Self-service - where to begin

All of the CPU board designs are targeted for a balance of serviceability, flexibility and robust resistance to external damage. Nonetheless, there are three dominant sources of damage that can affect the CPU out in the field; a bad display, a bad driver board (and/or corroded connector), and shorted switches on the playfield or at the front door.  We encourage field service on the part of our customers, and provide parts for the CPU directly from Ni-Wumpf.


Remarkably, except for the technician sticking a screwdriver into the wrong place underneath the playfield, the only place the switches consistently get shorted, is on the front door, where there is a lot of well-grounded metal for the coin switches to get twisted up on.  If your self-test button stops working, this is a good place to start looking for bent metal shorted to the door. You're also going to want to check beneath the playfield where bare metal of the playfield GI wiring may become tangled with the switch-matrix wiring. And lastly, be aware that the connectors to the CPU from the playfield and the front door may have become brittle and/or broken inside the nylon shell of the connector - look carefully there.

If something does get through to the CPU, it is going to wind up damaging the switch strobes of U1 (a 74156) on the original System 1 board, Z14 on the new board, or Z19 on the System 80 board. It is also possible to damage the return lines on the System 1 board through U2, only by mis-wiring, or shorting voltage to the switch matrix - and this type of damage can also affect U15 of the board - both of which are parts from Ni-Wumpf. To date, the System 80 return logic is too over-designed to show where it may get affected in the future!


With the System 1 board , if you are looking to diagnose what may be a bad display, you're likely to wind up with a blown chip on the board.  And that's not such a bad thing to do - you just have to be prepared for it.  The steps to make it easiest on yourself in this case, is to socket the chips that may be damaged when diagnosing displays, and to have a supply of chips on hand to quickly replace those that are damaged in the process.  74LS04, 74LS14, 7404, and 7414 are all valid chips to use for U21, and U22, (however, U23 can only be a '14 part).  These are all 14-pin sockets, (while U16, and U20 are 20-pin sockets).  And the chips in this group drive the digit's on the display, so a failure with these chips means that a digit on the display is either too bright, or never active.  Conversely if there is something wrong with a segment on all of the digits in bank A (displays for player 1 and player 2), you have fried U16.  And with bank B (displays 3 and 4), you'll have a bad U20.  The digit drivers are available just about anywhere fine electronics are sold while the segment drivers (U16 and U20) are PALs, and not available through normal electronics shops (they must be programmed).  And again, we do sell you what you need here directly.

With the System 80 board, things are a bit more simplified - all of the outputs to the displays are simply buffered through a 74LS244 buffer/driver chip without any logic at all, and these chips are commonly available at most electronic supply companies. The trick is figuring out which chip does what - the segments of display bank 'A' (players 1 and 2 displays) are buffered through Z9, the segments of display bank 'B' (players 3 and 4 displays) are buffered through Z10, and display bank 'C' (timing and bonus displays) through Z11. The digit strobes are buffered through Z12 for digits 1 through 8, and Z13 for digits 9 through 16.


Lamps run off of a matrix of four lamps on each lamp latch enable line. As such, it will be pretty easy to determine if the issue you are facing is with your CPU or with your driver; if the other lamps on the latch that controls that lamp transistor are acting up (refer to the game schematics for the driver board to determine this grouping of lamps), you are likely facing a strobe line issue that can be associated with the CPU, or more commonly, the wiring harness between the CPU or the driver. If not, it's a simpler issue with the transistor on the driver board, or the connections coming out of it. Please note; Ni-Wumpf CPU's do not work with Rottendog driver boards - there is a component incompatibility with the chips on that product. If you are having issues with the tilt or game-over lamps, this is the most likely source of the problem.


If you have lost a solenoid (it will no longer energize when it should), you will have to trace back the logic to the corresponding drive on the board.  On the System 1 board, each drive circuit is protected by a zener diode, and they protect the CPU from a broken driver board.  One good way to determine if you have blown one of these diodes in this case, is to remove it. 

However, before doing so, it will be necessary to correct the problem that blew this circuit to begin with, because after removing the diode, the circuit protection is lost, and a damaged driver board will then blow ever more complex components.  If, by removing the diode, you can restore solenoid operation, you can replace it with a 5v nominal zener diode.  If this is not the case, check the operation of the 7416 inverters.