Repairing Bally 6- & 7-Digit Displays

This page will help you repair your Bally 6- and 7-digit displays. The information is focused on the AS-2518-21 display. Since the AS-2518-15 display is interchangable with the "-21" display, everything mentioned here will apply to both displays. Bally 7-digit displays (AS-2518-58) work the same way, they just contain additional components to support the additional digit. If you don't know anything about electronic repairs, and you'd like to send them to me for repairs, Click Here for info.

First off, let's make sure your malfunctioning display is even repariable. If the glass display itself is shot, there's not much that can be done except to replace it. Here'a a couple of photos of things that can go wrong with the glass display.

Burn spots on the digit segments
You can't fix these but you can live with them

Broken glass nipple
This display glass is ruined

Then there's the circuit board. Most if not all of the components can be replaced without much effort, but if there is damage to the circuit board itself, it may be impossible, or unreliable to repair.

Burn marks on the driver board

Underneath - The traces are a mess

So we're assuming you have displays that do not fall into the categories above. If your problem is with the digits not being displayed properly, and it's NOT the MPU's fault, then it can be fixed, and fairly easily too!

FIRST THING - Before you go any further, there are two things you should do to ALL your displays, even if they seem to be working fine. First, reflow the solder on the joints for the header pins. On the bottom side of the circuit board, just heat each header pin solder joint with your soldering iron, then add a little fresh solder. Be careful not to create any solder bridges between ajoining pins, unless there is already one there. Next, replace the six 100K ohm 1/4 watt resistors with 1/2 watters. They are the ones with the color code brown-black-yellow, and are labeled R1, R3, R5, R7, R9 and R11. You'll find seven of them, but only the ones I just listed need to be replaced. For 7-digit displays there will be eight of them, so replace R56 too, along with the six I just mentioned. 1/4 watt is just too small for their circuits and often they are overheated which causes them to change their resistance. On the photo of the burnt circuit board up above, the area that was burnt was under 3 of these resistors.

SECOND THING - Conectors, connectors, connectors! There are lots of signals going from the MPU to each display driver, and if these signals are not making it due to bad wiring or connectors, then your displays will misbehave. Check all the display driver connectors for loose or broken wires, burnt pins, busted connectors, and replace or repair anything that looks bad. Remember that some of the signals are daisy-chained from display to display, and a bad connection at one display can effect the others "downstream". Then check the J2 connector on the MPU, since this is where all the signals come from. Next check the J3 connector on the solenoid driver board. There's a high voltage regulator used by the displays on this board, and J3 is where it gets the 230 VDC from the power supply (pins 6 & 3) and feeds the regulated 190 VDC to the displays (pin 8). Finally, check the J3 connector on the rectifier board, since this is where the high voltage comes from. Connectors are always a big pain!

Header pin solder joints. Notice the cracks around the
last two pins on the right
   Header pin solder joints after being re-soldered. No cracks

Can you find the six 100K ohm resistors (brown-black-yellow)    Here I've replaced them with 1/2 watt resistors of the same value (100K ohms)

Now we'll look at come common problems and briefly describe the cause, but I assume you've already replaced or checked the 100K Ohm resistors and re-soldered the header pins:

All displays are blank

Most likely cause of all blank displays is a lack of high voltage. First off, check F2 on the rectifier board. It's a 3/4 Amp slow blow fuse and it's test point on the rectifier board is TP2. You should read 230 VDC at this test point. If it's OK, then check the fuse on the solenoid driver board. The high voltage regulator's output is fused and if this fuse is blown then there's probably a short somewhere on one of your displays or the wiring harness. The high voltage regulator fuse is a 3/16 Amp, but it's a bit smaller (8AG) than a standard fuse, and a hard one to find. If the fuse is not blown, then the next thing to do is verify that at least 170 VDC is making it from the regulator to the displays. Check TP2 on the solenoid driver board. It should read between 170 and 190 VDC. If it does, then the regulator is working. While you're there, if your display glasses are not new, use the small trimmer pot on near the fuse and adjust the high voltage to be 170 VDC. Older displays work just fine on 170 VDC and it's less stressful.

