I recently had a problem with the ball release solenoid not operating on Gottlieb Genesis I was repairing for a collector. My problem was the ball release gate would stop openning intermittently. Sometimes after about 20 times in a row, it would start to miss a couple of times. The solenoid would stop for a short while and then start working again.
I needed a way to reproduce the fault so I could troubleshoot, so the easiest way was to press the outhole switch first, then the ball release switch, then continuously keep pressing the outhole switch. As long as a target was not, hit then the game would want to keep returning the ball to shooter lane.
The switch always tested fine during self test and I could hear the music change to signal the switch was being triggered so I was sure the switch was fine. Double checked the switch with a meter and examined the connectors and diode was present.
Also checked the wiring to the ball release solenoid, the under playfield transistor, the diode on the coil. I measured the resistance of the coil and compared this to the correct type on
http://www.flippers.com/coil-resistance.html. Coil checked out fine. While measure the coil, I continuously triggered the outhole switch and found that the coil was not being grounded or fired.
I didn't have the schematic for the game so I traced the underplayfield wiring back to the driver board and confirmed the correct wire with a continuity test. I now knew the transistor responsible for the ball release coil. The schematic for system 80 driver pcb is on
www.pinrepair.com The actual driver transistor measured fine when tested using a diode test on a meter. I then tried grounding the tab of the transistor and the coil always fired. So now the driver transistor, coil, playfield transistor, wiring all appeared to check out.
I then started to probe the actual 74175 controlling the ball release solenoid and could see that the output would stop, while continually triggering the outhole switch. So it appeared the 74175 was faulty. I changed this but still had the same fault. Even though the MPS-U45 driver transistor measured fine, I decided to replace this to rule it out.
At this stage I was suspecting a ground fault or the solenoid/lamp driver 6532 on the CPU was faulty. I did have another driver board which someone had given to me. The board was missing a few parts and had some damage but the section I needed to test was still intact. With this board installed the ball release solenoid worked every time so I now had confirmed the fault was with the driver board.
When looking at the driver board schematic, each 74175 (x 12) is connected to a common 4 bit bus and is selected by triggering a separate clock pulse on each chip. I probed the 74175 which drove the solenoid again and could see the clock pulse correctly but one of the 4 data bits would drop out intermittently. This was the data bit controlling the solenoid. This pointed to something else on this buss which was dragging the signal down. I carefully cut through the D3 leg on each 74175 using a good pair of side cutters and repeated the test. The solenoid worked fine and then I gradually re-introduced each chip back onto the buss by soldering the legs together until I found the affect chip. Replace the suspect 74175 and game works fine now.
