Pinball Rehab

pinball repair and restoration

Repairing WPC Dot Matrix Controller Repairing WPC Dot Matrix Controller Hot

Display problems are typically the result of failing DMD's, bad ribbon cables or connectors, a cable installed one pin off (or reversed) or the high voltage section of the Dot Matrix Controller (called DMC herafter).  Once you have confirmed the problem is with the Dot Matrix board (either logic or high voltage)  this article will assist you with the troubleshooting process.

For informational purposes I will also address some common symptoms that indicate problems other than the DMC board.

You can either test the board in the game or use a test fixture as explained in Building a WPC Test Fixture (see references).  If you are having high voltage problems, most of the troubleshooting can be done without the board needing to be under power.

Dot Matrix Display

The DMD is a plasma display and as it ages will begin to outgas.  This can result in a partial display of brighter dots, flickering and/or decreased brightness.  An outgassed display will often have to warm up before achieving full brightness or will remain blank for a few seconds and then work.

The second most common problem with the DMD is one or more rows or columns of pixels going out.  While this can be caused by logic problems it is most often a break in the pin that goes from the board to the display glass.  There are several articles on the Internet that explain how to repair this type of problem.

Image Gallery

Repairing WPC Dot Matrix Controller
Repairing WPC Dot Matrix Controller
Repairing WPC Dot Matrix Controller

DMC Theory

The DMC board provides the power and logic for the Dot Matrix Display/Driver board. Of all the boards in a pinball game this is one of the more difficult to troubleshoot since it takes a solid  understanding of electronics.  For most people it is best just to send the board out for repair.

A bit mapped image is written by the CPU into the RAM on the Dot Matrix Controller board.  The bit mapped image correlates to the dots on the DMD (128 by 32).  The RAM can hold 16 display images at one time.

There are three 74LS175 page registers that interface between the CPU and the RAM.  The high and low page registers are directly accessed by the CPU.  Each page register points to one of 16 memory sections for the CPU to read and write from.  The third page register points to the memory section that is currently being displayed. 

One last page register provides information to the CPU about which row of the display the controller is currently updating.  The DMC automatically updates the display according to the data in RAM.

U1 and U2 (74HCL138) are decoders.  U1 selects whether to access the RAM or registers and U2 selects which register to access.

The RAM circuit consists of nine chips.  U33 and U35 (74LS175's) control which page the system accesses.  U31 and U32 (74LS175's) control which page is displayed.  U25, U256 and U27 (74LS157's) control access to RAM by the controller and system based on the status of the E clock.  If E is low the system has access and if E is high the controller has access. U29 (74HC165) is a parallel to serial shifter.

The control circuit consists of nine chips.  U12 (74HC161) starts a row scan.  U13, U14 and U15 (74HC193's) generate addresses for the sequence of bits on the serial port that goes to the display/driver board.  U22 (74HC374), U21 (74HC688) and U5 (74HC74) generate the row interrupt.  U23 (74HC27)and U6 (74HC04) serve as a row 1 detect circuit.

The DMC provides regulated 62 volts, 12 volt logic,  -125 volts (anode) and -113 volts (offset) to the display/driver board.  At some point William's changed the anode and offset voltages to -112 and -100 volts respectively.  What is important is that the offset voltage is 12 volts less than the anode voltage.

Board Versions

There are three version of the Dot Matrix Controller: WPC-89 games with a DMD, WPC Security and WPC-95.  For more detailed information see WPC Board Versions.  In general these test procedures will work for the WPC-89 and WCP Security boards.

Common--and Less Common--Display Problems

  1. Slow Animations
    In addition the sound will appear to be synced to the animations.  This is the result of a manufacturing defect on pre-DCS boards where a bad solder mask resulted in a solder bridge between pins 31 and 34 on J601.  Remove the solder bridge and you're set.
  2. SuperScript or SubScript Characters
    This problem is caused by a poor connection between the ASIC and it's socket on the CPU board.  You can re-tension the pins on the ASIC as described in this article by Ed at Great Plains Electronics.
  3. Fuse F601 or F602 Blowing
    This is typically caused by a fault in the bridge rectifier, which can be tested using these instructions.  Some versions of the dot matrix controller routed BR1's AC trace too close to the lower right screw that secures the board to the backbox (see Image 2).  Over time the star washer will dig into the solder mask and cause a short.  Either install a nylon screw or a nylon washer.
  4. Cloudy Dot Matrix Display
    This is typically caused by heat related problems.  Refresh the heat sink compound on Q1, Q3, Q6 and Q7 and make sure they are tight against the heat sink.
  5. Horizontal Roll
    Typically caused by C6, C9 or C10 on the dot matrix controller.

High Voltage

High voltage is routed to the DMD controller through an 8 pin connector.  The easiest way to test the voltages is to lay the plasma display on the playfield glass and measure them at DMD controller.  Note: Be extremely careful when testing since these are high voltages.  The following table provides the expected readings.

Note: The offset voltage will read about 20 volts less than the anode voltage if the display is not connected.  In normal operation the offset voltage should be 12 volts less than the anode voltage.

Function Pin  Spec Notes
-125V in early DMD machines.
-113 in early DMD games.

If the high voltages are off by more than a few volts, perform the following test.  Disconnect the power cable and test the voltages at the J604 on the DMC.  If the voltages return to normal then the zener diodes are probably fine, but the surrounding resistors and transistors may be failing.   Check all resistors to make sure they are within tolerance (10%) and show no signs of overheating.

When replacing the large sand resistors raise them slightly off the board to aid in cooling.  Also, you can use generic transistors, but they must be rated at 150 volts.  The MPSD02's can be replaced with a 2N5551 and the MPSD42's with a 2N5401.

A defective DMD/controller can also pull down the high voltages.  Other than swapping out the DMC there is no definitive way to tell which of these issues is causing the low voltages.

The high voltage circuits create a lot of heat and it is not uncommon to find burns on the board and burnt components.  Because of this advanced soldering skills are required to remove components without lifting traces.  Cold solder joints due to overheating are common.  In Image 3 you can see several obviously cold joints, but all of the joints in that area need to be re-flowed.

Great Plains Electronics sells a high voltage rebuild kit.  The kit does not include capacitors, which can be added separately as needed.  This is one of the only times I would recommend rebuilding a circuit rather than troubleshooting it.

RAM Failure

After the high voltage section, the most common failure is the 6264 RAM chip at U24.  This chip is very susceptible to ESD damage and is often blown by improper handling.  There is a RAM test in the WPC diagnostics if you can decipher enough of the screen to navigate through the menus.

RAM failures typically result in missing dots, or dots that are on when they shouldn't be.  Although most of the time one or more rows or columns will be affected it is possible to end up with a completely random display that looks like a bunch of braille characters.

Logic Failures

While other board failures are extremely rare, they are also very difficult to troubleshoot unless you have advanced electronics skills.  The best approach is to start at the output to the DMD controller (J603) and then work backwards.  Pins 1 and 5 should be high with low pulses, pins 3, 7, 9 and 11 low with high pulses and pin 13 low (the even numbered pins are ground).

Because many of the signals go to multiple chips it is easy to get lost in regards to which chip is using the signal as an input and which chip is actually creating the signal.  This is a good time to become familiar with IC datasheets since they will show you which pins are inputs and which pins are outputs.

Most of the failures I have seen are the result of low (i.e. - 3 volts peak to peak) rather than dead signals.  Therefore watch carefully as you work your way through a circuit for any low readings.  Most of the time it is an IC pulling a signal low rather than a chip providing a low signal.