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CPU Board Debugging - White Screen

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fri0701:
Hi all,
I'm working to fix a CPU board of mine that's been broken for quite some time. I'm wondering if anyone has any thoughts.
The power button works and the light comes on. The screen is all white, with no beeps or clicks.
(it sounds very similar to this issue: https://68kmla.org/bb/index.php?threads/whats-the-problem-with-this-apple-lisa-2-10.43757/)

However, after probing around with my oscilloscope, it's a bit different than the linked issue.

I have a clock signal that looks fine, but the CPU seems to be doing... nothing. /RESET and /HALT are both high, the clock is on the clock pin, and the only other changing output is pin E of the CPU at ~0.5MHz, which seems expected after reading through the data sheet. There is no activity on any of the pins of the SRAMs - some are high, some are low, but none change.

I can confirm that all other boards in this Lisa work - I'm just swapping in the dead CPU card.

Any ideas would be appreciated! I unfortunately only have a few more days with access to this machine for a while, so I'm trying to get as much testing in as I can.

patrick:
Check or replace the electrolytic capacitor next to the 68000. This is a common failure. It gets leaky and the Reset is not generated properly.

fri0701:

--- Quote from: patrick on December 31, 2023, 04:21:54 pm ---Check or replace the electrolytic capacitor next to the 68000. This is a common failure. It gets leaky and the Reset is not generated properly.

--- End quote ---

I’ve tested (and replaced) that capacitor with no change. The reset line seems to stay low, then jump high as expected when I turn the system on.

sigma7:
What is the activity on pin 6 "/AS" after you push reset?

This goes low at the start of a bus cycle, and then the 68K would wait forever for a response from the hardware via /DTACK (pin 10) or /VPA (pin 21).

Rather than hanging forever, the CPU Board hardware will time-out the bus cycle with /BERR (pin 22) and if there is a double bus fault, the CPU will halt. See Troubleshooting Bus Errors

Any activity on those signals after reset?

Good luck!

fri0701:

--- Quote from: sigma7 on December 31, 2023, 07:30:28 pm ---What is the activity on pin 6 "/AS" after you push reset?

This goes low at the start of a bus cycle, and then the 68K would wait forever for a response from the hardware via /DTACK (pin 10) or /VPA (pin 21).

Rather than hanging forever, the CPU Board hardware will time-out the bus cycle with /BERR (pin 22) and if there is a double bus fault, the CPU will halt. See Troubleshooting Bus Errors

Any activity on those signals after reset?

Good luck!

--- End quote ---

/AS goes high momentarily when I push RESET, then goes low again shortly after.
/DTACK and /VPA are both high, regardless of the RESET button.

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