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Repair Logs

Pac-man (Midway) PCB repairs

I've added the Pac-man fixes page which shows screen shots and solutions for some interesting problems.

All Ms. Pac-man boards are really Pac-man boards with new character roms, and an extra Auxiliary card with ribbon cable replacing the Z-80 CPU. The easiest way to rule out the Aux card, is to remove it and install a Z-80 on the main Pac-man main board. If the board runs fine as a Pac-man, you can generally figure the problem is with:

  • The ribbon cable
  • The Aux card
  • Or the CPU socket on the main board

But, as with all things, this is not always the case.

General note about Pac-man boards: I've found that many times the cheap sockets on the boards cause trouble. When I start with a board that is constantly resetting, I usually replace the 28-pin sockets for the 284 and 285 cards with new dual-wipe sockets. Another thing I am now doing regularly is to replace any of the big yellow sockets they used sometimes for the CPU.  I've noticed when desoldering them, that many of the pins just fall off.

While we are on the subject of sockets and 285/284 cards, DO NOT plug these directly into the socket on the main board, YOU WILL RUIN THE SOCKET. Use another dual-wipe style socket on the pins of the daughter cards, and insert it's legs into the socket on the board.

If the logs below and the Pac-man fixes page above do not help you with fixing your board, please take a look at Lawnmower Man's Toolshed. It an excellent site with a ton of information on fixing Pac-man boards and many common board problems can be solved with the information there. 

If all fails and you need your board repaired, we can repair or exchange Pac-man and Ms. Pac-man boards for a reasonable rate.


Problem: strange garbage on the screen

Tools: multi-meter, soldering iron

The game would not get through to initialization sequence. When I went to reseat the Sync Buss Controller, I noticed a gouge out of a trace on the bottom. Testing it with the ohms setting on the multi-meter showed the trace was broken. I jumpering over the broken trace and plugged it back in. The game then powered up and played normally

Solution: Fixed a trace on the Sync Buss Controller


Problem: Missing Blinky from attract screen, Pac-man is invisible, no GAME OVER in red in the middle of the screen. Ghosts appear orangish.

Tools: Soldering iron, solder wick

It took several hours to find, but it turned out the 6 pin 1K resistor network at RM1 had broken legs. It appears to have been bent too many times. When I wiggled the part it would sometimes show all characters and colors correctly. Replacing the part fixed the problem.

Solution: Replaced 1K resistor network RM1


Problem: Pac-man and Ghosts stay only in the top half of screen

Tools: multi-meter, soldering iron, solder wick

It was like the sprites thought the maze was only 1/2 as tall. Pins 12 was stuck high on the 74LS161 at IC 1E.

Solution: Replaced IC 1E a 74LS161.


I started using a new tool for diagnostics on Pac-man pcbs today. It's called the ABC Diagnostics module from Two-Bit Score. It works pretty well, if you at least have the CPU running and the scratch ram is working. If the board is completely dead this board will not help. But it is a useful tool, none the less.


Problem: Screen has random letters and beeps, board would not get through the boot process

Tools: ABC pac pcb diagnostic module

This was my first repair using the ABC pac diagnosis kit and I must say, if your board is semi-working, this rig makes it easy to find problems. I replaced the CPU with the daughter card and got a screen telling me there were some RAM problems. I replaced the rams and the diagnostic program ran perfectly and detected no problems. So, re-installed the CPU and fired it up. Still garbage. This told me one thing, there were problems with the row 6 eproms, since the ABC kit doesn't use them. I removed the eproms and several of them had legs so corroded that they broke off in the rom sockets. :( I burnt new eproms and installed them. The board then worked just fine.

Solution: Replaced RAM and all ROW 6 Eproms


Problem: Random letters and numbers on the screen

Tools: ABC pac pcb diagnostic module

I was so successful on the repair above with the ABC, that I immediately went back to it for this one. This board went through the RAM tests and passed all diagnostics just fine. So, I knew there was an eprom problem again in ROW 6. I replaced the ROMS and found that I would then get a BAD ROM 0 message in test mode, and the board would not boot in normal/play mode. I reseated ROM 6F and the board worked. Simple fix and easy to find with the ABC Kit.

