Troubleshooting Early Bally and Stern Pinball Machines

A. Caution
B. Solid State Troubleshooting
    1) Early Bally / Stern
    2) WPC Bally / Williams
    3) Williams 11
    4) Williams 3 – 7
C. EM‘s

This covers Bally pinball machines known by the shorthand -17 which were made from 1976 – 1979, -35 which were manufactured from 1979 – 1985.  Also covered were early Stern SS pinball machines known as M-100 and M-200

Prior to starting, check the tools required and cautions.  

Understanding the Layout

The pinball machine must have power at the right voltages in order to start up (boot).  Only three voltages need to be present for the machine to start (boot):

1) +11.9 VDC (nominally +12)
2) +5.0 VDC (between 4.0 – 5.2)
3) +43 VDC (solenoid voltage)

The other voltages have to be there for the game to play.  But those do not have to be present for the game to boot.

To troubleshoot, the power flows as follows:

Images 1 – 3 show Bally & Stern pinball machines where the transformer is below the playfield, in the cabinet.  Images 4 – 6 show those where the transformer is in the backbox (head). 

Testing the Power Supply – Images 1 and 4

In this step, the power comes in from the wall (usually 120 or 220 VAC), through a fuse and filter, to the power transformer and to the ‘rectifier board’.  Boards located in the backbox are referred to as “-18” boards.  Those located in the cabinet near the speaker are called “-49” or “-54”.

What can go wrong

1) Main fuse is blown.  Unplug the power, remove and check the fuse in the can in the cabinet.
2) Transformer fails.  Extremely unlikely.  Will be checked under voltages.
3) Connectors burned or fail.  Common.  Visually look for burned connectors.  Will be checked further under voltage testing.
4) Rectifiers fail.  Common.  Will be checked with voltage testing.
5) Board failure.  Burned traces.  Bad connector pins.  Common.  Will check visually and resistance.
6) Fuse blown.  Common.  Can be a bad fuse.  Or another issue like a blown rectifier.

Backbox -18 board ground
-18 Board Ground

Look for burned connectors.  These are especially common on machines with the transformer in the backbox (Image 3), but can also occur with cabinet transformer models.  If the connectors are visibly burned, replace the plugs and board connectors.  To replace the plugs, you will have to crimp new plugs

If this is a backbox transformer, rather than repairing this board (which is doable, but a pain), just buy a new board.  Cabinet boards are easier to repair, but replacement boards are available, too.  Some replacement boards come with new plug housings and crimp connectors!

For this game to boot, it must have good 11.9 V DC and 43 VDC (solenoid).  The others should be checked, too, but those two are crucial.



-49 Rectifier Board
-49 Rectifier Board

1) Turn power off.  
2) a) Unplug all the connectors on the other boards.
     b) Image 1 (-49 or -54 boards in cabinet).  Unplug all connectors except J6.
     c) Image 4 (-18 boards in backbox).  Unplug all connectors except J2.

See caution section above.  Voltages are above 200 volts and can kill.  Do not proceed unless you are qualified.

3) Connect voltmeter (DVM) black lead to ground.
     a) Image 1 (-49 or -54 boards). Connect to TP marked ‘GND’.
     b) Image 4 (-18 board) connect black lead to ground side of R2 or R1 (picture).
4) Turn on power.
5) Make the following measurements (note one is AC, the rest are DC):

TP1 – Image 1 (-49 or -54 boards). 6.5 V DC (5.5 to 7.2 is good).  Switched lamps.
            Image 4 (-18 boards).  5.4 DC (4.5 to 6.0 is good).

TP2 – 230 V DC  Display Voltage**.   200 to 250 V DC is good.
TP3 – 11.9 V DC.  Crucial.  11 to 15 is acceptable. 
TP4 – 7.3 V AC.  6.3 to 8.3 V AC is good.  Note that it is AC, so set the voltmeter.
TP5 – 43 V DC.  37.6 to 48.4 is good.  

** Note if you have LED displays installed, this fuse should be removed as high voltage is not needed.  Plus it is dangerous.


If these voltages are good, proceed.  If not, diagnose the problem.  Typically fixing this is to replace the board AND the plug connectors.  If replacing the board, always replace the plug connectors.

If a fuse blows on this board, it is usually a shorted rectifier.  If the voltage is low, it is usually a failed (open) rectifier.

Turn off the power and plug in all the plugs on this rectifier board.

