Electronics > Repair
Pinball CPU data lines pulled up to 1.2V
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drhex:
Hi,
I have a pinball CPU on my desk where D0-7 are being pulled up to about 1.3V with CPU, RAM and EPROM removed. Lots of logic chips on the data bus, already identified and removed a leaky buffer which helped somewhat. Any smarter way to identify culprit(s) than snipping off 5V from all chips and see what happens? Nothing gets hot or looks particularly untoward. Some battery damage but affected chips aren't in the area. Post removing the buffer signals look better but there still are times when I get undefined states.
Thanks,
Florian
floobydust:
It's not easy when so many parts are connected together on the bus. All you have left are the peripherals?
But in removing the CPU careful the address bus is not floating and making new symptoms that are a goose chase.
Bus contention can also be cause by a bad address decoder, where it is enabling two or more peripherals at once. Look at the chip selects/enables as well. And that the (non-driven) address bus is OK too. I wouild use a weak pulldown if it's CMOS when probing.
You can give us the model/schematic for better help.

If it's ugly, I use freeze-spray on each of the IC's.
up8051:
What pinball machine?
drhex:
It is a Sega board used in X Files and Lost World e.g. Without CPU all data lines are at 1.2V constantly, with CPU in it seems like they are at 1.2V whenever the lines aren't actively driven. Something is wrong. Anyway, there are about eight logic chips on the data bus, will cut their power pins one by one and see what happens. Surely the last one :-)... Board seems to be somewhat running as there is traffic on data and address. Also, the run LED is lit, sadly there is no test EPROM or any debug information (blink codes or such) for this board. I have a display connected so should see when it does something sensible. Chip addressing is happening via a PAL which also isn't particularly helpful. That is new though so should work.
up8051:
Try adding Pull-Up resistors and check if the voltage is forced to 5V.
The voltage on the unconnected input of the TTL gate can be about 2V
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