Solution: Replaced ROW 6 Eproms


Problem: Board constantly resets, but will boot occasionally

Tools: ABC pac pcb diagnostic module, soldering iron, wick and solder

The diagnostics would give me ram errors and would display garbage if I pressed Start 1 to continue. I knew this board would run sometimes so the ram must be good. I pulled the diag board and put back in the Z80. When I tried to boot it back up, nothing on the screen at all. I tried a different Z80 and got some garbage. Pressing down on the CPU allowed the board to boot. Looks like a bad socket. I replace the socket and all was well.

Solution: Replaced 40 pin CPU socket.


Problem: Board works fine in JAMMA adaptor, but dead in a standard AC Pac cab.

Tools: VOM, Soldering iron and solder

The game powered up and played perfectly using a Pac to JAMMA adaptor, but would not power up once it was plugged into the cab. I tested the voltages on the un-banded sides of D7 & D8, this read 15 VAC, then tried the same on the un-banded sides of D3 & D4 reading was 24.56 VAC. Both are fine. Then I checked the DC across the large electrolytic caps C7 & C8, each should read about +5 to +7 VDC. These were fine too. Next step, check the voltage at the Base of the Q6. The reading should be around +7 or so. What? It's +1.23xxx, Hmm. What is going on? After replacing both Q6 and IR1 I still had the same problem. Just for grins I checked the 5 Watt 50 ohm resistor at R52, since it was the only thing left on that section. Open! The resistor was OPEN! Replaced the resistor and tried the board back in the cab. Powered up a played perfectly. I fixed 2 boards this evening with the exact same symptoms and both had bad R52 resistors.

Solution: Replaced R52, a 5 Watt 50 Ohm wire wound resistor.


Problem: Board works fine in JAMMA adaptor, but kept resetting in a standard AC Pac cab.

Tools: VOM, Soldering iron and solder

After replacing just about every component in the power section with no change, I changed out the large 10,000 uf @ 10V electrolytic caps out of desperation. The board then worked fine.

Solution: Replaced C7 & C8, the two large 10,000 uf @ 10V electrolytic caps.


Problem: Sound would fade out then back in.

Solution: Replaced the audio amplifier, the LM1877.


Problem: Maze area of the screen is white, game still plays

The board ran just fine most of the time, when it was moved, the entire maze area of the screen would turn white, but the top area with the high score and the bottom with the number of credits would stay normal. If I would move the board at all, it would go between normal and the white screen again. Finally the problem was tracked down to the 6 pin resistor pack RM2. Pin 1, leg that is soldered to the power bus, was broken and would intermittently make contact.

Solution: Replaced RM2, a 5x 1K ohm resistor network.


Problem: Music and sound was wrong, missing parts of sounds.

Two resistors were smashed, R2 & R3.

Solution: Replaced R2 & R3.


Problem: Board appears dead, no video, resets constantly.

The board would just constantly reset. Pressing down on the CPU and wiggling the 285 would show just a half dozen or so Zeros on the screen. Replacing both sockets left the board at least showing the zeros *sometimes*. I decided to try a known good eproms set and when I tried to take the 6H and 6J roms out, the socket came with them. I replaced the sockets and put in a known good eproms set, and tried the board again. I came up this time. I reinstalled the original roms and the board still worked, so the problem was bad sockets at 6H & 6J.

Solution: Replaced 4 Sockets for the CPU, 285, 6H and 6J.


Problem: Board show the left half of the screen twice and ran at double speed, sound was double speed too.

This board had been worked on before and had the 74LS161 at 3S replaced. Using a meter I found pin 16 was shorted to the trace that runs next to the chip on the bottom of the board. This trace runs to pin 12 of 2S.

Solution: Removed extra solder that was jumpering pin 16 of 3S to pin 12 of 2S.


Problem: Game would not play. Joystick and coin were not working properly.

