General Category > Lisa Troubleshooting and Repair

Help please for my Lisa 2/5

<< < (7/8) > >>

pintoguy:
Thanks James. You helped me solve this problem today. As I was carefully measuring voltages on all the COP421 pins, I found out that pin 19 had a significantly higher standby voltage (1.33V) on the failed board than on the good one (0.66V). So I followed this trace to D5 that connects to the +12V line. I quickly realized that this +12V line was open near the edge connector ! This explains the ~1s ON state from the PS that got all the right signals, but then quickly noticing that there was no load on the +12V line, and switching to OFF  ::)

So now I'm back to Status #3. #1 was no power at all. #2 was power and IO error 57. #3 was blank screen with letter "H", indicating good CPU. #4 was where I was yesterday.

Never giving up  :)!

pintoguy:
Second update: Major progress! I put a brand new 8T97 chip at U2F, and I'm back to Status #2: IO error 57  :)

So at the end, I had two different problems: a faulty 8T97 and a bad +12V trace. Back to where I was a month ago   8)

ried:
Thoroughly impressed by the investigative work happening in this thread. Kudos.  8)

sigma7:

--- Quote from: pintoguy on September 04, 2023, 04:34:22 pm ---Major progress! I put a brand new 8T97 chip at U2F, and I'm back to Status #2: IO error 57  :)

--- End quote ---

Awesome!

I take it that the new 8T97 is not getting hot; if so that problem should take priority over the IO error.

I'm not clear on some details of status #2. Does the ROM version show as H/xx where xx is random, or is it H/ with nothing following?

pintoguy:
Yes, the new 8T97 is not getting hot.

And yes, as I described earlier in this thread, the ROM version shows H/9F, and sometimes H/91 (not H/A8). As I was playing around today, I notice that the activity on the data lines of the 6504 is quite erratic. Upon multiple resets, it sometimes shows something, and sometimes not. But in both cases, it still shows error 57.

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 ?

Oh, and the memory issue I had earlier (gibberish when I had the two 512k boards) solved itself. I put the second board in, and still got gibberish. But after I hit the reset button once, everything was OK. Not too sure what happened though.

Thanks again !

Navigation

[0] Message Index

[#] Next page

[*] Previous page