Fishman Loudbox Mini guitar amp white noise

[james_s]

I've got this Loudbox Mini amp on the bench, it belongs to my girlfriend's dad so I offered to take a look at what I assumed would be a simple problem. He says it worked perfectly the last time he used it then he turned it on a few weeks later and there was a loud continuous static noise. I found the schematic (attached) and started poking around, eventually tracing the white noise as coming from the CH2 Dry node in the preamp which is the section that handles the mic input. The mic input does seem to work but the noise is there, and it is affected slightly by the gain pot on that channel and by the high and low tone controls. I desoldered one side of C29 to isolate the input and confirmed that made very little difference, then I desoldered one side of C71 and now the white noise is completely gone and the amp is working, . Ok so I decide U4 must be faulty since it's the only IC in that block so I scrounged around and found one on a scrap board and swapped it in, no change, the static is still there! 

A friend suggested checking the gain pot so I did, it seems to be fine, ranges from about 2R to 48k when I rotate it which seems about right for a pair of 100k tracks in parallel, why they used a double pot I don't know. Scoped the supply rails and they look ok, nothing out of the ordinary there. This has got me pretty well stumped, there's only one active element in that entire block and I already swapped it, I used a salvaged IC so I suppose it's possible it's also bad but it seems unlikely I'd have one with exactly the same fault. I'm wondering if there's some other path I'm not considering where having C71 in the circuit could cause white noise.

[james_s]

So as soon as I asked about this I fixed it! I shot some control cleaner into the pots, worked them all a bit and then hooked it back up and everything is working. It's really bizarre because I've been twiddling all the pots all along and none of them were noisy or had any apparent issues yet cleaning the CH2 pots made the static go away. Either that or dripping some control cleaner on the PCB somewhere fixed it. 

[james_s]

The noise came back while I was soak testing it tonight. I took it apart again and while the power was on I squirted some control cleaner in each of the pots and when I hit VR8 it suddenly vanished. I'm kind of suspicious that that pot is just bad in some way but I'll wait and see if the static comes back before I take more drastic measures.

[elecdonia]

The noise came back while I was soak testing it tonight. I took it apart again and while the power was on I squirted some control cleaner in each of the pots and when I hit VR8 it suddenly vanished. I'm kind of suspicious that that pot is just bad in some way but I'll wait and see if the static comes back before I take more drastic measures.

Can you unsolder VR8 and remove it so that it can be inspected internally? What if a tiny spider got inside it and constructed a web? There could be some foreign material in there which isn’t getting totally removed by the contact cleaner. Worst case it might be necessary to replace that control.

[srb1954]

A friend suggested checking the gain pot so I did, it seems to be fine, ranges from about 2R to 48k when I rotate it which seems about right for a pair of 100k tracks in parallel, why they used a double pot I don't know. Scoped the supply rails and they look ok, nothing out of the ordinary there. This has got me pretty well stumped, there's only one active element in that entire block and I already swapped it, I used a salvaged IC so I suppose it's possible it's also bad but it seems unlikely I'd have one with exactly the same fault. I'm wondering if there's some other path I'm not considering where having C71 in the circuit could cause white noise.

Scope the O/P of U4A and U4D to check for excessive DC offset along with the noise. The DC level of both op amp O/Ps should be close to 0V.

If there is significant DC applied across a pot element this can produce extra noise if the pot wiper is not in the best condition.

[james_s]

The fault was VR8 which is the "High" tone control in the ch2 input section. I didn't try disassembling it yet, I gave the pot a good dousing and then twiddled it until it felt nice and smooth and then I put it back together and returned it to the owner. I told him if it acts up again I'll take more drastic measures and remove the pot for disassembly and inspection. At that point I'd also be tempted to try tacking a couple of fixed resistors between the wiper and end of the pot to hopefully prevent it from being so sensitive to contamination or whatever. I doubt it anyone would ever notice the difference in the tone control.

[Audiorepair]

I found a faulty pot in a Fishman that would very intermittently cut the volume drastically, took me ages to find the problem.

I discovered that the wiper was intermittently shorting to the metal pot body.

Don't know if there was maybe some metal swarf in there or something, but I have read that in some models Fishman recommend compressed air to clean their dodgy pots, NOT any kind of cleaner.
I wonder if this also is some kind of metallic contamination floating around inside.

[james_s]

I suppose it could be. If that's the issue then they really ought to have posted a service bulletin and been taking care of it. When I encountered the problem I looked online and found a bunch of old posts describing the same problem but no solution posted anywhere that I found.

[Audiorepair]

Forgot to mention that the faulty pot I found was also in the EQ circuit.


And here is the alleged advice from Fishman:
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/49840-