General Category > Lisa Troubleshooting and Repair

Help please for my Lisa 2/5

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sigma7:

--- Quote from: pintoguy on September 04, 2023, 07:41:57 pm ---I noticed that the P6A "PROM State Machine" socketed chip (U4B) is also quite hot. If I remove it, I have activity 100% of the time on the 6504. Could the failure of this chip explain code 57 ? What's the purpose of this chip ?

--- End quote ---

U4B is a bipolar PROM and will naturally run warm/hot as its power consumption is about 1/2 Watt. The video PROM on the CPU board is the same type (with different programming) so you can compare to its temperature.

P6A is the same PROM as on the common version of the Apple II Disk Controller board. It is used to encode/decode the bitstream to/from the floppy disk drive. The explanation of The Logic State Sequencer in chapter 9 of "Understanding the Apple II" is an explanation of its function (perhaps there is a better one, dunno). This state machine is indicative of the talents of Steve Wozniak... if you find it interesting or even challenging to understand, imagine coming up with it!

The "HardInit" portion of the I/O ROM reset routine will return an error if the state machine doesn't respond appropriately, so I suppose a failure of U4B could cause error 57. See this topic: Troubleshooting self-test Error 57 for a suggestion on checking for this situation.

pintoguy:
Thanks so much ! I also noticed that the +5V line of P6A (pin 20) is open vs. all the other pins. Shouldn't resistance always show some finite numbers (say a few Mohms), like it does for all others pin-to-ground values on that chip? In any events, I did buy a spare one on eBay.

And thanks for the troubleshooting info. Interesting that on the 2/10, they changed the P6A/LS323 chips to a pair of LS85.

I'm off for a trip abroad, so I probably won't report again until after Sept 16. Much appreciate your help again, as always !!

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