Fishman SA220 bad channel - repaired

[drewl]

So had a Fishman SA220, two channel PA on a stick, channel two was bad, causing a bunch of white noise.

Plug into the loop and it's ok from there, power supply and power amp both good.

You could see dc levels and noise jumping around the preamp IC's

While researching I saw a thread where someone got the main preamp from Fishman for a low price. Contacted them and they no longer had them.

The tech was extremely helpful, sent me schematics with component locations which my schematics didn't have.

He also suggested I blow out the pots with air, saying that's the main fix in most units. Well I had already cleaned the pots with a little DeOxit, but figured I'd try his suggestion.

Sure enough, that cured the problem!

Must have been some contamination or something when they were assembled?

Also channel two was missing a cap in the input pad switch circuit, so the pad was on all the time, straight from the factory you could see there was nothing ever soldered in that place.


So, just a quick tip to try before digging into a preamp repair.

[g1]

I thought the fix sounded familiar, seems to be their 'go-to' fix for issues of this nature: https://music-electronics-forum.com/...-repair/49840-

[drewl]

Weird.

[Jazz P Bass]

Yup!
I posted this before when I had the issue.
They must have used a plastic / carbon ink track in the controls.
I do know that DeOxit didn't touch the problem.
Compressed air did.

[J M Fahey]

No dust/lint/cigarette ash inside?

Excellent as it is, Deoxit is not a cure-all ; doubt, say, cotton lint is much affected by it.

[drewl]

The unit is sealed pretty well

It had to be some kind of contamination either from when the pots were made (they are pretty odd, not a common manufacturer) or when the unit was built.

[El Dedos]

Having worked on 11-12 of these over the past 11 years I can say that Fishman was very helpful in obtaining schematics and "tech help". However the compressed air and cleaning
did not always fix the "static" problem Back when they were available the board replacement took care of the problem for 3-4 units I worked on. The compressed air trick did fix
2 or 3 but 4 of the ones I worked on required replacing a MC33079DR surface mount IC. That was the only thing that fixed them. Just saying.