PacMan Board - Garbage on Screen

Michael Roma

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I have a PacMan board that gets the below upon power up. Any suggestions?

PacBoard01_01.jpg


- Mike
 
I have a PacMan board that gets the below upon power up. Any suggestions?

I like the display !

First simple things to check and do, make sure you're getting a good 5V from the regulator,
test one of the TTL chips, like 7L ( 74LS02 ) test between pin 14 ( +5v ) and pin 7 (GND). You should have right around 5v there.

Swap out the z80 with a known working one, check the sync-bus-controller (SBC) at 6D and make sure no pins are missing, or fallen off if it's the original NV285 and that the socket isn't buggered. Swap with a known working SBC to be sure.

Then if that's done then check the ROMs 6E, 6F, 6H, 6J, if they're the original mask ROMs, not eproms, make sure non of the legs are missing.

-James
 
This one's a helpful link if you don't know of it already:

http://www.arcadegameover.com/pactrouble.html

I have a feeling it's like the link says, 4L video RAM bad. I just ran across that in my Ms. Pac not too long ago. If anything happens to the chips at 4L, M, N, or O, that's when you start running into these issues, in my experience.

Keep us posted,

VG
 
I have a feeling it's like the link says, 4L video RAM bad.

If the display is completely static and just showing random garbage, it's very unlikely to be just the RAM.

Here's the rationale for that.... because, right after a dead z80, this points to a problematic sync bus controller. If that goes bad/missing/bad socket etc. the z80 will never be able to write out to the video RAM so the system appears frozen. There's also the possibility of 7M ( 74ls139 ) being bad but that, in my experience, is less likely than a problem SBC.

When the SBC is working and you have bad RAM you can normally see the system clear the screen, or attempt to, and start the repeating patterns of the RAM test being written on-screen and not just random static characters.

- James
 
Could also be a pesky socket problem on the Z80... They used some craptastic sockets during that era.
 
Thanks for the suggestions... I'll check it tomorrow. I have a spare, working Ms Pac board that I can pull the Z80 and SBC off to test on this board.

- Mike
 
JROK got it... a faulty sync bus controller!

I pulled the Z80 and SBC from my working spare Ms Pac, tested it and got the following...

PacBoard01_02.jpg


I pulled the Z80 out and put the old one back and it still worked.

The chips at 5E and 5F were Ms Pac ROMs so I burned some Pac ones. Replaced and was greeted with this...

PacBoard01_03.jpg


PacBoard01_04.jpg


Now question is... Can the faulty SBC be fixed?

- Mike
 
If it's the board type, yes. Chip type, no, unless it has a rotted pin that you can replace. (This may take a little work with a dremel tool)

If it's the board type then check for bent/broken pins then you can either try to troubleshoot it with a logic probe or simply pick up a bunch of the 7400 series logic chips and replace them all at once.

RJ
 
Absolutly, I bought a bunch of "untested" ones off ebay a while back and fixed them all. Out of about a dozen, only one was working.
 
Absolutly, I bought a bunch of "untested" ones off ebay a while back and fixed them all. Out of about a dozen, only one was working.

So did you just shotgun the SBCs and replace all 7400 chips? Could I piggy-back the chips and then just remove one at a time in an attempt to narrow it down to a single bad chip? Or in your experience is it usually more than one bad chip?

- Mike
 
So did you just shotgun the SBCs and replace all 7400 chips? Could I piggy-back the chips and then just remove one at a time in an attempt to narrow it down to a single bad chip? Or in your experience is it usually more than one bad chip?

If you're going to shotgun, it may be a better use of your time and money to just by a replacement board...

In any case, I've found the 7474s to be the most common failure on SBCs...
 
You can't always get the piggy-back method to work. If the gate is shorted a piggyback chip isn't going to do anything to show you the problem chip.
 
If I recall, most of these only had one bad chip on them and it was usually the same one. I should have documented it but didnt so I have no recollection which one it was. Obviously these ones I had were plugged in backwards and toasted. Some of them had the tan socket installed backwards so they probably just looked at the socket notch and installed it on the board. You can try piggybacking one at a time but success is probably about 25% with that method. It really depends on how the chip failed. What mark said it true, the time and cost it would take to replace ALL the chips you would be ahead of the game to just buy a board.
One thing you can check that I have seen on alot of these is the bottom of the board will have scratched traces from people prying under them. Take a look right under the socket area where one might pry the board up to get those tan sockets off the header pins.
 
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