I used the ABC Diag board for this one. When I would test the joystick using the board, all 4 directions where toggled at once. Left, right, up and down were all tied together somehow. The LS367 that handles these inputs had already been replaced, but I removed the chip an socketed it. Then tried a new chip. Same result. Well, all the inputs are still stuck together. RM8 might have a problem. I unsoldered RM8 and when I got the first 5 pins unsoldered, that 1/2 of the resistor pack fell out of the board. Ok, so it's broken in half. Replacing RM8 with a new 1K x 9 resistor pack fix the problem.

Solution: Replaced bad resistor pack RM8


Problem: Dead board. Plays but no video.

Solution: Bad LS20 at 3E


Problem: Messed up maze.

Solution: replaced bad socket on the 284 card at 5S


Problem: Garbled sound, really tinny.

Let me first say, I hate working on sound problems on Pac boards. Seems I always end up replacing the entire sound section and still get the same results. This board was no different. I first tried replacing the rams at 2L & 2K, no change. Next I tried replacing the 74LS174 at 1L, no changes. Next I replaced the 4066 at 1N, no change. On to the 74LS283 at 1K, no change. On to the 74LS273 at 2M, no change. What!!?!?! Ok, let's look at 4F. Pin 8 of the LS86 at 4F should be pulsing, but it's stuck high. This pin feeds pin 5 on both the LS157 at 3L and the LS158 a 3K which then feed the data to the sound rams at 2L, 2K. I socketed this chip and still, no change. Then it dawned on me. There is a tiny little glass capacitor that goes between pin 8 and ground before it connects to the other pins. I unsoldered the cap and had sound! This cap was way too large. I replaced it with a 220 pf ceramic. I had sound now, but it was like all the bass was missing. I noticed that if I pressed on the board around the sound section I would get nice sound, but only briefly. Looking at the schematics I started checking  the work I did earlier around 2M. Checking each pin with a meter and checking where they were supposed to be connected. Doing this on pin 11 to R9 showed open. I jumpered the connection on the back of the board and fired it up. Sound was perfect then. So it seems I had broken the trace when I socketed that chip.

Solution: Replaced wrong value capacitor C34 with the correct 220 pf cap, and fixed a broken trace between pin 11 on 2M and R9.


Problem: Dead board

The crystal was oscillating, but the 74LS161 at 9C, pin 15 was chirping, meaning the watchdog was resetting the cpu every second or so. Probing pin 26 of the Z80 showed it was floating, it wasn't high and it wasn't low, the logic probe didn't light up at all. This board had one of the large yellow 40 sockets for the CPU. Removing the top of the socket, it appeared the pins were not making contact so I replace the socket. No change. Next I traced back pin 26 to the 74LS02 pin 13 at 7L for continuity. The trace was fine. I powered up the board and probed pin 13 of the LS02, it too was floating. Checking the inputs of the chip, the were both OK. Replacing the chip fixed the board.

Solution: Replaced the 74LS02 with floating output on pin 13 at 7C


Problem: no sound

I check the sound amp by running my finger over the solder side pins. Usually this will result in static, but it was silent. I replaced the LM1377 sound amp and the sound was fixed.

Solution: Replaced faulty LM1377 sound amp at 11A


Problem: Squished picture horizontally, board runs super fast

This was an interesting fix for me, because I used a tool I had never used on a Pac-man board before. I built a video probe that displays data on the monitor. A video probe is a wire with a  470 ohm resistor on one end and an alligator clip on the other.  The bare end of the resistor is the probe and the alligator clip is clipped to one of the color outputs of the board.  

First, I used my logic probe to check the outputs of pins 11, 12, 13 and 14 of the 74LS161 chips at 2R and 2S thinking that an output was sticking.  They appeared to all be doing their job because all the pins were pulsing. This had me puzzled, I expected that either 2S or 2R where faulty because they generate the timing signals used to draw the display horizontally, but they appeared to be fine.

I had just been doing some Galaxian repairs and had built a video probe to help troubleshoot a problem with a Galaxian board. So I thought I'd give it a shot on this board. I clipped it on the output side of R33, then  probed the outputs of 2R, 2S again. What should I should have seen, when I moved  from the lower number pins to the higher number pins, is the number of vertical bars display on the screen double. When I moved to pin 12 or 2S, there was no bars on the video display at all . The output of the 74LS161 at 2R, pin 12 was wrong.  Changing the chip fixed the board. How the chip failed in this way, I don't know. Generally they fail by an output getting stuck high or low.

