Asteroids - VSM woes

I've gained a lot of experience fixing Asteroids boards over the past few
months, but one area that has now defeated me is the dreaded VSM!

I've been messing around with this board on and off for a week now and it's
driving me nuts. When in test mode, it beeps about twice a second, and in
game mode the LEDs flicker rapidly and you can't play a game.

In test mode, grounding the watchdog stops the beeps, BUT if I press the
reset button I get one long beep (for about a second) which does NOT equate
to the normal low tone you'll get to indicate a faulty RAM, and it's also
not the same as the short 'bip' you get to indicate the RAM test has
passed. Pressing any of the CP buttons produce a slightly longer beep.

Looking at the screen with the watchdog disabled you can see the normal
test mode image, but it's lower down on the screen than usual, flashes a
lot and is accompanied by a fair few rapidly flashing diagonal lines.

 From doing some comparison checks with the ICs in the VSM area against a
known good board, I can see that certain lines are *slowly* pulsing (about
2 pulses per second) - many of these are HALT lines, but not all. Some are
inputs, some outputs.

 From reading the schematics and messages elsewhere, I decided to try and
inject a pulse onto the HALT lines (pins 9 and 10 on the LS109 at A9). By
doing this the watchdog stops resetting the CPU, the LEDs pulse at their
normal speed, and I can play a game, but ....... the normal game screen is
mixed up with the test screen, along with the usual few extra diagonal lines.

I'm not really sure how or what to test now - does all the above maybe
indicate of a vector RAM failure, a short, broken track, failed/failing IC
- or maybe any of these???

In theory, if the vector RAM had a problem, shouldn't the self test
indicate as such?

Thanks,
Phil

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Received on Wed May 1 04:30:00 2002