Game: Original Bally/Midway Pac-Man pcb.
Purchase Description: Non-Working/Parts and Repair.
Fault on Receipt: Missing all masked proms/eproms, no custom IC’s/daughter cards and no Z80 CPU. Unable to test “as is”.
This is the second board from a small batch of PacMan pcbs that I recently acquired. I note that this particular board looks like it has had all the electrolytic capacitors recently replaced, so hopefully there will not be any cap issues to look at. However, just like the first board I repaired, it’s been robbed of a lot of the socketed parts; no proms/eproms, no custom chips/daughter cards and no Z80 processor. On the plus side, the 4 small bi-polar proms are still there and so are all 6 of the 2114 rams.
I notice the Ground and +7v (+5v) pins on the edge connector on this board are a bit burnt up. This is quite common to see on PacMan pcbs, so I need to get this sorted out first.
With the refurbished edge connector, a set of new eproms, Z80 processor and reproduction daughter cards fitted, plus the 6x 2114 rams tested (they all tested good) I’m now ready to power the board up.
Switch on…..it’s dead. Just a black screen showing nothing.
First thought; the sockets. The board still has all the original Midway sockets, none of them have been previously replaced. I get my multi-meter and check the socketed chips (with the chips and daughter cards still in situ) and check for continuity on every pin to make sure I’m getting a connection. I soon discover that I’m getting a short to GND on pin 27 of the Sync Buss Controller. Looking at the schematics, pin 27 comes from pin 7 of the 74LS139 at 7M.
The track doesn’t go anywhere else and with the Sync Buss controller removed, I'm still getting the short. There doesn’t appear to be any damage to the track itself so the only obvious conclusion is that there must be an internal short inside the 139 chip. I remove the chip and fit a new replacement.
With a new 74LS139 fitted, the game boots right up!
….however something doesn’t look right. Some of the colours are wrong and there is no sound (only a very quiet amount of static coming through the speakers).
This looks to me like Pac-Man Plus colours, so I take a closer look at the 2 colour proms at 4A and 7F. The copyright stamped on them is 1982 (AFAIK all original PacMan proms are marked 1981).
These are the proms that were in the board, so I can confirm that they actually are Pac-Man Plus proms. So this board was either previously a Pac-Man plus conversion or someone has stuffed a spare set of Plus proms in the sockets. I’ve not got the Pac-Man Plus black epoxy resin add-on kit to convert the game to a Pac-Man Plus (a shame as I’ve not got this version). So I will have to convert it back into a standard Pac-Man.
Now onto getting the sound problem fixed. The static gets louder/quieter when adjusting the volume pot so the problem is more than likely in the logic chip area.
From previous experience, usual suspects for sound problems are the two 7489 rams at 2K and 2L.
These chips are not socketed and I think the easiest way to check these is with a logic probe. The output on these chips are pins 5,7,9 and 11. I put the game into sound test and probe the outputs of the two ram chips. You should normally get a smooth high/low audible sound through the probe on the outputs, however the outputs on 2K sounded really scratchy and funky. I’m certain this chip is faulty. Even thought the chip at 2L seemed ok with the logic probe, they are both of the same brand so I decide to pull both of them to test. Sure enough the one at 2K is bad, the one at 2L is good so I’ll re-use it.
With new ram fitted at 2K, this restores all the sounds
So that’s PacMan board number 2 now bought back to life. Not as many problems as I originally thought.
I’ve got yet another PacMan pcb to fix, so that will be coming up next…
Summary
* Fitted new Z80.
* Fitted and burnt 6 new HN462532G Eproms.
* Fitted reproduction 284 Video Ram Addresser custom pcb.
* Fitted reproduction 285 Z80 Sync Buss Controller custom pcb.
* Bad 74LS139 at 7M had internal short to ground causing the game not to boot up.
* Bad 7489 ram at 2K causing lack of sounds.
* Wrong colour proms at 4A and 7F (PacMan plus were fitted) giving incorrect colour palette.
Parts needed for this repair
* Replacement 284 V-Ram Addresser daughter board
* Replacement 285 Sync Buss Controller daughter board
* 1x Z80 CPU chip
* 6x HN462532G Eproms
* 1x 74LS139 chip
* 1x N82S123N colour prom for 7F
* 1x N82S129N colour prom for 4A
Purchase Description: Non-Working/Parts and Repair.
