Trouble with my pacman PCB (scrambled screen)

[2PacMan]

Can any pacman experts help me?  I bought a nonworking pacman, and this is what comes up when I turn it on (see pic).  I put in my known working pacman PCB and it's fine, so it's gotta be something on the PCB.  I compared it to my other PCB and everything seems to be there.  This particular screenshot is not on any of the pacman troubleshooting websites.  Any ideas on what to do to fix this?

[HaRuMaN]

What are those wires soldered on the bottom?  Also... check for broken traces.  Are there ROMS that have come loose?

[D_Zoot]

Never ran into that specific problem before on a pac, but it looks like there's no sync from the game board to the monitor.  If the game doesn't play blind, and there is no sync, then it would indicate that there is a clock problem with the board. 

I'd start by probing around the clock circuits....


D

[2PacMan]

I'm not sure what those wires are, they look sort of like a hack I found on one of the web pages.  I tried the board with the wires, and with the wires cut (right now they are cut) and it doesn't seem to make a bit of difference.

The game doesn't play blind or anything.  That garbled screen just sits there and there is no reaction to moving the joystick/buttons.  Everyone once in a while the garbled sprites will shake a bit.  And there seems to be a slight squeal from the speakers when I turn it on.

D_Zoot - Where are the clock circuits at on the board?  The traces seem ok, except someone tried to do an edge repair job on the first two pins where the connector hooks into the board.  It looks like he soldered a bent piece of metal over the first 2 pins and then soldered it to the board.  It looks like he did a decent job, would that have anything to do with the scrambled screen?

[D_Zoot]

The clock circuits start with a crystal and some logic chips and go out from there.  The clock is needed for CPU and board timing, the sync for the monitor is derived from the clock as well.

Look at the Pac schems (I can't refer to them at the moment, I'm not where I can pull them up),  find the crystal and you should be able to follow the clock path out from there.   A quick check to see if there is clock or not would be to poke a logic probe on the clock input of the Z80 (pin 6).

The "garbage" on the screen looks like static garbage (would be the random contents of the ram at power up) along with no sync,  that's what makes me think the cpu isn't running and the lack of sync implies it's a clock problem (cpu won't do squat without a clock signal).

D

[D_Zoot]

Don't worry about those wires quite yet, hook them back up and  try to get some signs of life into the board first.

I can't suggest what the wires are for, I can't really make out enough detail in the pics to see.  Could be one of the hacks,  what's in the board for eproms? 

[2PacMan]

Well, i'll tell you the circumstances of how i got the board.  I bought a Pacman Plus cabinet (to use for MAME) and that board was in there, along with the original pacman marquee and 7 or 8 chips laying in the bottom of the cabinet.  The board didn't have the PacMan Plus glob daughter board, so i'm assuming maybe someone hacked the board to play pacman plus without the extra board.  And then i thought the extra chips at the bottom of the board were the original pacman roms and the chips that are in there now are the pacman plus roms.  Other than those jumper wires, which i cut, it doesn't appear that any traces are broken or anything like that.  I can't find any sites that show a hacked pacman plus board.  But in any event, the board was like that since I got it, so it must have worked for a while, and then just one day went bad.  That's why i'm thinking the roms and everything must be OK.

[D_Zoot]

I've never heard of a pac plus hack without the aux board.  It might be possible, just I've never heard of it.

Still, even with who-knows-what for roms in it (do you have any way to dump the roms and see what they are?) the board should still be doing something.  So hook the wires back up and focus on getting the clock/cpu up and running and go from there. 

No point in trying to sort out a hack on a dead board,  try to breath some life in the board first.  If you can get it so it's bringing up something usefull on the screen it will be easier to sort out the hack.


D

[CheffoJeffo]

Is the squeal from the speakers or monitor ?

Everything tha D_Zoot says makes sense to me (and has been noted for future reference!). Your screenshot looks something like one with the following diagnosis (which also jives somewhat with what D_Zoot is saying).

Fast moving short vertical lines covering screen. High pitch squeal from monitor. Caused by shorting 6 MHz clock signal to other chip pins. Also caused by PCB frequence pot not adjusted.

2B shorted 6 MHz clock signal p14 to p9,10,11,12,13 or 15. OR 2A short p14 to pad next to it. OR 1E,2E short p2 to 1,3,4,5,6,8

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

I have no idea what to say about the hack and ROMs though ... I look forward to hearing (and learning) more.

[2PacMan]

Hmm, that picture does look like a similar problem to what i am having.  I will have to take a closer look at those chips (2A, 2B, 1E and 2E).  That looks like a good place to start!

[Witchboard]

The wires look like it's a Ms. Pac 2532 hack.