Lunar Lander 'lightning' display

Some of you may recall that a few days ago I posted a message here about my
Lunar Lander board that WAS working in test mode but now has a corrupted
display.

A picture can be seen here:

http://www.morris0.fsnet.co.uk/ll_disp.jpg

Just to keep the testing simple it's only running with the B1 EPROM and the
screen is even more mangled with 'lightning' (AND the text that should be
there) with the others inserted - the square box with the cross is, as
you'll probably know, because I don't have N/P3 installed. Installing R3
though causes the screen to go completely blank! The screen as you see it
flashes very rapidly, and the diagonal lines you see also scroll rapidly in
roughly the area you see them covering in the picture. As noted, the
display is worse the more ROMs that are inserted with more 'lightning'
covering larger areas of the screen, more jagged, angled lines, etc.

What is even more curious is that the second Lunar Lander board I have
(from the same source) has EXACTLY the same problem.

The RAM passes it's self-tests but I swapped the Vector RAM on one board
anyway to no avail.

The monitor is fine when using it with Asteroids or Asteroids Deluxe boards.

The cabinet is in fact a Lunar Lander cab, BUT the wiring harness has been
slightly modified many years ago to accept a modified Lunar Lander board
(modified to run Asteroids). Even so, all the important signals are getting
through on the correct wires.

The AR board is supply the correct voltage (I should say that I slightly
modified this board according to some instructions I found on the 'net
which enables it to play Asteroids Deluxe with no sound distortion, the
only side effect being that you have to turn up the volume pot when playing
Asteroids or, presumably, Lunar Lander). I can't see as this would have
caused the problem though, and remember that one board was at least working
for a while.

I've tried swapping out the VSM PROM at C8 with a known working one to no
effect.

Could it be the EPROMs I'm using? Doubtful I guess - I've tried various
types/makes/speeds of 2716's and even some 2732's (with the image doubled
up before being loaded).

I'm 100% stumped. I ASSUME it's one of the ICs in the vector circuit, but
which? Maybe it's something else? I don't know, all I do know is it's
giving me a headache and causing me to go prematurely grey. :)

Any help is appreciated.

Thanks,
Phil

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Received on Sat Feb 23 07:04:09 2002