Re: Battlezone repair question

>For instance, when the CPU has a clock, and all address and data
>lines are pulsing, and each RAM chip is being selected at some point, as is
>each ROM, and the watchdog is barking, what's a good place to start? The
>watchdog is barking presumably because the CPU didn't write some data to
>the
>appropriate memory location in order to reset the timer. But, why? Memory
>access "appears" to be good. Anyway, it's this type of stuff that gets
>hairy, for me. What have some of you done when you run into this sort of
>road block? Replace all the RAM anyway? Then what?

Well, I'd do the following:

1) Take a 'scope and look at the clock signal. Just to be sure that it's
really the same rate as the crystal and looks "normal". (sine wave at the
crystal, 50% duty cycle on the inverters.) Weird clocks can still run the
CPU well enough to rattle lines, but not run the game.

2) Check the ROMs. Obvious, but not to be overlooked. I'm especially wary
of the EPROM sockets on BZ and Asteroids. They're quite old now, and were
not great to begin with.

3) This is where a Fluke 9010 or similar really starts to shine. If you
have one, do a RAM and ROM test. If they come up OK do a manual write to
the watchdog location and tell the Fluke to "loop". No put a logic probe
(or scope) on the pin on the watchdog and see if it moves. If so and the
watchdog is still barking, it's possible that the Fluke's "loop" isn't fast
enough to clear it. What I do in this case is hand assemble a little test
program-- for Battlezone:

0000: sta $1400 = 8d 00 14 (watchdog is at $1400)
0003: jmp $0000 = 4c 00 00

Hand "write" these values into RAM at 0000-0005. Then tell the Fluke to
"run at 0000". This will pound the hell out of the watchdog and it should
*never* trigger. It's it's still triggering, the counter is hosed or
something-- replace it. If it stops triggering the AD is probably OK, and
I'd be back at the bad ROMs or socket premise. Even if the Fluke says
they're OK I'd still replace them.

>On another topic, anyone have an estimate for the price to produce a 2
>sided
>PCB, same size as Battlezone's? Yes, I'm still thinking of laying out a
>PCB
>and populating it with new components. Unless the price is ridiculously
>expensive. It seems to me that making a new board would give reliability
>at
>least as good as they were originally, and with better components of today,
>the boards should run properly, longer - in theory. :-)

Hmmmm. Well, that's a BIG board to prototype. Maybe 12x24? I don't think
AP Circuits can go that big, but if they could it'd be around $240 each for
prototypes (minimum two). For a soldermasked/screened board probably around
$70 each in qty 100 out of China.

If you really want to improve reliability, start condensing things down into
FPGA's or CPLD's and make a board that's about 5"x8" instead... ;-)

-Clay
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Received on Wed Jan 12 18:52:25 2000