- misc pinball fixings and non commercial pinball help

Attack from Mars: Game resets when freeplay knocker fires

Issue: Game resets when freeplay knocker fires. This does not happen always, but most of the time.

Solution: First, voltages for +5v and +12v where checked, and they are both rocksolid, even at reset time. Power Driver was swapped with a known good board, issue remained. CPU was swapped with a known good board, issue disappeared - so we know the issue is on the CPU board or with some intermittent connection. Swapped original board back, issue returned.

It seems the problem is vibration based - the knocker causes the components to vibrate a bit. Using a small piece of plastic, i started knocking slightly on components - it turns out that by slightly knocking U4 (the 6821 CPU) the reset would occur.

Taking the board to the workbench and removing U4 reveals corrosion that seems to be a remainder of an improperly cleaned battery leakage.

I desoldered the socket, and you can clearly see the corrosion on pins 4,5,6,7. The other pins dont look to good either


Other side of the same socket, no corrosion visible


Soldered in a new socket for U4, cleaned the pins of the 6821 and reinstalled in machine. Issue no longer appears. Another one done!

W?D: Fixing the slot reels reloaded

Problem: After my first attempt to fix the bloody Who Dunnit slot reels, the rightmost reel started acting up again recently - it spins freely, but stops after one or two turns max.

Solution: Checking into problem, i noticed that the slot index opto of the right slot reel wasnt registering. Time to have a look at the switch matrix.


As we can see, two wires (ORG/GRY, which is in reality WHT/GRY and GRN/YEL) are involved. Connecting the wires from the right index opto to the middle opto didnt change anything either, so that ruled out the opto board itself. Following the wires, a broken cable was discovered on the WHT/GRY wire at the BLACK target. Soldering the wire back to the target fixed the issue. Problem solved.

WPC Anti Ghosting Eproms

Issue: In all modern pinball machines (WMS/Bally), the use of LED's usually leads to ghosting, a condition where other LEDs also are lit or flickering while they shouldn't be.

Solution: While there are special non-ghosting LEDs out there, these are expensive and just not needed. The issue stems from a timing problem within the lamp driver code when using LED's instead of conventional light bulbs.

Luckily there's an easy fix - and its called WPCLP (WPC Lamp Patcher), and was kindly made available on GitHub by Eric Conner, programmed by John Honeycutt -

This utility basically takes an existing WMS/Bally WPC ROM and modifies the lamp driver code to behave with modern LED's. If you want to try it for yourselves, pull a copy off of github, compile on your favorite platform (i'm using gcc but mingw should work just fine) and give it a spin.

Here's a quick snap of it in action:


Take that patched ROM file (in that case a T2), burn it, and you are good to go. If you don't want to do the work, feel free to contact me for any EPROM needs you might have.

W?D: Fixing the slot reels

Issue: On Who Dunnit, the slot reels are either stuck, move only sometimes, or are just jittering.

Solution: First of all determine if the problem is of mechanical or electrical nature. If you have one working reel, try to swap the driving pcb around between the working and the non working one. If the reel is now working, you have an electrical issue.

If you find your issue to be mechanical - the motors are prone to failure due to dirt. Instead of buying new ones, just clean the old ones - a detailed instruction guide is available here -

If you find your problem to be electrical and have ruled out the usual - no voltage, connectors, opto not working... - then there's a good chance of the LM339 being faulty. For whatever reason the LM339's on these boards seem to fail quite commonly.

Happy Slotting!




Demolition Man - Row 6 of switch matrix not registering

Issue: The sixth row of switches on DM was not registering at all. A fault on the CPU was suspected, as it was recently cleaned of battery leakage damage.

Solution: Responsible for row 6 is the white-blue wire, as per ops manual -


Looking at the schematics for the wpc-89 cpu board, we see the following components involved coming from J209-7 -


We can see J209-7 to D8, cont to R61/R62, to a LM339 at pos U19 pins 8,9 with an output at pin 14, cont to LS240 at U13 pin 4 and R32.

Using an DMM, checking for continuity as by the abovementioned path revealed a bad trace between U19 pin 14 to R32 and U13 pin 4. Soldered a wire jumper from U19 pin4 to R32, switches are now registering. Issue resolved.


Monster Bash - all flasher are really bright and capacitor C10 explodes

Issue: On Monster Bash, all flashers are really bright and fail after a couple of flashes. Furthermore, after 10 minutes of running, smoke comes out of the backbox and C10 on the PDB explodes.

Solution: Checked for voltage on flasher sockets and found +50v instead of +25v. The cap in the 25v section is rated for 35v, therefore exploding when prolonged 50v were applied to it. Found that a previous owner attached the power wire for flashlamps to 50v instead of 25v.

Cleaned exploded Cap, replaced C10, and connected flashlamps to the proper 25v power wire. Issue resolved.

Judge Dredd - flippers weak, even after a full mechanical rebuild

Issue: On JD, even after a full flipper rebuild, the flipper fingers feel weak.

Solution: Checked for correct components and correct coils, ok. Measured voltage coming to coils, which should be around 70v dc when no flippers are pressed - only got around 55v at the coils.

On JD, power for the flipper fingers is generated via a bridge rectifier (br1) on the fliptronics board, with AC power coming directly from the transformer's secondary winding to j901. Measured the bridge's diodes - all ok, voltages at j901 were ok as well.

Looking at the board, C2 was missing, which Williams did on some boards, and installed on others. Installing C2 on the board (i used a 100uf 100v axial capaciror) bumped the voltage right up to around +68VDC. Flippers were now snappy and powerful. Issue resolved.


Monster Bash - all motor driven gadgets no longer working

Issue: All motors are no longer working (Frank up/down, target bank up/down, dracula)

Solution: Expected the +12v to be missing for the motors, measured, but 12v was there. Probed the upstream ic's on the power driver board with an osciloscope, and found that the output pins were not pulsing. Probed the input pins for pulses, found that they were not pulsing either. Those pulses are supposed to come from the CPU.

On a hinch i reseated the short ribbon cables between CPU and the Power Driver a couple of times, and motors started working again. Issue resolved.

TAF - Settings are not being retained

Issue: Occasionally, on startup, TAF would present a factory settings restored message. This wouldnt happen everytime, but often enough to be annoying.

Solution: Easy things first - removed battery holder, soldered in new external battery holder with new batteries, issue still persistent. Checked for 4,5v at RAM chip U8 at pin 26,27,28 - measured ok. Changed RAM chip at U8 with known good 6264 - issue still present. Checked for 4,5v again - it was reading 0,xx suddenly. Tapping the diode with the finger at position D2 restored the 4,5v sometimes, sometimes not. 

Seems that D2 had some whacky internal fault which causes it to react on vibration, and tapping it caused it to fail at times. Replaced D2, issue resolved.

TAF - Enter Test Menu button not working

Issue: Cannot enter test menu on TAF, button not registering.

Solution: Checked actual button, measured ok. checked wirking for broken cables, measured ok. As the the test menu button is a dedicated switch, used alligator clips to jumper the pins directly on the CPU (j205), button still not registering, therefore isolating the fault to the CPU.

Measured the way up to the logic ic's from j205 and found a broken trace not visible with the bare eye, propably a leftover from a previous battery damage repair. Soldered a jumper wire, button is now working.