This circuit creates a unique 5 volt power source for the MPU board. Resets happen on WPC pinball machines because the Driver Board’s 5 volt power supply is minimally sized for the many functions it needs to drive. WPC series MPU Boards have a watchdog circuit that shuts down the board if the voltage on the 5 volt line drops below a cutoff voltage (around 4.7 volts). When this happens the machine appears to unexpectedly turn off – immediately ending any active games. Interestingly, the watchdog shutting down the MPU board often sufficiently reduces the draw on the 5 volt power supply such that the voltage quickly recovers and the machine starts booting as if all were well once again. Unfortunately all is not well – in some cases re-energizing the MPU board just as quickly pulls down the 5 volt power supply and the watchdog again turns off the MPU – creating an endless on/off reset cycle about every two seconds. Even if your machine doesn’t enter this rapid “reset loop” but if your machine resets, your machine will experience additional resets – the periodicity of your reset loop may be minutes (or even an hour or more) but you are just waiting for the next reset.
The WPC MPU Power Fix eliminates watchdog resets by moving the MPU load off the 5 volt line to the 12 volt line. If you look at the WPC MPU Power Fix board, several pins are missing from the wire-side connector… this is because the Driver Board 5 volt line is no longer passed to the MPU. Instead a switched regulator generates a very stable 5 volt supply that is used only on the MPU. This reliably satisfies the watchdog circuit and therefore the MPU board is not reset.
Other parts of your pinball machine still use the original 5 volt power supply so best maintenance practices such as replacing failed capacitors, burnt connector pins, and non-functioning rectifier diodes still must be performed. At long last, however, you no longer have to worry about every hundredth of a volt when conducting these repairs.
Manufactured by Kahr Circuits
