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Bally & Stern Solenoid Driver Board Modifications

Many of the Technical Articles on our site contain information and directions involving electronics and circuit board repair. They are authored with the assumption that the reader has adequate experience and knowledge required to do the work being described.

If you do not feel qualified, or are in any way uncomfortable doing any of the work described in any of the articles, then we strongly recommend enlisting the help of a qualified repair person or shop who can do the work for you. It may save you cost, time, and further repair work.

We (Action Pinball & Amusement, LLC) are not responsible for any damage to you, your game, or your property, from doing any work on your game related to any of the articles listed on this site.
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This article deals with improving the design of the solenoid driver / voltage regulator boards used in Bally and Stern pinball machines from 1977 through 1984.

There are basically two "weak" spots in the design of these boards. Each will be covered separately below, along with individual modifications.

#1) 5 Volt Regulator Connections:
The 5 volt regulator circuitry on the solenoid board requires a lot of current, but only has one single pin with small gauge wire for each of the positive and negative lines between the solenoid board and rectifier board. This is a huge bottleneck, and is the cause of connectors scorching and burning up on both the solenoid driver and rectifier boards.

To modify this circuit it is necessary to provide a better (larger) path to ground from the 5vdc regulator circuitry. The only pin originally designed for ground return from this circuit is in connector J3, pin 10. This pin is commonly scorched, and sometimes the whole connector housing is burned or broken in half at this point. To make a better path to ground, a jumper wire is installed between the negative lead of the C23 filter capacitor and any of the block of multiple ground pins at connector J3, pins 18 through 22. These pins all lead back to chassis ground on the rectifier board (as well as the original line at pin 10), so this modification essentially "widens" the ground path for the 5vdc regulator circuit.

C23 leads - before

Scrape copper trace clean for soldering

Solder jumper wire
(Images shown above are for the more common "late" solenoid board. See below for info on the "early" board.)

While making this modification, it is STRONGLY recommended that you replace your C23 filter capacitor, if it appears to be an older, or original part. This is a common failure part on this board, and is the main cause for games going "dead", operating intermittently, or resetting during operation. We replace the C23 capacitor on ALL of the older Bally and Stern games (1977-1985) that come through our shop- along with fixing bad connectors and doing the modifications described in this article, it is the only way to make this circuit reliable and ensure better game operation.

NOTE: We have a C23 Capacitor Kit available, along with other associated electronic parts and connectors. Click here for more information and online ordering.

Late Boards:
Late Bally boards have a wide ground trace that runs right below the negative plate-through hole for the C23 capacitor, when looking at the board from the back side (solder side). To make the modification, scrape off a small section of coating over the ground trace, right below the negative lead for the C23 capacitor. Then install a short jumper wire (or even solder wick) between these two points. Modification complete.

Early Boards:
Early Bally boards did not have a ground trace that was as conveniently located as the one on later boards, so for early boards, simply install a longer jumper wire on the back of the board between the negative point of the C23 capacitor and the ground pins 18-22 on the J3 connector.

We recommend using a heavier wire for the jumper wire- 18 or 16 gauge wire is fine.

Solenoid Board Connector J3:
It may be necessary to replace the 25-pin J3 connector housing on the solenoid board (upper right corner) so that you have more reliable connections at this location.

This is more likely the case in later (1981 & up) Bally games, as they used the low-quality IDC press-in connector housings, where wires were simply pressed- or staked- into the back of the connector housing without requiring soldering or crimping. The J3 IDC connector housing, when heated up over time, will get brittle, and break right in the middle. IDC connectors are also notorious for allowing wires to wiggle loose over time from vibrations/old age. Even if the wires look like they're secure, they can wiggle and cause intermittent connections.

Earlier games used the older-style Molex crimp-and-solder connector housings with crimp pins, which are much more reliable. We suggest using the Molex connector housing with crimp pins if your old J3 connector housing is in poor shape. We also recommend using a tiny bit of solder on each crimp pin (after you have crimped the wire) to ensure a firm, reliable connection.

Older games that use the Molex housing at location J3 can have their old original internal crimp pins suffer from corrosion, oxidation, and metal fatigue from decades of past use. So while the exterior of the connector may look fine, the pins inside may be in poor shape. We have seen many of these where the pins inside will simply break from old age and parts of them will fall out of the housing. If your housing is in good shape, you can remove the pins and replace them with new ones into the original housing.

