Asteroids tech stuff

Hello list,

I've been working on some Asteroids/Deluxe boards recently and I thought
that somebody may find these notes of use...

Alternative Asteroids speedup hack:

I got hold of an Asteroids board with the following modifications to the
74161 at C5:

Pins 3, 4 and 5 are clipped and lifted from the PCB. Pins 3 and 5 are
connected together and pulled high via a 1k resistor. Pin 4 is connected via
a wire to M11 pin 4 (a 74259, but the schems say it's at M10). This is the
SAUCRSNDEN signal.

These mods result in a nice brisk game with turbo nutter UFO's :)

Sound mod to use an Asteroids board in an Asteroids Deluxe cab:

The audio levels from these two boards are different, meaning you have to
crank up the volume to almost full when you swap in an Asteroids board in
order to hear the sounds. Unfortunately this also boosts the background hum
through the speaker to an unacceptable level.

Fortunately the fix is simple - we can change a single resistor on the
Asteroids board so that the sound output level matches that of Asteroids
Deluxe. The component we need to change is resistor R86, which is situated
next to the LM324 at P11. Pull out the 1k resistor fitted here and replace
it with a 10k and you'll no longer have to change volume levels when
swapping boards.

Note 1: This should also work the other way around - when swapping an
Asteroids Deluxe board into an Asteroids cab (with an unmodified original
A/R board). On Asteroids Deluxe it's also R86 (hey - consistency!) which is
a 10k device. Pull this and swap in a 1k resistor and you should be set.

Note 2: There's another mod floating about which involves modifying the AR
board, but that just fixes the sound clipping experienced when using Deluxe
in an Asteroids cab - you still have to alter the volume control when you
swap boards.

Asteroids Deluxe repair notes:

Rev 01 board.

Tools used:
Scope
DMM (with logic test mode)
Logic Comparator

Before powering up any unknown Asteroids/Deluxe board experience had taught
me this: first of all, check the rectifier diodes near the edge connector.
Failure to do so may cause you much embarrassment as you shower the games
room with the guts of an exploding electrolytic capacitor. Especially when
it isn't *your* games room. Brown trousers are optional but probably a good
idea in case you are in close proximity to the cap when it goes.

A bad rectifier diode may cause the death of most of the voltage regulators,
the TLO8x's and, rather spectacularly, the LM324's which may begin to do
impressions of a roman candle.

Anyway, with the diodes tested OK and all voltages present and correct we
may begin. Don't forget to unplug the monitor if you're working out of the
back of the cab.

The board was completely dead when powered up. No clock observed so the
12.096MHz xtal was pulled and replaced with a 12.0MHz one kindly donated by
a bootleg WWF board :) Clock pulses were now seen but the reset pin of the
CPU was merrily pulsing away.

The 7442 at L5 was socketed and pin 1 pulled up to isolate DMAG0. The board
was still resetting indicating an MPU problem.

The ROMs were pulled and verified. They were all OK. I noticed that all the
program roms were version 01E - Mame doesn't seem to support this set. It's
either a lot harder than the later revisions or I'm a really bad Asteroids
Deluxe player ;) The CPU was swapped with a known good one but still no
change. The program rams at L1 and M1 pulled and verified. No change. The
74245 at E2 was replaced and bingo - the game no longer reset and could be
heard playing blind. The sounds didn't seem right though - explosions were
OK but the various beeps were just clicks so the pokey was swapped with a
known good one and the correct sounds were then heard.

OK, the game was now playing blind, but I didn't even have to press start in
order to begin a game. It was acting as if the start button was held in - a
new game could be heard being started as soon as the previous one finished.
A quick probe around and it was observed that /SINP1 which reads the player
controls was stuck. This is generated by the 74139 at E3. This was replaced
and the game now waited to be started as it should. Progress!

Pin 1 of L5 was reinstated so the VSM was again active. The board was again
constantly resetting. The watchdog was disabled by grounding the appropriate
test hook and the board put into test mode. A single beep was heard
indicating that the tests were OK, so we know the remaining RAM is OK.

After much probing around it was noticed that many of the ADMAx signals were
static. These are generated by the 74193 counters at F4, H4 and J4. Using a
logic comparitor showed that H4 and J4 both had incorrect outputs so they
were replaced. The watchdog was re-enabled. Success! No more resets.

The X and Y outputs were then attached to a 'scope in X-Y mode. Y deflection
looked OK but X didn't. The X-probe was moved to pin 7 of the associated
TL082 - picture looked perfect. Probe moved to pin 6 of the TL081 - picture
still OK. (These tests basically check the output at each stage from the DAC
to the output pin). Probe moved to the output of the 4016 - picture
knackered again. 4016 replaced. Now we have a good picture from both the X
and Y outputs. Time to hook up the monitor.

The game now sits in attract mode asking me to insert coins. In Spanish.
Language dips were changed but had no effect. Use of the DMM revealed the
dips were working OK so the 74253 at P5 was replaced. The language then
changed as expected and the other dips on that bank now had the appropriate
effects on the settings too.

The game seems to play OK but the high scores are not saved. The game
briefly displays "FFFFFF" for each of the top 3 scores all drawn on the same
line then immediately goes back to demo mode. Hoping in vain the EAROM isn't
dead I checked the -29v was OK (I had a board in the past with a dead 555
which had the same symptoms) but it was OK. The 74374 and 74244 for the high
score circuit were changed but still no change. Finally I bit the bullet and
removed the EAROM and replaced it with one borrowed from a Dig Dug - which
luckily was already socketed - and now the board saved the top three scores
as it should. (Are EAROMs still available, or is there a retrofit
available?)

The board now seems to work fine but with so many dead chips I wonder how
long it will be before something else dies during burn-in.

Hope somebody finds this useful.

Chad

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Received on Wed Sep 3 08:48:43 2003