New topic ‘cause I didn’t want to clog up Hilary with what I knew would be a lengthy ramble.

I struggled with an issue on my RFM after replacing some bulbs beneath the playfield. I powered on and started a game. Shortly, I started smelling burning and saw a wisp of smoke and immediately shut down. I got it in time, but it took a couple hours to figure out the problem. My methods may not be perfectly logical but they worked. Here is what I did...

After noting which bulbs had fried and cracking open the manual, I determined that it is column 5 in Lamp Matrix A that is overheating. I inspected the wiring and the lamp sockets looking for shorts. I disconnected the sockets, inspected them again and reconnected them one by one after replacing the bulbs. I powered up. The issue seemed to be better except Capture Zone Active in column 5 row 4 would remain steady on and over powered when I turned the machine back on. The DMM showed a steady 21V. We know there should be a pulsing 18V there when the lamp is on, but the meter shouldn’t even catch much more than 6V due to the pulsing. I continued disconnecting and reconnecting socket wiring with no help so I left the bulb in Capture Zone Active removed. Then I went through testing all lamps together and then individually using diagnostics. Everything, including column 5 except for Capture Zone Active, seemed to work fine. That is until I started testing column 6. Each time I tested a lamp in column 6 the lamp next to it in column 5 also came on. When testing that same column 5 lamp the lamp next to it in column 6 did NOT come on. At this point it seemed to indicate a bad diode somewhere in column 5. Unfortunately, all the diodes checked out fine with the DMM after removing the bulbs. Just to be thorough, I went ahead and checked the column 6 diodes and they were fine as well. (I have a pretty good idea as to what the problem is at this point, but I don’t want to accept it.) I powered the machine back up and every socket in column 5 came on and stayed on with too much power – very bright. I quickly shut down. Wow! Before it was just Capture Zone Active that stayed lit. (Or was it? I’m second guessing myself.) I thought that this behavior would also be caused by a bad diode, but they had all tested good. I hadn’t tested them yet, but I did a visual inspection of the transistors on the driver board and they appeared to be fine. I know looking fine doesn’t mean much, but in the past I’ve seen them blown apart on my F14-Tomcat. I don’t want it to be a transistor. I would mean pulling the driver board.

I flipped my DMM’s dial around so I could test continuity and stuck one probe on the grounding strap and touched the other probe to this circuit to see if it was shorted to ground. No. Power is off by the way. I clipped one probe to the bad circuit and went around touching other circuits checking for continuity to see if they were shorted together. Nothing I could find. Okay, time to face reality and check the transistor. Q21 is the driver for Lamp Matrix A column 5. I make some room and clip the ground probe to the transistors metal tab. I touch the other probe to one outside leg. 0.5 and change. Good. Test the other outside leg. 0.003. BAD. Test the center leg. 0.003. Okay. Gotta replace the transistor.

This is a developer prototype previous owned by Wally Roeder at Bally. He had marked the connectors on the driver board to be sure they would be correctly reconnected if disconnected, but had marked one wrong. I knew I wouldn’t remember this five minutes from now so I took some pictures to make sure I get things back together correctly. I removed all the connectors, loosened two screws, removed the parallel cable and removed the driver board. I test the transistor again with the same results. I clip it off and use an iron with built in suction to clear out the holes. Then I wait because I don’t have any TIP107’s or compatible replacements in my stash of parts. I ordered transistors, other spare parts, a couple of pieces that production machines have but were missing on the prototype and stuff that I just wanted – and I wait.

After my order arrived, I soldered in a new TIP107, reinstalled the driver board and powered up. All is good in the Martian world. I clearly moved or bent something when changing bulbs which likely caused a short that blew the transistor. The transistor stuck ON, sending 20V to everything in column 5. I’ve never found the cause of the problem, but I suspect I unknowingly corrected it while troubleshooting since I was messing with the same bulbs and sockets. I meticulously inspected to be sure nothing was touching or close to touching. Well, everything is close, but you know what I mean. The most exhausting part of the ordeal was constantly waiting for the machine to boot when powering off and on. But, you do what you gotta do.

Hopefully there won’t be a next time, but if there is ever an issue with an entire column in the lamp matrix I’ll listen to my brain and go straight to the driver transistor.

Looking back it’s kind of scary. What if the transistor had given up when I wasn’t around? Anything from repairable damage to the machine to my house burning down. Just because I changed a light bulb. I try not to think about it.

I’d like to thank all the people on Pinside who helped me out when I had issues with my F-14 Tomcat. I am far from the best pinball technician, but I took what I learned and applied it to the RFM. It took me two hours to diagnose the problem, but if I had listened to myself and just checked the transistor then it would have been quicker. Add another hour for the repair, putting things back together and going over everything to make sure it was kosher. Three hours! In the past it would have taken a week or more of pulling my hair out to figure out what was wrong. This time, instead of posting and hoping for help, I fixed it and then posted. Maybe my experience will help someone even more clueless than me one day.

I’m going to admit to one stupid thing I did although I’m sure the experts can come up with more. I was playing and realized many of the lamps in the top half of the playfield were out. I figured it would be a fuse and it was. I replaced it (correct fuse) – without investigating why it had blown. I paid a price.

The End