Pac-man (Midway) PCB repairs
I've added the Pac-man
fixes page which shows screen shots and solutions for some interesting
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
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
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
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
Solution: Replaced ROW 6 Eproms
Problem: Board constantly resets, but will boot occasionally
Tools: ABC pac pcb diagnostic module, soldering iron, wick
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
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
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
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
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
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
Solution: Replace the faulty 74LS161 at 2R
Problem: Entire playfield was covered in horizontal red
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
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
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
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
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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