Next thing to check is to be sure that the 170 volts is making it to the displays. Check connector pin J3-8 on the solenoid driver board, that's where the 170 VDC leaves. Check for a good connector, and check the solder joint on the header pin. The high voltage hits all of the displays at connector J1-1 and is daisy-chained from display to display. Check these connectors and the header pins too.


If you have 170 VDC at all of the displays, and they are still blank, then there is probably a problem on the MPU or the MPU connectors, such as a "stuck-on" blanking signal, etc.
All displays are missing the same digit
If all the displays have the same digit out, then the problem is most likely caused by a bad connector at the MPU. The MPU daisy-chains the digit enable signal to all the displays, and if the same digit is out on all, then it's either an amazing thing that they all have a problem w/ the same digit, or the digit enable signal is not present. Check the connectors on the MPU and all of the displays. If all seem well, then one-by-one, disconnect each display and see if the problem goes away. If it does, then there's a short or a bad connector problem on the board you just removed. Be sure to turn the power off between swap-outs, and wait 10 seconds for the high voltage filter capacitor to discharge before you remove any connectors.
One display is missing the same segments on all the digits
Each display has it's own decoder chip, which takes four inputs from the MPU and outputs seven signals to light the seven segments of a digit. If you're having problems with one or more segments on all the digits, then it's either the segment driver transistor or the decoder itself. I'm sure there's a way to test the transistor but I just go ahead and install a new one. If that doesn't fix it, then replacing the decoder will. I've not had a segment problem that was not fixed by replacing both segment drivers and decoders (other than MPU problems). Be sure to orient the transistor and/or IC in the proper position. You'll notice a flat side on the transistor and a small notch on one end of the decoder IC. Be sure they line up just like the one you removed, and that the wires of the transistor go into the same holes that the old one came out of. Also, if you're replacing the IC, use a socket, so it'll be easier to replace it next time. Here's a handy chart that shows which segment driver transistor drives which segment, and another diagram of how the segments are labeled. Just find the segment that's giving you trouble on the chart, then look up it's corresponding driver transistor. Then find this transistor on the circuit board and replace it.

Segment Segment Driver
a Q13
b Q14
c Q15
d Q16
e Q17
f Q18
g Q19

One digit is out on one of the displays
If you have a single digit out, then it's probably the the level shifter and digit driver transistors for that digit. Just to be sure, check the connector on the display at pins J1-4 through J1-9. These are the connectors that supply the digit-enable signals from the MPU. Your problem may be that the display driver is not getting the enable signal from the MPU. If it is, then just replace the two transistors and you should be back in business. Here's a handy chart that shows you which level shifter and digit driver are associated with each digit. You'll find the transistor labels printed on the circuit board, or look at the diagram below. Just look at this chart, find the digit that's blank, and then locate the two transistors listed for that digit. Find them on the circuit board and replace them.

Digit Level Shifter
Digit Driver
1's Q1 Q7
10's Q2 Q8
100's Q3 Q9
1,000's Q4 Q10
10,000's Q5 Q11
100,000's Q6 Q12
1,000,000's Q20 Q21

One digit is very bright, and the rest are out
I which I could remember the name of the guy that told me this, but I can't - sorry. Anyways, if you have one digit that's very bright and all the rest are out, simply replace the digit driver and level shifter transistors for that digit. Turns out sometimes a short transistor will draw so much current that there's not enough left to drive the remaining dights, and so they all go blank. Just use the chart above to determine which transistors to replace.
Display(s) flicker
This is almost always caused by broken solder joints on the header pins. If you already re-flowed the solder, double check them, then look for other broken solder joints and check all connectors for tight fits. Something is loose that's making them flicker.

That covers the most popular problems. I hope it covers yours. If you don't have the manual for your pinball machine, click on any of the thumbnails below for a large picture of the component layout of the all three boards. The 100K Ohm resistors are the ones colored brown-black-yellow.

AS-2518-21 6-Digit Display

AS-2518-15 6-Digit Display

AS-2518-58 7-Digit Display

Want a lot more info? Check out Clay's Bally Repair Guide

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Created 2/18/03 - Last Modified 1/19/10 - Steve Kulpa Mail Icon Nolensville, TN
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