Replacing crimp pins in J3 or any other Molex connector housing is a time-consuming, intricate, tedious job. Neatness absolutely counts. If you're not up to the task of doing this confidently, we strongly suggest enlisting the help of a qualified service person to ensure the job is done right. If doing it yourself, we recommend doing 1 pin at a time (to make sure you don't get a pin/wire in the wrong position during installation), and being very careful to neatly crimp and solder each pin. Don't use too much solder or you may solder closed the tension tab on the bottom of the pin that holds it in place in the connector housing. Or the pin won't go into the hole in the housing if too much solder is present. Crimped/soldered pins must fit effortlessly into the connector housing, and give an audible "click" when they snap into place. Always buy several extra pins in case you end up damaging some during installation (very easy to do).

Make sure also that the header pins for J3 are clean and in good condition. They may be oxidized or scorched from problems in the past, or have plating worn off the pins from vibrations or from having been disconnected/connected hundreds of times over the decades, in which case they should be replaced.

Rectifier Board Connector:
We also recommend replacing the connector pin for the positive lead of the C23 circuit down on the fuse/rectifier board. This pin is at connector J3, pin 8, on the fuse/rectifier board (long bottom connector) and is a small gauge orange wire. This connection is a bottleneck for the C23 circuit, and tends to run hot due to being under-designed. You will usually see signs of burning/overheating at this pin on J3, and it's not uncommon for the J3 connector to break in half and fall apart at this pin, due to this problem.

We recommend replacing connector pin 8 in the J3 housing with a higher-quality Trifurcon pin, and properly crimping and soldering it to the wire.

This is assuming your fuse/rectifier J3 connector housing is a Molex style housing that uses crimp pins. Some later games may use an IDC connector housing instead, in which case the whole connector housing has to be replaced if one pin is burned/damaged. We recommend replacing the IDC housing with a better-quality Molex style housing with crimp pins, for better reliability.

Make sure also that the header pins for J3 are clean and in good condition. They may be oxidized or scorched from problems in the past, or have plating worn off the pins from vibrations or from having been disconnected/connected hundreds of times over the decades, in which case they should be replaced.

Connector Pin Solder Joints:
The back sides of the connector pins on all boards (and displays) should be inspected for cracked solder joints around the individual pins. This is the #1 most common problem on these old games. Solder should be added/reflowed on cracked pins to remedy this problem. We also strongly recommend adding DeOxit anti-oxidant/lubrication compound to the connector pins for added reliability and to reduce stress on the pins and board.

Circuit Overview:
Power (positive) comes from the rectifier board at connector J3, pin 8 (J3-8), in a small orange wire. This wire connects to the solenoid board at connector J3, pin 12 (J3-12), and usually burns/scorches at J3 on the rectifier board.

Ground (negative) comes from the rectifier board at J3-17, in a small white wire with brown trace. This wire connects to the solenoid board at J3-17, and usually burns/scorches at J3 on the solenoid board.

#2- Solenoid Driver Logic Buss:
Current for the logic buss in the solenoid driver circuitry on the solenoid driver board is supplied from the 5 volt regulator circuitry on the same board. However, for some unknown reason, the path for this current is actually routed off the board, in the J3 connector, then back into the board, also through the J3 connector. Essentially, there is just a small "loop" or jumper wire in the J3 cable connector housing where this path leaves the board, then comes back- specifically, between pins 13 and 25.

This path basically connects the two test points TP1 and TP3 together. The problem with this design is that if any poor connections develop in the J3 connector (dirty connector pins, scorching, or burning), or if any solder joints on the backs of the pins develop cracks (common in older pins), then the driver buss won't have it's full share of current (or no current at all), and this can cause coils to either cut out altogether (die), or can cause some coils to operate sluggishly.

To modify this circuit, simply solder a jumper wire between TP1 and TP3. This can be done on the top side of the board, or on the back side, to hide it, and keep it looking neat. This added jumper connects the driver buss to the regulator output directly, and bypasses the factory J3 connector "loop". This modification simply provides better reliability for the coil driver circuitry. Problems don't occur with it often, even on un-modified boards, but if they do, it's better to be covered, and this is a quick, easy modification to make, especially while you've got the board out of the game.

TP1 and TP3 Connected

IMPORTANT NOTE: Be sure NOT to mistakenly connect TP5 instead of TP1. TP5 is located just above TP1 on most Bally boards, and if you connect TP5 instead of TP1, then you will be sending unregulated 12vdc (13.5-16.5vdc) down the 5vdc line which can destroy components and chips on ALL of the boards in the game. Be sure to ONLY connect TP1 and TP3.

NOTE: Test point locations can differ from the above photo, depending on what revision Bally board you're working on. They're also in different locations on the various Stern boards that were produced over the years, so don't go by the picture alone- be sure to identify and connect the correct locations on your particular board.

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