FIXED: Data East DMD Driver faulty PAL?

[BomberHQ]

Folks,

I have a faulty DMD board (520-5055-00) which is probably best demonstrated by a video ...

When fully cold the corruption starts after a couple of minutes. When warm it starts after a few seconds. The display locks up or blanks completely after a few more seconds.

I have a spare working board so I am sure its not the PSU, CPU, DMD or cables.
Nothing on the board is getting over-warm.
I have touched up all the solder joints.
Ive put a scope on the crystal oscillator and its working fine.
Unfortunately I dont have a logic analyzer, so cant get into the 0s and 1s

When I put localised heat from a heat gun over U16, a PAL16L8, it seems to accelerate the fault, so Im going to start there ...

Does anyone have a copy of the code in this PAL?
Does anyone have experience fixing other faults with this board before I start taking it apart?

Thanks for any help!

Kevin.

 

[BomberHQ]

Update: After hours trawling the 'net I finally found the JEDEC files on ipdb.org
Time to find some PAL equivalents to program with the trusty TL866II Plus ...

 

[drhex]

interesting, looking forward how this turns out. Should be able to program these if you run into issues. Could also lend you a LA if needed.

 

[BomberHQ]

Well, after an adventure into PAL programming (a rabbit hole as it turned out!) I invested in some freezer spray, which worked a lot better than trying to apply local heat.

U18 74LS374 was the cause - corrupting the data bus. Replaced with a 74HCT374 as the LS wasnt available. Its adjacent to the PAL!

Incidentally, when I was a young engineer freezer spray used to cost about £5 a can, but ambersil is nearer £30 now! Thankfully Rapid do what seems to be an equivalent for a fiver!

 

[drhex]

Good thing you found it. Learning about PALs surely didn't hurt ...

 

[BomberHQ]

Good thing you found it. Learning about PALs surely didn't hurt ...

Yes, fascinating ... I can prevaricate for hours over anything technical, so learning how to reverse engineer and program PLDs was a bonus! I really appreciate the amazing work others do to keep pinball and other retro electronics working.

Downside (financially) is I now have an IR camera on order - it was just too tempting!

 

[drhex]

You won't regret it. Bought a Fluke Ti480Pro myself at some point as that does everything from finding underfloor heating to broken components and will last me the rest of my life.