Marshall SV-1 chorus hum

[buzzy]

I'm trying to trace a hum on a Marshall Supervibe - it's a unit that came to me totally dead but I found some bad soldering (someone's been at it) on a chip with some legs bridged, sorted that and now the effect does all it should but there's a low hum when the effect is switched in. It doesn't sound like traditional mains/PSU hum (but probably is) and I'm now running out of electronics knowledge and I don't own a scope so I was going to try just changing components until the hum stops - should I start with the caps first do you think? ...or might it be a grounding issue? It's currently birds-nest style on the bench would the absence of the metal case make a difference? Also I've extended the jacks with wire about 50mm and changed the led would this matter?

Thanks in advance!
Buzz.

[ashcatlt]

Most pedals won't even turn on without a cable in the input jack. Is that cable plugged into a guitar when you hear the hum? Are you sure it's not the guitar itself? Turn the guitar's V all the way down. Does it get better or worse?

Are you running it from a battery or a wallwart?

The shielding of the metal case will definitely help if the problem is the circuit itself picking up electromagnetic interference. In fact, once you've eliminated the guitar as the problem it will be difficult to diagnose further until it's shielded.

[JohnH]

Ig you are running off a wall-wart, then I'd 99.9% suspect that, and the way to check is to try it with a battery only. You need a smoothed stabilized wall-wart to run pedals, and some that are sold for effects are not, and some pedals are more sensitive than others.

Does the guitar sound fine through the pedal, whether its on or off? And do you know if this is a true bypass pedal? - Im thinking it's not (not a problem, just for info)

John

[buzzy]

Yes the hum happens when the effect is switched in though the chorus effect is there too - when it's bypassed (passive bypass on Marshall fx) the hum gies and the guitar sounds fine.

Yes it's a wall wart (a Zoom one from a 1010 actually) and all the other fx sound ok with it though I must admit I haven't tried another yet, I will but I really am convinced it's a circuit problem.

[buzzy]

It's not the PSU - I didn't think it was but hooked up my super duper bench PSU to prove it anyway.

I have noticed that Marshall arrange the PCB tracks so that the metal case fixing posts pick up on the negative tracks and with everything bolted into the case it would all link ip with the ground on the jacks, jack PCB and 9v supply/battery negative so maybe I should try linking these with wire in case it's a grounding problem but I'd still appreciate a nudge in the right direction circuit-wise if anyone has the knowledge please? Cheers, Buzz.

[buzzy]

Note to self; next time register as a hot single 22yr old cheerleader so people are more likely to help FIX THIS DAMNED HUM!!!!

[newey]

Buzzy-

No one's ignoring you, I don't know that anyone has enough experience with the circuit to suggest anything other than what you mentioned, i.e., running separate ground wires.

[JohnH]

Did you try it with a battery?
J

[4real]

Nov 6, 2012 15:29:02 GMT -5 JohnH said:

Did you try it with a battery?
J

In order to troubleshoot this, you need to do some work and be a bit more explicit about it all, the chain, was it always like this, does using a battery change the issue.

Don't jump to conclusions, I did a google search to try and work out what this pedal even is, if it is digital, anything like that time delay wise is extremely sensitive to voltages and current and require extremely regulated supply designed for that pedal.

Without the details, how can we get a feel for what is going on. Ground loops can be a real problem, not just with the pedal board, but the amp and all kinds of things. I have with some set ups have to use two stereo DI's to get around mains hum. Running things from a laptop with the power supply unconected fixes things...problem identified and a solution found. You got to do the work, otherwise it's just guessing.

It could be some kind of circuit fault, hard to say, you know the history perhaps and did the repairs and we don't know how that went or why it originally died, or if this was a fault originally a part of the design perhaps, who knows!

Insulting the members is never a good idea, nor is theer much indication that one's sex or age would have that much of an impact...though, I am not aware of many 22yo cheerleaders on any forum, so you'd likely just get deleted as spam in the first place!

[buzzy]

Sorry to hear you got no sense of humour dude I thought it went with being a guitarist....? Pity about the cheerleaders too - ah well

Anyhow I 'did the work' as you suggested (btw I was originally referred because there were electronics experts on here hence my meant-to-be humorous comment!) and there was one leg of an IC on the jack board dry where I looks like someone's replaced the cap next to it. I re-soldered it and the blasted hum has finally left the building! Hurrah! So I've replaced the pots (need to be splined for the knobs I want), extended the jack wires, changed the led for a green one and now waiting for it's new housing to arrive.

Y'know some Marshall pedals sound pretty good it's just the packaging that sucks - put it in a decent enclosure with knobs you can see and (in the case of the GV-2 and JH-1) split up the stupid concentric pots into separate ones and they're nicely useable too!

Cheers, Buzz.