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WPC CPU board repairs.

This repair method can be used for cpu boards of the WPC-dotmatrix, WPC-fliptronic and WPC DCS type pinballs. To know wich machines are in these category's look at under "repairing Bally / Williams WPC pinball games , Game list. here ...

Luckely it is a rare thing that CPU's are going bad.When it happens there are a few faults that are common, and these faults are treated in this article.

The cpu board has some bigger chips on it, these are , the game rom ( U6) well know and the image of that rom is different for every machine, the images are to find at different places on the WWW. Also the processor chip ( U4) type 6809. Then the memory chip ( U8) only one and not to sensitive, therefore rarely faulty. About in the middle of the board we find the square ASIC chip, that one is specialy made for Williams , i did annonce this article already more then a year, and was in the mean time searching the datasheet of this ASIC chip, but the copyrights still forbit that and the secret is still kept.... Sad otherwise it would be possible to write a test for this cpu board ...The ASIC does lot of different things . The selections of the memory ,the game rom, the coils , the lamps and so on , the clocks are made here, there are probaly several timers, for the " watch-dog" circuit , this circuit will prevent burning coils when the game program stops for one reason or another, Atari being the first to use that technologie, Gottlieb did add an extra little board on their cpu's ..But i will not go into the technologie of the ASIC , but let's focus on some common faults of this board.

Sadly enought the most common fault is still leaking battery damage..Please remove those battery's from the board and use a remote battery holder far from the board. If ever you have battery corrosion look at how to handle that. Also when you have to solder on a CPU board be aware that the runs are very tiny and easely come lose . Use a regulated soldering station other wise the runs will come off the print!!

When you start the pinball , the led in the middle must start flashing rapidly afther three seconds..If that does not happen , the cpu is not started. First remove all connectors , only leave J210 who is the power connector ( +5 and +12 volts) In many case a fault on another board will prevent the cpu to start because one or more of his output signals are grounded by the bad connected board.

If all connectors are removed led D21 is "on" and the middle led D20 is still not blinking , the ony thing that you can do is to replace the 6809 and/or the ASPIC or change them with those of another board.If that does not help, stops there and hand over the board at a experiented repair man ..


Here the placement of the major chips and the led's


D19 Blanking,

D20 Diagnostic

D21 +5VDC

D19 - At game turn on, this should be on for appx 3 seconds, then should go out AND STAY OUT! This indicates that the blanking circuit is disabled and therefore the machine cannot energise solenoids etc.

D20 - After D19 has gone out, this should stay flashing permanently while the game is on. This indicates (when flashing continuously) that the CPU is running.

D21 - This should always be on. This indicates that the 5VDC power is on the CPU board.

Whenever you have the rare case D19 ( blanking) led stay's "on" look at pin 3 of U21 there starts the blanking signal coming from the ASIC; If it is "low" there you have a problem in de amplifier chips U21 or U5. You can follow the signal trought both chips easely using the schematic . If it is already "High" at U21 pin 3 the signal coming from the ASIC if bad, Again change the ASIC or 6809, if that does not help, stop here, and start to look out for an experiented technician....

The schematic of the CPU board

From here we suppose you have a board that has a flashing diagnostic led ( D20) .

The first thing to do is to meaure if we have the +12 volts coming to the cpu board.. ( J210 between pin 1 and 7) The board can have a flashing D20 without the 12 volts present !!! This is a trap in wich many are fallen...

From here we suppose that you have a flashing D20 led and the 12 volts present on the cpu board. But other ( mostly switchmatrix) problems

The switch matrix.

Preliminairy test you can do if you have doubts about the good working of the switch matrix .


Use the switch test ( in the pinball) ) deconnect the following connectors J205,206,207,208,209 and 212, if there are still indications ( little squares) on the display indicating there is still a switch closed , then you have a problem at the input drivers of the cpu board.


If during the switch test , with all connnectors connected normally, you see that a whole row of switches is missing ( mostly 8 ) you have a problem with the output driver on the cpu board.

Start repair of the switch matrix

Take the cpu board out the machine an continou on the bench . We need two tensions, +5 and +12 volts, an old computer power supply will deliver these. Connect on J210 pin 1-2 is ground, pin 4-5 is +5 volts and pin 6-7 is +12 volts.

