Sponge Board corrossion??!!

     I recently ran into something I haven't seen much before, and thought
I'd share it with you all. Asteroids Deluxe board would unpredictably reset -
could happen within 5 minutes, sometimes would take an hour or so. Socketed
and replaced ALL ram, ALL rom sockets, image verified roms, used SA on the
prom to make sure it was fine, replaced all bus drivers (LS244's and 245's),
socketed CPU, checked all clocks, thoroughly checked the reset button.
    I knew the problem was not the harness, audio reg, or power supply. Board
passed bus test on Fluke, even tried using CAT box....etc.. Needless to say
this problem was a BITCH to find becuase it was intermitant. Board could sit
in test mode forever without resetting. No error codes. Would allways happen
in play mode, so I started to suspect the state machine, vector timer,
program counter... Checked many traces and inspected the board for any damage
- all seemed good.
     However, the decaying sponge on the back of the board was making a mess,
so I decided to peel what was left of it off. I noticed a that several solder
spots on the back of the board had a slight whitish corrosion where the
sponge had been for 21 years. I am familiar with nicad corrosion (from
restoring many pinball MPU's, Omega Race boardsets, Berzerk/Frenzy boards,
etc....) but this was different. It seemed to be a salt-like residue. It
really didn't look that bad at first, but after looking closely, the solder
seemed pitted, and I'm thinking "Here is the SOB!!!....." Component side of
board looked perfect, the only suspect areas were where this crap was under
the sponge on the solder side.
      From what I saw, it was affecting the vector memory latch (LS273 @ H6),
the decoder in the vector timer (LS42 @ E6) as well as the counter (LS161 @
D6). Pulled all suspect components, removed all piited solder, treated the
board (50/50 vinegar / distilled water, followed by 91% iso-alchohol). Then,
I tested all traces in the area to make sure they were good. Socketed and
replaced above components, no more reset problem. I'll probably recoat the
back of the board using clear nail polish to prevent the corrosion from ever
returning.
      Sorry for the ramble, but has anyone seen this before??

MK

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Received on Fri Oct 4 09:42:57 2002