Diagnosing a dead Asteroids board - questions

Firstly, in order to resolve the following problem I've already looked at
various resources such as the Asteroids Repair Encyclopedia, this list and
searched through the google newsgroup archives, etc. :)

Okay, so I have a dead Asteroids board (revision 5). On powering it up, all
I get is a continuous roaring/white noise from the speaker with maybe the
odd intermittent burble of sound f/x (these are rare though and usually on
power-up, where the roar goes on, and on, and on ....... ). The P1/P2 LEDs
are lit steady (P2 LED is slightly dimmer than P1 LED which isn't the case
with my good working Asteroids board. There is no response from any
buttons, either in game or test mode. No video signal either (the spot
killer is on). It's definitely the board at fault as another board works
just fine.

I've done the usual visual inspection of the board to check for bad solder
joints, touching pins, cracked traces, missing components, etc. Also
reseated any socketed chips, cleaned the edge connector, etc.

I've also:

Replaced the CPU (the original was dead as a dodo)
Added a crystal (the original had been cut off once upon a time)
Replaced a badly overheating 74LS259N at location M11
Discovered a burned track from edge connector pad 'D' to the Collector of
the 2N6044 at Q12 (although continuity is fine, as is resistance on this
track - it's for COIN CTR-L)
Replaced ALL the EPROMs/ROMs (some of the originals were bad, so I thought
I'd do a new set)
Replaced the 74LS245 at E3 and one of the 74LS244's next to the CPU (I'll
do the other 244 soon maybe)

The watchdog is resetting like there's no tomorrow
I've tried the suggestion to remove and socket the 74LS42 at L6 and lift up
pin 1 to ascertain whether the VSM or the MPU is likely to be the cause of
the problem. This didn't resolve the issue so the problem seems likely to
be on the MPU side of things. The only thing remaining to try here is to
replace the RAM chips one by one, but surely if one or all the RAMs were
dead the machine wouldn't be in the state it's in now? I mean, shoudn't I
at least get a beep or two when putting the machine into test mode?

I'm also considering replacing all the ROM/EPROM sockets, and maybe even
the CPU socket.

Remember, I'm a relative newbie at all this and don't as yet even possess
an oscilloscope, let alone a decent logic probe. I do hope to remedy that
soon though. :)

Is there anywhere a decent diagnosis routine for determining the cause of
dead Asteroids boards? I mean, when the experts here have a board in such a
state, what things do YOU do and in what order? I've had a look at the
schematics (and managed to fix a sound problem on my other board the other
day just by studying these) but with a dead board, where do you start?

I'd appreciate any help and advice you can all give, and I'll endeavour not
to clog up the list with 'thank you' messages as I have done in the past.
Hey, I can't help being polite. ;)

Cheers,
Phil

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Received on Tue Feb 5 13:21:17 2002