Print

[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   

[ried]

Thoroughly impressed by the investigative work happening in this thread. Kudos. 

[sigma7]

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

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 !

[sigma7]

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 ?

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 !!