The Champion Pub – Switch Matrix Problem (Ground Short Row #)

It is not unusual to come across an error message GROUND SHORT ROW #. It is very likely caused by a damaged output of the switch matrix column driver ULN2803. Replacing this IC will in most cases fix the problem. What is sometimes more difficult and time consuming is to find the reason why the IC got damaged in the first place. The consequence of not making this effort can lead to repeated problems and more serious damage to the CPU board going way beyond the ULN chip.

The damage usually happens due to a short circuit between the switch matrix and one of the other voltages used in the playfield. It can be the +50VDC for the solenoids, or +20VDC for the flashers, or short between the switch matrix and lamp matrix which works of 18VDC or general illumination which uses 6.3VAC. 

Don’t be quick in replacing the ULN2803. We need first to narrow down the problem to what particular column got damaged. Of course you can use your CRO to do that but you may not have it with you.  If you switch the machine off and I would suggest that you also unplug the switch matrix connectors from the CPU board, you can measure the resistance of each ULN2803 output against the ground. You can measure it on the IC pins or straight on the pins of one of the column connectors. The damaged output will usually read low resistance. Once you know what output is damaged you know what column it is and this makes difference. You can concentrate on a particular column and the associated 8 switches in the playfield. 

You can try to measure voltage on the faulty column wire, switch the machine on but leave the switch matrix connectors unplugged from the CPU board so no further damage to the CPU board can happen. Most likely you will not find any strange voltage as these shorts are intermittent and often caused by vibration in the playfield. Try to induce some vibration to the playfield. Maybe you’ll be able to detect something which could be a useful hint for your further troubleshooting (remember to keep the safety switch in the coin door closed).

If the troubled column is used for slingshots there is a good chance that the ball got trapped behind the rubber and shorted the lamp with the slingshot switch and that could be all what happened. But look for other reasons too. Sometimes the column wire is pierced by the sharp edge of the solenoid lug or is pinched under lamp holder. Follow the wire through the playfield and check all the associated switches with that column as well. Look for damaged insulation or any possibility of short circuit. It is sometimes hard to spot. All it could be is just a bent terminal of the switch occasionally touching a lamp holder in near proximity.

I recently worked on The Champion Pub which could serve as a good example. There is a weak spot in the Speed Bag assembly and I can imagine that this problem can happen in other machines as well. The solenoid lugs can touch the metal assembly, connecting the solenoid voltage to the back support bracket of the speed bag switch (see Speed Bag Assembly - Picture 1).  There is an insulation between the solenoid lugs and the metal bracket which should prevent such short but it is not covering the lugs properly. You can also notice broken off insulation in the leaf switch which is there to prevent electrical contact of the switch with the back bracket. Picture 2 shows how the switch can get in electrical contact with the back bracket which carried the solenoid voltage of +50VDC (or more than +70VDC when not loaded) and this caused damage to the CPU board far beyond the ULN2803.