electrolyzing a TB-303 (weird repair story)

[Heiko van der Linden]

Hi all,

I had a pretty weird repair this weekend.

One of my friends has a TB-303 which got rain water inside after his roof
started leaking.
When he came home he turned on his TB-303 which functioned ok for some 5
minutes
then it went completely crazy with flashing leds on and off so he turned it
off. When he
picked it up to inspect it he saw quite a lot of water running out of it.
So he found the hole in the roof and also another synth that was exposed to
some rain
water.
So when I came around he asked me to look at his broken TB-303. I opened up
the
case and expected to find some white residue close to the microcontroller
and RAM chips
and maybe some burnt out chips due to the short-circuits that the water had
caused.
I found that the defec was indeed around the microcontroller and the RAM
chips but
instead of finding some white residues due to the rain the whole area around
the RAM chips
was covered in red residue. It looked pretty horrible.
We set out to buy some stuff to clean the pcb (ethyl alcohol and
demineralised water).
After coming back I desoldered the switch board assembly from the main
board.
(pretty cool to see the owner's face turning white ;-)
Then after desoldering one sixth of the main board I cleaned it and the red
residue was
rust. So the next thing was to check where the rust came from. And here
comes the weird
part: two of the RAM chips had both their pins 20 eaten away. Since pin 20
is the plus pole
the only thing that comes to mind is that the rain water was electrolyzed
during the 5 minute
turn-on period. This caused acid to be generated at the plus pole which in
turn attacked the
chips' pins and causing the red residue and the failure.
The reaction would be: 2H2O ----> 4H+ + 4e- + 02.
I have never seen this kind of "weird" stuff but it seems to me that it's a
good hypothesis because
only the plus pins were eaten (nothing else on the whole board).
After cleaning, resoldering the whole thing and reassembling it. I plugged
the wall wart into it and
it functioned again. And my friend became very happy again instead of
looking at the wall paper the
whole time.

Has somebody else seen this defect ? Is this common to electrical devices
that have been exposed
to water ? It must be I guess.

Greets and thanks to listening to my story (maybe we should start a
synth-diy cafe for this kind of bar talk ;-)

Heiko