Line 6 Spider 4-75 Master volume groaning?

[allante666]

Hi..
This came in with the usual 'everything works apart from there's no audio from the guitar or the headphones' fault. Usual suspect (codec) cured it.
However, It has a strange fault that was there before I replaced the codec and is still there now.

Best I can describe is that the amp works exactly as it should for everything. Except.. when you turn the master volume pot, with or without a signal, it 'groans'. thats the best way I can describe it.
Imaging a creaking door, if you open it very slowly you get the slow steady clicking but if you open it quickly you get the 'creak' or 'groan'. well that's what the pot is like. Volume of groan stays the same.

At first I thought it may be some digital volume control signal that was leaking through from somewhere but on browsing the forums it seems that the master pot is the only pot that isn't digital and works just like an ordinary analog volume pot.

My main problem is that I cant find the correct schematic for this? I have diagrams for the spider iv 75 but they all seem to be a slightly different variant. The one I need has the master volume pot labelled as R723. Anyone shed any light?

When the pot isn't being turned it works just fine and controls the volume as it should. The noise only happens whilst turning.

[allante666]

So, small update. I have found on the DSP board a small 14 pin chip labelled as R109. This is a MCP421 which is a digital potentiometer. I found that with a dab of a damp finger shorting out pins 6 and 7 it would cure it. Until it was switched off and then on again.
I took various measurements on this chip (only going by a data sheet I found so some are educated guesses) and the voltages seemed ok so I was toying with the idea of replacing it. Whilst messing about taking measurments etc it seems to have cured itself and I cant replicate the fault now. I am 100 percent certain there is no bad joints on it etc so am a little stumped now.
The fact that it reset itself after switching off an on Im guessing it was sending a signal somewhere to some sort of latching control?

Still havent found a schematic.

[Jazz P Bass]

Pin 5, 6 & 7 are for the volume potentiometer.

Sounds to me like that MCP42100 needs to be reflowed.

[allante666]

Hi Jazz, yeah I'm going to replace it. For the sake of 70 pence I might as well! That way I can check under it for any cracked tracks etc.

[allante666]

Well that was a waste of time. replaced it, still the same. really need a schematic!

[allante666]

OK so I may have found it. Coming off pin 6 is a cap. A surface mount one. I removed it and the fault seems to have gone. (I may stand corrected on this later if the damn thing comes back!). My problem now is i haven't a Scooby what value its supposed to be. there no markings on it. Am I ok to just replace it with any old low value smd cap?

[allante666]

So I replaced aforementioned capacitor (C176) with a 470pF as that was the only new ones I had to hand and all seems to be well.

[Jazz P Bass]

As I said, pin 5, 6 & 7 go to the volume control.

So maybe it's a bright cap?

Does the cap run in parallel with pin 6 & 5 or 6 & 7?

[allante666]

Hard to tell without diagram and its double sided board but it looks to come off pin 6 then go to a smd resistor then goes off to who knows where!

[olddawg]

I don't know if you do this with Line 6 amps. My apologies if it's obvious. And... People tend to get pissed because they loose their "customize" presets. But there is a hard reset proceedure in owner's manual of all of these amps, usually involving holding 3 or more buttons while powering the unit on. It returns the amp to factory data. I've acquired several Line 6 amps that were either dead or had various weird problems that this hard reset cured immediately.