Amp noise - am I missing something obvious?

[Mick Bailey]

I have a Fishman Loudbox Mini in right now that makes massive amount of hiss. Otherwise the amp works fine. I quickly isolated it to channel 2 and in particular pin 14 of U4D, which also had 0.47V DC, despite no DC (or noise) on either input pin 12 & 13. So I removed the opamp and checked for DC on the pads and all was well. Reinstalled a new opamp and back to exactly the same situation - no noise or DC on either input and DC/noise on the output.

To further isolated the problem I removed C71 and connected the output to my bench amp. Lots of noise which can be altered with VR7 and VR8. No noise on any other part of the circuit - just pin 14.

I had to hand a fresh delivery of TL074s so just out of interest switched the IC yet again to rule out the remote possibility of the first replacement having a fault. No change.

Can anyone shed any light on what I'm missing?

[nickb]

Can I suggest you lift the U4 pin 1 end of R32 and tie it to the nearest ground to isolate it further?

If it still hisses, remove C33 then C35 to go deeper.

[Mick Bailey]

Cheers - will take a look tomorrow now as it's been a tiring day. One of those days where everything moves slow due to constant interruptions and nothing gets done.

[J M Fahey]

If no news with above tests,with amp off and preferrably with one leg of R32 open, check:
* U4 pins 12-13 are not shorted to each other (test with amp OFF)
* U4 pin 13 is not grounded , again with amp OFF
either of those defects might leave U4 D basically open loop and own noise would be amplified a lot.

Just for kicks:
* solder a 100k resistor straight from U4D pin 12 pad to ground
* just for testing: short pins 14 and 13 , that turns U4D into a unity gain buffer.

Any change? .... this with amp ON ofcourse

As usual, besides "bad parts" (which I do not discard) I am suspecting/checking broken tracks and such.

Good luck.

[Enzo]

Scope it, looking for RF oscillation there.

[Mick Bailey]

No oscillation.

But a breakthrough - a varying/unstable 86 ohm resistance to ground off U4 pin 13. R35 checks OK. So desoldered the connected legs of VR7 (no change) then VR8. Removed VR8 and no noise. I uncrimped the pot to discover the shell is misaligned and in slight contact with one leg (which connects to pin U4 pin 13).

Phew. Glad I posted. It's a lonely job being a solo repairman and sometimes you get stuck in a groove. Thanks for the help.

[Mars Amp Repair]

Hello folks,
I know this thread is a bit old, but adding info for folks that might be searching for this issue. OK it's well known that the Fishman pots get some kind of debris in them that causes resistance to show up between the pot and the case of the pot. The Loudbox mini I had had what sounded like a ground loop issue only when I turned up the mic channel volume. I inadvertantly discovered it was a ground loop issue by loosening the ground screw on the power supply board & the hum reduced substantially.
I then discovered if I removed the preamp board to isolate the pots from grounding on the chassis, the hum went away.
Having experienced the shorted pot syndrome on other Fishman products, I measured the resistance between the individual pots in the Mic channel to ground. Turne out the mic volume pot had the lowest resistance (around 30ohms). I blew it out with compressed air while monitoring the meter & the resistance went to infinite.
It's interesting that this issue reminds me of the 'whiskers' issue that occurs in the old Germanium transistors. You can Google that one.
So with ANY weird issues that come up for these Fishman amps, you should see if blowing out the pots resolves the issue.
glen