MISSILE COMMAND white screen - update

[vernimark]

I've this MISSILE COMMAND board

- switching on the game I can see a white screen
- in test mode I can hear 8 "tic" but no sound so actually I dunno if the problem is within the RAMs
- I haven't any activity on the data lines of ROMS in line 1
- ROMs are ok
- most of 244 have been replaced
- addressing seems to be ok
- watchdog doesn't trig and CPU pin 40 is always high

before proceeding to remove all RAMS I'd like to know if you have any other suggestion

regards

[philmurr]

Pin 40 of the CPU is reset. what happens to that pin when you press the reset button?

And can you tell me what the address and data bus lines on the CPU are doing?

[vernimark]

hi,
I replaced in any case all RAMs because I could see one was physically damaged (PCB has been probably crushed) cutting some data lines

Anyway you were right, a couple of D lines (D2 and D4) were always high. The responsible was a 244 near the Pockey.

regards

[vernimark]

Update:

I replaced all RAMs and the faulty 244. Signals (data and addresses) seem to be good.


Switching on the game in test mode I have the following behavior:

CASE 1, removing one RAM chip (just to verify the test mode works)
After few seconds of white screen a low raspy tone is followed by a low beep, then high beep
A repeated series of 8 beep starts separated by a low raspy tone.
Depending on the removed RAM one of the tones is low, the rest are high
In any case the screen is always white all the time


CASE 2, complete
After few seconds of white screen a low raspy tone is followed by a low beep, then high beep
the screen is always white all the time

Switching on the game not in test mode the screen is white all the time and the game doesn't react to any button.

Actually now I don't know what to check

regards,
marco

[vernimark]

I could see the ROM sockets had some problems so I replaced them.
Now I have good signals on ROMs too and
SWITCHING ON:
After few seconds of white screen I have a low raspy tone
then nothing. The screen is always white.

That's all. Probably I'll store the board as not working.