We are going to examine the switch matrix and drivers easy and straight forward..

During the now following matrix test you need a electronic probe , you can buy that at " Radio sSack" or make one yourself as indicated on this website under "Make an electronic probe" .

Left the probe from Radio Shack , right the self made probe .

The outputs

When the both tensions are applyed the board will start up, and led D20 will flash constantly. Using the probe we look at all 8 pins of J207. All 8 have to show a pulsing signal. The yellow led on the probe is "on" and the red one will be very bright, where the green will be faint. This because the pulses are very small and negative . See the image of the scoop .

Colum drive pulses (= Output)

If on all 8 pins you have pulses the output of the switch matrix is ok

We miss one ore more pulses or all of them..Using the probe go directly the U20. This is the chip that delivers the pulses; Inputs to this chip at at pins, 1,2,3,4,5,6,7and 8 here you have to find pulsing signals , again the yellow led, but as the signal is inverted , the green led will be bright and the red faint. If there are pulses missing at the input replace U 14 .If all are ok control the outputs of U20 at pins 11,12,13,14,15,16,17 and 18 if you miss one there , replace U20 ( most likely scenario) .



Very sadly there are some models of machines who do not use all outputs at J207!! A reason why during CPU boards repairs many did replace needlessly U14 and/or U20... In time i will list here the machines that are exceptions. If you find missing ouputs , use a game rom from one of the ok machines listed here, wich i have tested myself up to now.

CFTBL is one of the exceptions...

Ok to use are , Twilight zone, Addams Family, White Water, Indiana Jones ,Terminator 2.

The input

The inputs are to control on J205 and J209

Make a temporally connection between pin 1 of J207 and pin 1 of J209. With this connection we put a pulse at the entry of switch matrix, and are going to control if it arrives on the databus of the cpu.. Use the schematic and see how pin 1 of J209 is connected at D3 from there via a resistor to the input of U18D pin 11 The pulse comes out of U18D at pin 13. Using the probe we look directly at the output of U18D at his pin 13, is the puls there then all is ok..Is the pulse missing , we then look at the input on pin 11 ( U18D) , is the pulse there then we replace U18. Is the puls missing at that input pin, then we go look for it at resistor R 51 or at the diode D3 the element where it does not pass has to be replaced.

We repeat this same scenario, for ALL inputs. Meaning that we put the pulse coming from pin 1 J207 at every pin of J209 and at all pins from 1 to 8 of J205. The result will be that we have to find pulses on the corresponding output pins of U16,17,18 and U19. An important remarque here ,there is a fault in the schematic , U18a and U18B are inversed the same for U19A and U19B ( example, meaning that input J209 pin 8 arrives at U19 pin 5 instead of pin 7 as on the schematic).

The temporally connection to test the input of J209 pin 1.

This way we will connect all the input pins on connector J209 and pins 1 to 8 of J205 and look for outputs at the different output drivers U16,17,18 and U19. Use the schematic to find the corresponding in and outputs ( easy) This terminates the complete control of the switch matrix.

What else can we do ?

There are some buffers we can check out, these are U3,U1,U2,U12,U7

The IC's U1,U2,U7 and U12, are unidirectional buffers from the CPU to the connectors.. All these IC's need a "low" on pins 1 and 19 to work. So control first if there is a low on these pins, if so then all pulses found on an input should be there on the corresônding output. Wich IC's are validated ( low on 1 and 19) depends on wich type of game the cpu you are testing comes from ( depending on the game rom) . The inputs are at pins, 2,4,6,8,11,13,15 and 17 , the corresponding outputs of these pins are ; 18,16,14,12,9,7,5,and 3 ( example pin 6 in gives pin 14 out, if 1 and 19 = "low")

Buffer U3 is bidirectional, pulses can go from the left pins to the rights pins or reversed.( see placement pins on schematic) , the direction depends of the tension at pin 1 , at "high " the pulses travel from A to B and at "low" they travel from B to A.( always at the condition that pin 19 is low) , The corresponding pins are alsways at the opposit side of the IC as well indicated on the schematic. Using this data you can control all these output buffer IC's .

Here the datasheets of these buffer IC's ....

If all this is checked it is unlikely there are still faults at your CPU board.