Testing the Voltage Regulator / Solenoid Board – Images 2 and 5

The Voltage Regulator / Solenoid Board serves two functions when it comes to power:  +5 V DC power supply, and 190 V DC power supply.

The 190 V DC powers the displays.    The +5 V DC power supply is crucial to booting (starting up) the pinball machine. 

What can go wrong

1) Connectors.  Plugs and header pins.   Fairly common.
2) 5 V DC regulator.  Common.  This LM323 (Bally E-00710) fails with age.
3) Electrolytic capacitor.  C23 dries up and fails with age.
4) The high voltage blows out.


Many people just purchase a new board.  For those who do not do board work, that is fine.  But please do not throw this board out.  Sell it to someone who can fix it.  If purchasing a new board, expect to crimp new plugs.  This is especially important with J3 and J4.

This board is exceptionally easy to fix.   The fixes usually involve simple steps like replacing the voltage regulator (LM323), C23 and/ or header pins and crimping new plugs.  Or resolder where the header pins meet the board and hope for the best.

Bally rectifier solenoid board
Rectifier Board
Ground Points

It is best to use a ground on the board under test.  Once again, Bally did not provide a ground TP (test point).   The most convenient spot is the (-) side of the large capacitor C23, but be sure to not short that capacitor.  Or the (-) lead of C26. Note that the (+) side of C26 has high (190 V) voltage.  Otherwise, use the ground on the rectifier board.

1)  Turn off the power.
2) Insure that all plugs are connected on the rectifier board (previous test).
3) On this board, unplug all plugs except J3
4) On the MPU, unplug all plugs.
5) On the Lamp Board, unplug all plugs.
6) On the sound board, unplug all plugs except those that go to the speakers (if present).
7) Unplug J1 of the Aux Lamp Board (if present).

See caution section above.  Voltages are above 200 volts and can kill.  Do not proceed unless you are qualified.

Bally Voltage Regulator
Bally Voltage Regulator

7) If using the ground on the rectifier board, measure ground on the voltage regulator / solenoid board.  Either the (-) side of C23, or (-) of C26.  Note that the (+) side of C26 has high (190 V) voltage – stay away!
This should read less than 0.1 V DC.
8) TP5 – 11.9 V DC.  Same as TP3 from the rectifier board, or at most, down 0.1 V DC
9) TP1 – 5 V DC.  4.9 – 5.2 V.    Higher than 5.2 can cause issues with some games booting.
     TP1 – 0 V AC.  (Close to zero.) This is rough measurement of how C23 working.
10) TP3, TP6 & TP 7.  All should be the same as TP1.
11) + C23 connector.  Less than 0.25 V AC. If higher, replace C23.

Testing the High Voltage

If using LED displays, your high voltage (HV) should be off.  If using the old plasma displays, it might be wise to check it.  But this could run 230 V and can kill you.  See Cautions, above.

12) TP2 – 190 V DC.  170 – 195 is OK.  Note that most try to adjust this downward to 170 to 175 to make the displays last longer.  There is a small pot (variable resistor) to adjust this.  The screwdriver handle must be  well insulated.


If all voltages are good, proceed.  

If TP5 is bad, you have a connector plug or header pin on this board or the rectifier board that is bad.

If TP1 is bad, your voltage regulator is bad.  Replace it.  Even better, purchase EzSBC as a replacement.  

Note that because of the way Bally designed this circuit, the +5 may run a bit high.  If it is above 5.2, it could cause booting problems, especially with aftermarket boards.  Try removing one end of R49 to see if that lowers the reading to about +5.0.  Do this in a way that you can reinstall this resistor if this does not work.

If TP3, TP6 and TP7 are low, you have a connector (plug or header pin) issue at J3 on this board.

If the AC at C23 is high, replace C23.

If the high voltage is bad (high or low) then rebuild the high voltage, put in LED displays and sell your old ones, or get a new board.

Testing the MPU Board

See images 3 and 6 (above).   The MPU board is the brains of this machine.  It holds a small computer that has to boot and run properly for it to work.

There only have to be three voltages for this machine to start up:  +5, +11.9 and +43 (solenoid).  +5 comes from the regulator / solenoid board you just checked.  

What can go wrong
Bally Stern MPU - 17 - 35
Bally Stern MPU – 17 – 35

1) Connectors (header pins and plugs), wires.
2) Board problems.