For more information on using/making a video probe, see the Galaxian Troubleshooting Guide Part II or the Timing section of Pac-Man & Ms. Pac-Man Troubleshooting Part II on the manuals page

Solution: Replace the faulty 74LS161 at 2R


Problem: Entire playfield was covered in horizontal red lines.

Game would play, but you could not see the center maze section below the High Score section and the CREDIT section at the bottom. Using a jumper, I checked each 2125 ram at 2A thru 2D and found that the only one that effected the screen when jumpered high or low was 2D. Replacing that ram fixed the board.

Solution: Replaced 2125 RAM at 2D.

NOTE: I had a similar problem with multicolored lines on the Pac Fixes page, it was also a bad 2125 RAM. So, it appears if one or more is bad, it may cause the playfield to be filled with any random color or multi-colored horz lines, but the Scores and CREDIT sections will still be visible.


Problem: Speckles are sparsely spread on the screen when the board is running in various colors all seem to be the same color at the same time. Looks like noise of some kind.

Figuring it was a ram problem of some kind, I piggy backed a chip on each of the 7489s at 3F and 3H. The problem cleared up! Removing one piggybacked ram at a time revealed that the problem was with the ram at 3F. Replaced the 7489 and the problem was gone.

Solution: Replace faulty 7489 RAM at 3F.


Problem: Constant loud thumping sound. Volume control has no effect

Solution: Electrolytic Capacitor C51 was open, 330uf @ 16V


Problem: Tinny sound

Solution: Replaced bad LS273 at 2M


Problem: Tinny sound

Solution: Replaced bad sockets for sound proms 1M and 3M


Problem: All characters stuck at the top of maze

Solution: Replaced bad LS161 at 2E


Problem: White screen, no characters or maze. Game plays, sound is fine

Solution: Replaced bad 82S23 at 7F.


Problem: Tinny sound

Solution: Replaced bad cap C1 .01 uF 50V cap


Problem: Dead board

The board was getting no power to any of the +5 rails. Since my test bench is using a JAMMA setup, I jumpered from one rail to the +5V side of D8 and the board ran. Testing the voltage output at Q6 showed there was no power getting through it. Replacing the part fixed the board.

Solution: Replaced bad Q6 (TIP31A)


Problem: Sprites jump in all directions, but not all the time.

This was an interesting problem. This board had 2 ghosts and pac that were fine until Pac ate a power-up. As soon as you ate the power-up (Power pill) the ghosts would turn blue and all would then have the same jerky behavior. It happens in all directions. This turned out to be pin 14 out of circuit (broken trace) to on the 7489 RAM at 3H

Solution: Fix trace from pin 14 on 7489 @ 3H to 16 on the 2114 at 4K.


Problem: Board was dead, power problem

Testing the power on the power rails on the logic section of the board showed only about 2.15 Volts on the +5 line to all the chips. I tested the voltage across C6 & C7 expecting it to be low also, but it was a steady +5. Moving on the the D44VM4 @ Q6 (big transistor with a heat sink) showed that the center pin (collector) was getting +5, but both the emitter and base were only reading 2.15. If I jumpered from the + side of C7 to any power rail, the board would boot. Replacing the D44VM4 with a TIP31A fixed the board.

Solution: Bad D44VM4, replaced with new TIP31A


Problem: Board would occasionally boot, watchdog going nuts

It would sometimes run the Two-Bit ABC Diag board with no trouble at all. Other times it would reset, other times I'd get BAD V RAM 0. I replaced just about every socket on this board, and still had the same problem. Pressing near the prom at 7F would occasionally make the board boot, but if you let go, it would again reset. I started looking at the LS367s in row 8. Piggy-backing them one at a time, when I got to the chip at 8D, the game booted and stayed running. Replacing the chip finished the repair. What a long way to go to find the culprit, but at least this Pac now has all new sockets and should run just dandy for quite a while.

Solution: Replaced faulty LS367 at 8D

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