pac-man video display moving garbage

JimD53

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I had an old thread on pac-man not being displayed properly and since I've done a lot of checking I decided to start this new thread as the symptom has changed.
The screen does respond (with different garbage) when i press the 1P button to start the abc diagnostic program so I'm confident that the processor is executing code. I wish I could post a video showing motion but. The area highlighted in yellow drop straight down from the top of the screen in columns represented by the junk spaced out in the yellow highlights. The area below in blue moves horizontally. Has anyone seen this sort of error.


IMG_5331.jpg.

What I've done so far is to replace or swap chips in the video stream starting at 3E & 3H and 1E & 2E across to the output at 7F. I've used my multimeter to check for continuity on all traces in those circuits or the buses they touch which was a week worth of fun. I also checked them against short to ground or +5. I verified that all pullup resistors were in place and of the correct value. I went back through the clock circuits on the left of the schematic to verify good clocking for the primary V & H clocks as well as the derived signals such as vblank, hblank, etc. All were good. I did check to make sure the processor was not being reset. I've looked the board over for solder bridges and none were obvious. I've looked at mikes arcade and lawnmower man web sites and didn't find a match to my problem.

I don't have a fancy logic analyzer, just a cheap 8 channel USB connected one. I have a digital oscilloscope, logic probe, and multimeter. Beside checking for signals that should have data that are either high or low what else should I look for, especially on the various buses. I understand how to verify operation of the single logic ICs like and, nor, nand, etc but not how to check the data buses are operating properly with the equipment I have on hand.

I've been working with digital circuits as a hobby since the Z80 first came out but this is only my second and last arcade repair that I'll attempt so advice from you arcade experts will be greatly appreciated.. How would you approach dividing and conquering the circuits to find the problem?
 
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No, for now I was using the diagnostic tool. I never had sound even when the diagnostic program would display properly all tests worked except sprite placement and sound. My plan was to get the video fixed before tackling sound. The diagnostic program has several sound tests I was going to use when I probed the sound circuit.
 
bad daughter card connections
Tomorrow I'll get my meter out and check each pin on top of the 284 card to all endpoints that it touches. I'll post a picture of the connector I use. It is more reliable than the original. The circuit board traces are another story. It is entirely possible that another one opened up. I've had to install several jumper wires but I think they are good or I'd found a problem when I previously traced all connections. Thanks for the suggestion.
 
Tomorrow I'll get my meter out and check each pin on top of the 284 card to all endpoints that it touches. I'll post a picture of the connector I use. It is more reliable than the original. The circuit board traces are another story. It is entirely possible that another one opened up. I've had to install several jumper wires but I think they are good or I'd found a problem when I previously traced all connections. Thanks for the suggestion.
he's saying monitor. you didn't specify if it's K4600 or G07.

H. Hold being out of adjustment is probably your problem
 
when you check the daughterboard I have had cold solder joints on the pins that were on the daughterboard
 
he's saying monitor. you didn't specify if it's K4600 or G07.

H. Hold being out of adjustment is probably your problem
Thanks, I hooked up my video test generator and the G07 monitor has a bad sync problem. It has been working fine for several months which is why I didn't consider it as the cause. I've been working through multiple problems on the pac-man board more recently.

First picture -- Now. It looks worse than it is. the lower half is black and blue blocks that are steady. The lighter text in the middle of the screen is messed up and moves horizontally. It looks like the lighter colors are confusing the sync circuit
Second picture -- In its better days.

Until I sort out the monitor problem I'll pause this thread
 

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Thanks, I hooked up my video test generator and the G07 monitor has a bad sync problem. It has been working fine for several months which is why I didn't consider it as the cause. I've been working through multiple problems on the pac-man board more recently.

First picture -- Now. It looks worse than it is. the lower half is black and blue blocks that are steady. The lighter text in the middle of the screen is messed up and moves horizontally. It looks like the lighter colors are confusing the sync circuit
Second picture -- In its better days.

Until I sort out the monitor problem I'll pause this thread
didn't you have another thread with this exact problem before?

treat your games like classic cars if you intend on keeping it, service the monitors.
 
sort of. The later iterations of that problem had static garbage on the screen that didn't react to starting the diagnostic program. I found a couple of chips that were bad. When replaced I just ended up with different garbage. Like I said earlier all of my work lately was related to finding and fixing board problems like several very minute solder bridges or broken traces so I've been looking for the next board problem that fixes the game. I did verify that the sync cable is wired correctly with the two sync wires jumpered together in the wiring harness. I checked and it was connected to the G07 pins (sync sync open).

This thread is closed for now until I can get a good signal on the monitor from my test generator.
 
Mecha you were correct. There was a horizontal sync problem. Went through the monitor and checked all transistors in the horizontal sync circuit. All were good so I resoldered anything in the signal path or in the horizontal pot over to the IC. I was able to get the picture to sync. Now I get this (first picture) which is a messed up combination of the second and third pictures from the abc diagnostics module. I can detect some of the words in the correct location. Thanks for your help.
 

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