...

Should I perhaps consider swapping out the carbon resistors and replacing them with the metal films or is the type unimportant?

Also, what would qualify a fixed resistor as "bad" - I mean do you mean they fail totally (either total or zero resistance) or do their values drift a bit? Or is it something else entirely? Can I measure this badness? If I could avoid replacing every single component that would be good hehe.

And regarding the pots specifically, I've got bad pots that will pass no audio until I sweep them around, and then I can find spots to park the knob where it will pass audio but it will be terribly distorted, or more clean but noisy hissing. Another thing that has been a noticeable cause of noise has been switches, either lever-type select switches or latching push switches. Applying DeoxIT to those and exercising vigorously has helped. Take the time to find out how to apply the cleaner so that it really gets into the switch. This may be a non-issue on your mixer, but the M-520 switches latching switches pretty much require me to pull the channel card out to squirt at the switch body...can't do it from the top at the actuator. Pots too...the resistive element in them...sometimes there are multiples in pan pots or stereo pots for instance and of course in stacked pots. I've found it to be important to be able to get to the body of the pot rather than trying to force cleaner down the shaft from which you risk getting grease in the body...the pots on my Tascam board have a hole in the side of the body...I can tip the pot so that when I spray the cleaner in it will drain away from the element(s), hopefully taking impurities with it rather than loading cleaner in there only to have the whole mess sit on the pot.

Some pictures and in-kind description here.

Hope that helps some, and good luck. Put up some pics of your board if you can.

Never heard of it.

It does help. Thanks a bunch. Can't see the pics in that post but I get the idea. I don't have any stacked or switched pots so it should be fairly easy.

Not surprised you've never heard of my desk, it's a dinosaur for sure. I'll get some pics in the next few days or so, but you'll laugh when you see it.

I can't actually remove channels separately which is a pain, so it's fiddly (luckily it's so simple). I've got the schematic though thankfully.

 

[sweetbeats]

My uneducated advice...take it with a grain of salt...I'd recap the master section and give all the pots and switches a good cleaning.

Good question about the resistors, for which I don't know. Can anybody speak to the advantages of metal-film resistors over carbon, and how can you measure badness? I'd say the answer is going to include an oscilloscope though. Also, you may know this already, but the drift won't likely effect noise, and to measure you'd need to pull one leg of the resistor.

Maybe you have a bad opamp? What kind are in the master section?

 

[slowmotion]

Maybe you have a bad opamp? What kind are in the master section?

Ahem yeah they are all 4558s throughout (on each channel as well as the master section), with a 748 on each mic input. I read these are very garden variety and probably a source of hiss, but original specs say -85db noise and I'm hitting about -55db (after a power section recap) and something is surely out.

I've ordered a bunch of metal film resistors and extra caps to get all the electrolytics replaced. If doing that achieves nothing I will try swapping in some new op amps, but I'd rather not replace... well nearly 70 of them (that's a lot of solder points!) - only the 748's are socketed (boo).

I might replace 1 channel worth and the whole master section and see what I can see.

But anyone with ideas for testing bad resistors in circuit I'm all ears. Lifting one leg and testing each one, I dunno, if I did that I may as well just replace it without measuring (i'd have to resolder the leg back down anyway!)...
I've also tried poking them with chopsticks, apparently bad or arcing resistors in tube amps will complain when poked, but I don't think I'm dealing with enough power to have that happen.

 

[dementedchord]

resisters can drift in value... but thats primarily a problem in hi voltage situations like tube gear... IME dirty pots (scratchy) are not necessarily noisey at rest... carbon film resistors are abit noisier than metal film... but that's thermal noise... also some crackling around pots can be traced to bad caps in things like eq circuits... the cap is leaking voltage to the pot (where it should be blockin dc) and it's that voltage trying to get through that causes the pops and such by intoducing offsets...

 

[slowmotion]

haha

So I got the cleaner out and removed a slider to give it a spray and see how dirty it might be. Ahem.

I think I have a little more cleaning to do.


The black stuff, to be fair, is deteriorated foam rather than actual dirt... but I guess I need to get them all clean. There could well be enough of the black stuff flaked off inside the faders to stop them from working (or getting back to full/zero resistance, I'm not confident they are operating for their full travel if that makes sense). Back to the spray can.