Fault on Receipt: Missing all masked proms/eproms, no custom IC’s/daughter cards and no Z80 CPU. Unable to test “as is”.
This is the second board from a small batch of PacMan pcbs that I recently acquired. I note that this particular board looks like it has had all the electrolytic capacitors recently replaced, so hopefully there will not be any cap issues to look at. However, just like the first board I repaired, it’s been robbed of a lot of the socketed parts; no proms/eproms, no custom chips/daughter cards and no Z80 processor. On the plus side, the 4 small bi-polar proms are still there and so are all 6 of the 2114 rams.
I notice the Ground and +7v (+5v) pins on the edge connector on this board are a bit burnt up. This is quite common to see on PacMan pcbs, so I need to get this sorted out first.
With the refurbished edge connector, a set of new eproms, Z80 processor and reproduction daughter cards fitted, plus the 6x 2114 rams tested (they all tested good) I’m now ready to power the board up.
Switch on…..it’s dead. Just a black screen showing nothing.
First thought; the sockets. The board still has all the original Midway sockets, none of them have been previously replaced. I get my multi-meter and check the socketed chips (with the chips and daughter cards still in situ) and check for continuity on every pin to make sure I’m getting a connection. I soon discover that I’m getting a short to GND on pin 27 of the Sync Buss Controller. Looking at the schematics, pin 27 comes from pin 7 of the 74LS139 at 7M.
The track doesn’t go anywhere else and with the Sync Buss controller removed, I'm still getting the short. There doesn’t appear to be any damage to the track itself so the only obvious conclusion is that there must be an internal short inside the 139 chip. I remove the chip and fit a new replacement.
With a new 74LS139 fitted, the game boots right up!
….however something doesn’t look right. Some of the colours are wrong and there is no sound (only a very quiet amount of static coming through the speakers).
This looks to me like Pac-Man Plus colours, so I take a closer look at the 2 colour proms at 4A and 7F. The copyright stamped on them is 1982 (AFAIK all original PacMan proms are marked 1981).
These are the proms that were in the board, so I can confirm that they actually are Pac-Man Plus proms. So this board was either previously a Pac-Man plus conversion or someone has stuffed a spare set of Plus proms in the sockets. I’ve not got the Pac-Man Plus black epoxy resin add-on kit to convert the game to a Pac-Man Plus (a shame as I’ve not got this version). So I will have to convert it back into a standard Pac-Man.
Now onto getting the sound problem fixed. The static gets louder/quieter when adjusting the volume pot so the problem is more than likely in the logic chip area.
From previous experience, usual suspects for sound problems are the two 7489 rams at 2K and 2L.
These chips are not socketed and I think the easiest way to check these is with a logic probe. The output on these chips are pins 5,7,9 and 11. I put the game into sound test and probe the outputs of the two ram chips. You should normally get a smooth high/low audible sound through the probe on the outputs, however the outputs on 2K sounded really scratchy and funky. I’m certain this chip is faulty. Even thought the chip at 2L seemed ok with the logic probe, they are both of the same brand so I decide to pull both of them to test. Sure enough the one at 2K is bad, the one at 2L is good so I’ll re-use it.
With new ram fitted at 2K, this restores all the sounds
So that’s PacMan board number 2 now bought back to life. Not as many problems as I originally thought.
I’ve got yet another PacMan pcb to fix, so that will be coming up next…
Summary
* Fitted new Z80.
* Fitted and burnt 6 new HN462532G Eproms.
* Fitted reproduction 284 Video Ram Addresser custom pcb.
* Fitted reproduction 285 Z80 Sync Buss Controller custom pcb.
* Bad 74LS139 at 7M had internal short to ground causing the game not to boot up.
* Bad 7489 ram at 2K causing lack of sounds.
* Wrong colour proms at 4A and 7F (PacMan plus were fitted) giving incorrect colour palette.
Parts needed for this repair
* Replacement 284 V-Ram Addresser daughter board
* Replacement 285 Sync Buss Controller daughter board
* 1x Z80 CPU chip
* 6x HN462532G Eproms
* 1x 74LS139 chip
* 1x N82S123N colour prom for 7F
* 1x N82S129N colour prom for 4A
