Asteroids Deluxe Problem

I'm having trouble diagnosing a problem on an Asteroids Deluxe board.
When the game is powered up, the Player 1 and 2 buttons flash too
quickly and the monitor's spot killer is on. The game does not play
blind. I have established the following:

1) Logic supply is a steady 5.25v. DAC supply from VR3 is 4.98(is this
too low?)
2) All roms were checksummed and are good.
3) Tried working Ast Dlx board in the cab and it works fine.
4) Swapped out the 6502 from the dead board in the working board and it
is good.
5) Replaced Y1 as it had broken off. Could not find a 12.096 mhz crystal
locally so I substituted a 12.000mhz.
6) Removed and socketed L5. Isolated pin 1 and checked pin 40 of the
6502 - was stuck low. I found this test here:
http://www.geocities.com/SiliconValley/Circuit/2940/asteroidtech/asteroidsrepair.html

According to the above page, if pin 40 is low, the problem could lie
within the MPU, the vector state machine or both. However, I found this
test inconclusive as pin 40 on the good board was also stuck low and it
works fine.
7) Checked E2 with a logic probe and compared results to the working
board. Both were identical.
8) Probed the 6502 on both boards. The only difference was that pin 21
on the good board was high and 21 on the problem board was pulsing.

Note: Dip switch settings on the good board are identical to those on
the dead board.

Any suggestions on how to proceed?

Thanks
George

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Received on Sat Jul 7 14:08:06 2001