Lunar Lander - obscure 'overlap' problem

One of my LL boards has just, err, 'developed' a most obscure problem (more
on the cause later!).

As an example, when the attract mode is running the text in the top right
corner that should read:

ALTITUDE 0
HORIZONTAL SPEED 0
VERTICAL SPEED 0

in fact reads:

ALTITUDE
0HORIZONTAL SPEED
0VERTICAL SPEED
0

The rest of the screen is fine.

The EPROMs are all fine and work perfectly in another board, and a known
'good' set of EPROMs exhibits this same peculiarity.

How did this happen? By the simple error of squirting the N/P3 and R3
EPROMs with freezer spray while trying to diagnose another fault. This
immediately caused some mangled characters to appear on the display, and on
removing and testing both EPROMs they now appear to be faulty.

The thing is, the damage to these EPROMs has obviously caused some kind of
knock-on effect, I'm just not sure where at present.

The Vector and Program RAM is fine.

I've spent hours trying to diagnose this most obscure fault, would anyone
care to give me any pointers please? :)

Thanks,
Phil

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Received on Thu Mar 7 13:56:42 2002