Thanks. I will check all of U14. Also, I was thinking about tracking down the the sound issues at start up, with the idea that it is related to the other issues and it seems it would be the simplest to diagnose. I have already bench tested the sound board, and it is functioning fine. After looking at the mpu schematic, it would appear that the buffer at U15 could cause all sorts of problems. Am I correct in assuming that the outputs of U15 should match their respective inputs? If so, it would be easy enough to check.

Would I expect to see increased voltage, or does it just allow for more current?

I checked the inputs and outputs of U14, as Quench had suggested. All looked good. Pulsing on all pins. Then I was going to try and test some chips of the output section of the board, but the badly tarnished pins of U19 have always bothered me, so I decided to change it out. Put the board back into the machine, turned it on, and was machine gunned for the first time! MPU led was off! Hit the start button, ball kicks out, and start tune plays. I played a couple of balls, then decided to go into diagnostics to check lamps, solenoids, and switches. While on the lamp test, it stopped. I thought the routine had timed out, but no. Restarting the machine proved I was back to where I was. To say I was deflated, is the understatement of the year. Then, out of disqust I suppose, I decided to replace all the remaining ic's on the board. I knew the problem didn't reside with these chips, but I did it anyway. Turned the machine on, of course it didn't start, but it did add the ricochet sound to the noise at startup. Something I hadn't heard before. So, something changed. At this point, my suspicions of a bad connection somewhere, were pretty much confirmed. I tried reseating, and slightly flexing the sockets of, each chip. Attempting to start after doing a couple at a time. After the 339's at U18 and U23. It started up. I was able start and end a couple of games. Diagnostic mode worked fine, and showed some bulbs out and some switches not working. Solenoids all fired, but a couple of the pop bumper switches aren't registering. A couple of the displays have a digit out. All Mickey Mouse stuff, compared to diagnosing the board issues.
So, I know there is a connection issue on the board. I have reason to suspect U18, U23, or the immediate vicinity. This is actually a little encouraging because that section of the board was badly damaged by corrosion. Even after thorough cleaning, sanding the traces, using plenty of flux, solder did not want to flow there. At least it is a place to start.
Sorry for the long winded post.

Tom

Freeze spray and heat gun?

I knew of freezing components to test, but never thought of trying it with traces or connections. Thanks!

Looking at the schematic, I should have realized this. Of course, you are right. They only provide input to the PIA. Well, there is no denying that the area around U26 is a disaster. It was hit hardest by the corrosion, with entire sections gone. It's okay, though. I now know the board can work, a gremlin may be annoying, but eventually it will be found.

Update. I haven't found the location of the intermittent connection on the mpu yet, but I haven't spent much time looking for it, either. It has started every time, so I decided to address the other issues. Everything was quite minor. It didn't take long to get all the lamps, switches, and solenoids working. Even the display problem was just a bad connection. Would you believe, there wasn't a single burned out bulb on the entire machine? A few of the lamp sockets are sketchy, and the pop bumpers badly need to be rebuilt, but it is a fully functioning machine. Now begins the teardown, cleaning, and rebuild. Maybe I'll start a new thread for that.

Quench,
I would like to take a moment to publicly, and personally, thank you for all the assistance you have given. You should know that the guidance and information you provided, has not been wasted or forgotten. Your contributions, and the research it prompted me to do, have given me a much better understanding of how these systems work. I thank you for that.

Tom

I have always wanted an oscilloscope for the audio stuff I do, so it will get used.