Anyway, sure enough there's a fuse blown on the power supply board. So I turn off the machine, replace the fuse, and try again. This time I get 7 flashes! But in the background I can hear humming and then the fuse blows again. At least I know it has something to do with an energized coil. I take a quick look under the playfield and notice the post-up coil is a little dark. This was one of the coils I replaced (along with the post down, the saucer eject, and a chime coil). I read up on the SDB and learned how it works. Basically there's a simple circuit for each solenoid and relay coil. Signals come from the MPU which tell the SD which coil to fire. I tested the circuits and discovered 4 bad transistors. This made sense since I originally found 4 bad coils. I bought some parts and replaced the 4 circuits (consists of a diode, a power transistor, and a resistor). Then I powered it up again, and things got better, no more blown fuses on the power supply board. Now the problem was that the wrong solenoids were firing.
Instead of playing the little song with the chimes, the knocker and other solenoids were going nuts. I put the machine in diagnostic mode and let it cycle through the solenoids. I noticed several were not firing at all, and some more than once. When it got to the ball eject solenoid, the under-the-playfield fuse blew. A quick check found that I had wired this solenoid backwards. That also blew the transistor on the SDB, so I had to replace it's transistor, diode, and resistor again.
Now no fuses blow when the diagnostic mode cycles through the solenoids, but I still have the problem of the wrong ones firing. Not all, just some, and some don't fire at all. I read that if you ground the tab of the power transistor on the SDB, the corresponding solenoid will fire. I did that for each solenoid and no problems, the proper solenoid fired as I grounded the corresponding transistor. It's like the MPU is confused and is telling the SD to fire the wrong solenoids.
So here I am with mixed up solenoids. I tried everything I could think of. All the switches were OK. If I put the machine in diagnostic mode and tested the switches, all were sensed OK, and no switches were stuck closed. I had lots of suggestions from r.g.p.'ers, but a couple suggested the same thing: The signals from the MPU to the SDB were messed up, or missing. I checked the connectors and all was OK. I removed both boards and re-flowed the solder for all header pins. Some were cracked so I hoped that fixed it, but it did not. Someone else suggested that I swap the two PIA chips on the MPU board (there's two, one to drive the SDB, and the other to drive the lamp driver board). I swapped them and still had the same problem. Then I had an idea. On the SDB, there's a 4-to-16 decoder chip that takes a 4-bit signal from the MPU and "converts" it into 1 of 16 output signals. This is how the MPU tells the SD which coil to energize. If you know anything about binary numbers, there are 16 possible patterns in a 4-bit number. That's what the decoder chip does, takes a 4-bit input pattern and turns on (actually turns OFF) one of it's 16 output lines. I went to the Fairchild web page and looked up the 74LS154 decoder chip. I got a diagram that shows the 16 different input patterns and which output signal they activate. Then I got my Power Play schematic and wrote down the corresponding coil for each input pattern.
So now I have a chart of each input pattern (inputs are labeled A, B, C, D) and which solenoid should fire. The decoder inputs (A-B-C-D) are on pins 6, 5, 4, and 3 respectively. The Up Post relay coil is driven by transistor Q17, but is not energized using the SD's decoder. The flippers are enabled by the relay on the SDB, which is energized by transistor Q15.
|1000||8||9||Q13||Left Pop Bumper|
|1001||9||10||Q14||Right Pop Bumper|
|1010||10||11||Q9||Center Pop Bumper|
|1100||12||14||Q12||Left Drop Target Reset|
|1110||14||16||Q16||Right Drop Target Reset|
|1111||15||17||Not Used||Not Used|
By looking at this chart I notice that all the solenoids that do not fire all have the "C" input to the decoder activated. Ah-ha! That's the problem, the signal from the MPU board that drives the "C" input to the decoder chip is not making it all the way to the decoder chip on the SDB. Sounds simple. I get out my handy DMM and start at the "C" input of the decoder and start working my way back to the MPU PIA chip that send signals to the SD. I pick up a little resistance, but that's normal since I'm going though components. I make it all the way to the PIA socket and still have continuity. So now I'm stumped again, and I thought I had it figured out. The PIA socket is soldered to the circuit board on both sides and I made it all the way to the underside of the board. Just for the heck of it I put my meter probe on the top side of the circuit board, just under the socket. Still OK. Then, again just for the heck of it, I put the probe on the pin of the IC, and THERE IS IS. The freekin' socket is bad. There's no continuity to the pin of the IC. And only on the pin that supplies the "C" input to the SD decoder. All 39 of the other pins are OK. Well, there's no I way want to remove this socket since all 40 pins are soldered to both sides of the circuit board, so I get some of those snap-off SIPP sockets and make two that have 20 pins each. I remove the PIA IC and put the two socket strips in instead. The pins on the strips are bigger than the IC pins and make contact on all 40 pins. Then I put the PIA IC into the socket strips and replace the boards back into the machine. I cross my fingers again and power it up.
Finally, I hear the start-up song on the chimes, for the first time. What a nice sound. I put the machine in diagnostic mode and have it cycle through the coils. No problem, all fire just as they should. Then I reset the machine and play my first game! I can't tell you what a feeling playing that first game was, after so many weeks of work, and the frustration I had debugging the solenoid problems, this game was pure pleasure.
After a few games I notice one solenoid is no longer working, the left pop bumper is dead. I reset the machine and it works OK, then after a few pops it quits again. Once again I remove the SDB and replace the transistor/diode/resistor for the pop's solenoid and it's fixed. I guess the transistor was on the edge of going bad.
Anyway, I fixed it and it works! I then invite the whole family down to the basement and we play it for hours on end!
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