(Topic ID: 146760)

Need some System 7 MPU Help

By stangbat

5 years ago


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Topic Stats

  • 15 posts
  • 7 Pinsiders participating
  • Latest reply 5 years ago by Orbaddict
  • Topic is favorited by 2 Pinsiders

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#1 5 years ago

I need some help on a System 7 CPU from a Black Knight. The game booted and played but coils intermittently locked on. Different coils at different times. First things first: replace the 40 pin connectors. I did that and the CPU will no longer boot. 0 and the LEDs are locked on. They don't flash and go out, just lock on solid. Replacing the 40 pin header is the ONLY rework I've done to the board.

Here's where it gets weird. Leon's test ROM runs fine. It also runs fine with a driver board attached (used two different driver boards, no issues). Leon's test ROM runs fine with the EPROMs in IC14, 20, and 26. Only when I remove the test ROM and put the game ROM in IC17 does it lock up. PIAs test good with Leon's ROM. RAM passes Leon's memory test. Yes, I've verified the EPROMs, see below.

Here's a synopsis:
-No corrosion on the board
-IC12, 25, 19 previously replaced. Work looks good and checks out with good continuity.
-I've tested the ICs that were replaced: IC 12 (7408), 25 (4020), and 19 (5101 RAM), all are good.
-I replaced the 40 pin header (This is the only rework I've done to the board). I can find no shorts or issues. I even removed again it to make sure I made no mistakes. No difference.
-I erased and reprogrammed the EPROMS. I verified they are good in another System 7 MPU.
-I've checked all traces between the four EPROM sockets (IC14, 17, 20, 26). I admit I haven't checked the unused sockets (IC21, 22).
-I've applied 5v to pin 40 of the CPU, no change.
-I've checked for shorts to ground on all IC legs

Figured I'd ask here before I tear into anything. I can't find anything amiss using my logic probe and Leon's test ROM. I have an oscilloscope but haven't broken it out as I'm not sure what to look for.

Thanks!

#2 5 years ago

I would probably re - check your work on the 40 pin connector. Did you replace both male and female parts?

#3 5 years ago

I'd expect a solder bridge on the interconnect somewhere, or a bad connection on one of the interconnect pins.

Also possible that you could have a problem with the address decoder circuit on the board.

Might be IC15, the 74LS139.

#4 5 years ago
Quoted from Andy_B:

I would probably re - check your work on the 40 pin connector. Did you replace both male and female parts?

The 40 pin connector is no longer on the board. I removed it to check that there were no bridges or mistakes. It is behaving the same with the (replaced) connector removed as when it was in place.

Yes, I replaced the header pins on the MPU and the sockets on the driver board. I've cross tested the driver board with a different MPU and the vice versa. The driver board and its connectors are not the issue.

Quoted from johnwartjr:

I'd expect a solder bridge on the interconnect somewhere, or a bad connection on one of the interconnect pins.

Also possible that you could have a problem with the address decoder circuit on the board.

Might be IC15, the 74LS139.

Thanks John, but as stated above, the interconnect pins aren't even on the board anymore. I removed them to eliminate this possibility when troubleshooting.

I'll check IC15 and report back.

#5 5 years ago

Would you like to try my Williams Test ROM? It includes tests for sockets IC14,20 and 26 and does a more thorough testing of the memories. Leon's test ROM is a bit unreliable with that. Send me a PM for the images if you are interested.

Marco

The video is obsolete already because I changed the timing and also included IRQ generator tests.....(is there a pulse at pin 4 at the 6802?)

#6 5 years ago
Quoted from MarAlb:

Would you like to try my Williams Test ROM?

PM sent! Thanks!

#7 5 years ago

Are you now trying to boot the CPU with game ROMs but no driverboard? That won't work. The game code needs to 'see' the driverboard PIAs there or it'll halt the boot process.
Leon's test ROM is handy for that very reason - it'll allow the CPU to continue to run even with dead/missing PIAs on the driverboard.

#8 5 years ago
Quoted from yendor0:

Are you now trying to boot the CPU with game ROMs but no driverboard?

I know it won't boot, but you'll at least get a flash on the LEDs. I've got nothing, they lock on immediately. Either with or without a driver board.

Marco's been helping me and we're making some progress. Still don't have it narrowed down yet.

#9 5 years ago

Break out your O'scope and just probe through all the address and data lines while it is running. Sometimes you will see something that looks different and that can be a clue that something is shorted together... or open/floating. Check at different points for the same line and make sure that they are exactly the same on all the chips that use those lines. Static checks can sometimes miss cross shorts to other active lines. It's easy for those cases to not be a factor with Leon's code but is killing the execution with the real game code running.

Mac

#10 5 years ago

Finally! Got it!

I kept going back to what did I change before it quit working. It was the 40 pin header. I couldn't find any issues or problems with my work or any shorts. But it worked before I replaced the header!

Last night after some more thinking it finally dawned on me: Are there any vias going from the top of the board to the bottom through the 40 pin connectors? Sure enough, Pin 8 on the 40 pin header has a via from the top to bottom. I must have bunged it up when replacing the header. Oops.

Kind of embarrassing, but whew! We all can make mistakes when doing board work.

Many thanks to MarAlb for the use of his test ROM and his patience with me during the troubleshooting. He's a very helpful guy!

#11 5 years ago

You are welcome Erik. Nice to read you finally won the battle

Thanks for your feedback.

Marco

#12 5 years ago
Quoted from stangbat:

Finally! Got it!
I kept going back to what did I change before it quit working. It was the 40 pin header. I couldn't find any issues or problems with my work or any shorts. But it worked before I replaced the header!
Last night after some more thinking it finally dawned on me: Are there any vias going from the top of the board to the bottom through the 40 pin connectors? Sure enough, Pin 8 on the 40 pin header has a via from the top to bottom. I must have bunged it up when replacing the header. Oops.
Kind of embarrassing, but whew! We all can make mistakes when doing board work.
Many thanks to MarAlb for the use of his test ROM and his patience with me during the troubleshooting. He's a very helpful guy!

Glad you found it.

Sys7 is unique as it has VIAs on the 40 pin interconnect. Sys 3-6 don't. And if you get one of the real late Sys7 MPUs like used on Firepower II, which don't have the DIP switches, extra sockets, status LEDs, etc, the traces seem to be made of some really crappy material that easily lifts.

When one of those come in, I remember to be extra careful, make sure I use flux, and make sure my desoldering equipment is squeaky clean and operating at its peak potential.

#13 5 years ago
Quoted from johnwartjr:

Sys7 is unique as it has VIAs on the 40 pin interconnect. Sys 3-6 don't.

I work on very few System 7 boards but I see a lot of System 3-6. Plus I operate a couple System 6 games so I'm pretty familiar with their boards. I admit I probably got complacent when replacing the header on this board. You can blow and go pretty quick on System 3-6 headers. Learned a good lesson on this one.

#14 5 years ago
Quoted from stangbat:

I work on very few System 7 boards but I see a lot of System 3-6. Plus I operate a couple System 6 games so I'm pretty familiar with their boards. I admit I probably got complacent when replacing the header on this board. You can blow and go pretty quick on System 3-6 headers. Learned a good lesson on this one.

I assembled a system3-6 test rom also, would you like to receive that one as well?

Marco

#15 5 years ago

I've been racking my brain on a Sys7 board from a Joust for a customer. It had what I thought was minor battery corrosion, but turned out to be in the sockets and the 40 pin connector. Easily have 6+ hours into it. I have Leon's test eprom installed, but I can't find the "how to" on what to look for. Marco, I'd be interested in getting your tester as well.

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