I fixed the switch matrix problem on the MPU and after about 30-45 minutes of use it failed again. I think I may have a prob with the game blowing the board.

I need some help deciphering these voltages.

I tested J8 and J10 wiring unplugged from MPU. Here is the results in DC with black lead on braided ground, and red on each wire with game in attract mode.

J8:
1- .1
2- .017
3- 2.226
4- .470
5- 1.307
6- UNUSED
7- 1.113 to 3.336
8- 2.538
9- .133

J10
1- .046
2- .208
3- .1 to .3
4- UNUSED
5- .048
6- .056 - .3
7- .072
8- .064
9- .023

Not sure what sense to make of it.....HELP

How can I make sure nothing on the game is blowing up the board. My pinbot does not have those high of voltages. They are all 0.0xx volts whereas this comet is 2.5v on some and 0.1xx and 0.3xx.

My thought is I put all new chips in the board and now they have failed again....

We were standing at the machine with it in attract mode and it tilted out for no reason. We went to play and any switch you touched would tilt and add player. In switch test wacky things were going on where all the switches would fire if you wanted only one. This was done with connectors were removed as to isolate problem with board only. Cold or hot, it would do this always. I pulled board and replaced both PIAs both 4011 buffers, and the main SN74LS244N decoder chip. Tested resistor networks and transistors. All were good. Reinstall board and all was well for a while. Now it is back to wacky things again.

I have shot gunned the entire switch matrix circuit on the board. So far:

-Replaced Both 6821 PIA's + Sockets due to cosmetic corrosion.
-Replaced both 4011 Buffer chips and sockets
-Replaced SN74LS244N decoder and its socket
-Replaced all (8) 2N3904's

You can typically fire it up cold and maybe get a half a game and in some cases a full game out of it before it starts to have phantom switches.

Safe to say im done with it. Unless anyone has some further advice.

I will check it. I do not beleive it could be that simple, but worth the try for sure.....

None of the shotgun parts I replaced tested bad. I put all the IC;s into a chip tester and they were fine. Other than the acid damage and cosmetic cleanup, it was all a waste, but now I know those components are good and are now socketed. The PIAs were rotted but still fine. During the unsoldering the legs fell off. So again the chips were fine, just the legs were not.

What I was not clear on, is when the board warms up (about 3 - 5 min) with the connectors for the switch unplugged unplugged, I can jumper for one switch and get all at the same time. I have disconnected the plugs therefore isolating the issue to the mpu and not the machine. J8 pin 7 and 8 have bounching voltages just sitting, and frankly all of them do, but I do not have anything to compare it to.

The problem happened when I removed the mpu for cabinet cleaning and about 2 hrs later it went wacky. I think flexing the board, disrupted a joint somewhere.

Actually not.

I unplugged the switch matrix connectors from the board. Powered on the game, while in attract mode, tested for stray voltages coming from the game on each wire hopeing to find and simple issue before condeming the board.

I have a oszifox probescope like clay used back in the day of his guides, but I honestly dont know how to use it to test this issue. I stick it on the board headers, and dont know what to look for. My pinbot works perfect and it too jumps all over the place on the probescope.

So to keep it easy moving forward, I want to power the game on, and test each wire on the connector WITHOUT them being plugged into the MPU. This way I can verify there is no lamp voltage and solenoid bleed getting into it roasting the chips. I assume it is as simple as leave the two off and booting the game up.