This is the final step of this process.  If the MPU will not boot (startup) yet the voltages are OK, then it is almost certainly a board issue.  These MPUs can be fixed, but that is advanced work.  Many people get aftermarket boards.


1) Turn off the power.
2) Plug in J4 on the MPU board.
3) Confirm that J3 and J2 are plugged in on the rectifier board. J1 does not matter.
4) Confirm that J3 is plugged in on the Voltage Regulator / Solenoid Board.

All Other Plugs Should Remain Unplugged from the Boards.

5) Turn on the pinball machine.  If it boots, you should confirm the voltages anyway.
6) Connect the voltmeter (DVM) black ground lead to TP4 on the MPU board. This is ground.
7) Readings:
      TP5 / TP1 – 5V DC (4.9 – 5.2) – same as TP1 on the Voltage Regulator / Solenoid board.
      TP3 – 21.5 V DC  (18.8 – 24.2) – half the solenoid voltage. 
      TP2 – 11.9 V DC (11 – 15) – The same as TP5 on the Voltage Regulator / Solenoid board.


If these voltages are good, and your MPU does not boot, then you have a problem with the MPU board.  Fix or replace.

If these voltages are low, but the testing on other boards were OK, you have a connector (plug or header pin) or wire issue.  The 5 V DC comes from the voltage regulator board (J3 pins 14 & 15).   The 21.5 V DC is the solenoid voltage divided in half.  It comes from the rectifier board (J3 pin 12).  The 11.9 V DC comes from the rectifier board (J3, pin 8) via the voltage regulator / solenoid board (J3 pin 12 through pin 13).

If the 21.5 V DC is high but it measures OK on the rectifier board, there is a problem with the MPU board.

If all voltages are low, it is likely the ground is bad.  Ground comes from the Rectifier Board (J3 pin 15 & 17) through the Voltage Regulator Board (J3 pins 15, 16, 17; out through J3 pins 18 & 19).  If these are bad, the boards can still be grounded through the screws that hold the boards in place and through the metal chassis.  But this chassis ground is not secure.  If these other wired grounds fail, you will find that your game may suddenly quit or reboot. 

Final Step

If your voltages are good and the MPU works then:

1) Turn the power off.
2) Plug in all plugs except J1, J2 & J5 on the voltage regulator / solenoid board.
3) Turn the game on.

Everything should work EXCEPT the solenoids and flippers.  The sound board has its own +12 V DC power supply from the solenoid power that we are not checking here.

4) If everything works, turn the power off.
5) Plug in all plugs, including those on the voltage regulator / solenoid board.
6) Turn it on AND listen to insure none of the solenoids lock on.  If this happens, you will hear a ‘thud’ as the solenoid plunger gets pulled into the coil.  

If a coil stays on, turn off the game immediately and troubleshoot.  If left on, the coil may burn up.


Comments, including suggestions, improvements, errors, etc. are welcome (see below).

If you have a specific question about your game that does not directly apply to checking voltages on an early Bally or Stern pinball machine, please see our FAQ section.

2 thoughts on “Troubleshooting Early Bally and Stern Pinball Machines”

  1. I am an electrician trying to help a friend troubleshoot a Bally 8-ball machine. I am getting 4 volts on the MPU board and the LED comes on and stays on never blinking. The self test switch doesn’t do anything at all but when you turn the machine on the back lights all light up but the displays do not work nor the solenoids. Any help would be appreciated

    1. Hi Matt. We generally cannot take the time to respond to specific questions anymore. But we can throw a few ideas out at you.

      The first thing to do is check all the voltages. This starts at the rectifier board located near the transformer. There are test points there for you to use. Be sure to see if you are supposed to get AC or DC as one test point is AC. Note that dangerous high voltage is also there, at 230 V DC and ~180 V AC.

      From there, move onto the voltage regulator / solenoid board. The +12 and +5 are crucial. If the (nominally) +12 is too low, then the +5 cannot be right. There is a voltage regulator on that board, the LM323, and that is known to fail. But there can be other issues, too, such as the large electrolytic capacitor. And the 0.100 and 0.156 connectors.

      Look out for the exceptionally high voltage on that board. There is 230 VDC present there.

      Once all those voltages are OK, then move to the test points on the MPU board. That board will not boot if the +5 is below about 4.8.

      All of these steps, problems, testing and solutions are outlined on this page for you. If there are sections that are not clear, please feel free to point that out to us and we will do our best to make it more understandable.

      For more information, please check out:

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