PacMan Troubleshooting Blind!

kb2umj

New member
Joined
Jan 27, 2010
Messages
30
Reaction score
0
Location
New Jersey
I'm in the heat of battle with a cocktail pacman that's dead all the way round. The monitor is dead (G07) and the board is lifeless (00B). The replacement monitor I had turns out to have issues. For now I'm working blind. I hope to have a wg4606 to use in a few days.

I've got all the schematics and guides the net can offer so I'm on my way.

The +5vdc is present and clean (o-scope) at all the chips.
The Z80 is not getting reset. Reset is high. Clock is clean and 3mhz.

I have found that the 128v, 64v and 32v signals from the 74ls161 @ 2S are trash. All other V and H signals are good.

None of the input buffers 8D, E, F and H 74ls367 chips are being selected. Pins 1 and 15 on all of them are high.

The D0 through D7 data bus lines are the thing that has me wondering if there is another issue beyond the bad TTL chip. All of the lines have for lack of a better term, a ramp type pulse. The pulse width is approx .64us wide. The pulse is positive going that ramps from 0 to 3vdc. The last .4us is flat at the 3vdc level..

My questions are: has anyone ever seen this kind signal on the databus? Could this be an effect of the screwed up V signals?

My plan is to order a few of the TTL chips that are used in more than one circuit so that I'm not paying $15 for a .55 74LS161 chip.

Sorry for the ramble.

Michael
 
Hi Michael,

Definitely replace the 2S '161 as that (should) at least restore your vsync and fix the vertical timing. IIRC the vertical timing chain is completely separate from the data bus and only used for timing / vram-address generation.

Most likely reason the '367 buffers aren't being selected is that something else has gone bad and is fighting with other signals on the data bus.

Hope this helps...

-Yvo
 
I've removed the daughter board that is closest to the Z80 (can't remember the name cause I'm not in front of the schems) and the ramp pulse stayed put. The plan is to replace all the ls chips that touch the missing V signals. We'll see how far that takes me. The Z80 and socket are also getting replaced as a matter of course. I'll update as it goes.
I'll post pics of the waveforms soon.
 
I received my Jameco order today. I replaced the LS161 @ 2S and fired it up. It started resetting..... From what I could gather from other postings and some of the t/s websites, the suggestion was to replace the Z80 socket. It took a while but I got it done. I tossed in a $2 Z80A along too. The machine came up! Waka waka waka!!
For good measure I replaced the main 7.5 vdc filter caps with 105 degree, 16wvdc versions. I'm still blind.... the monitor is next on the hit list. The G07 is getting a rebuild kit from www.therealbobroberts.net and we'll see if the pac gets the WG4600 or the G07. Now that it's working I can take pics of the waveforms and get 'em posted. Sounds like a good idea for a webpage.......
 
Riptor here is good with the pac stuff, he also has a web store with a handful of pac related parts n such. Not too far away down in Maryland.

PS I'm 15 mi WEST of Philly :)
 
Update: The cap kit, HOT and F901 are in and it came right up! My issue at the moment is that the B+ stops short of 120vdc. I can get 118 max. Anyone have a suggestion?
 
Back
Top Bottom