WPC 5v Power Reset Fix PRO – 5 Volt Daughter Board

The Pro daughterboard implements the same reset-preventing solution as the classic daughterboard but also includes additional circuitry to monitor your Power Driver Board.

The 2017 edition monitors the driver board’s 5v and 12v supplies!  Simply inspect the LEDs to quickly determine PDB power supply functionality.  New in 2017 is additional indications… How to read the LEDs:

• Green LED lit means both your 12v and 5v power supplies are stable
• Flashing Green LED 5v supply exceeded 5.6v
• Red LED lit means at some point since turning your machine on the PDB 5v power supply dipped below the voltage that would have triggered a reset had the daughterboard not been present
• Flashing Red LED means the 5v supply fell very low and needs maintenance
• Blue LED lit means the 12v supply is weak – at some point since turing your machine on the PDB 12v regulated supply dropped below 9 volts
• Red and Blue LEDs lit means both supplies on your PDB have triggered alerts.

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