Confusing Apple IIc Plus Behavior

[oldpcguy]

I have an Apple IIc Plus system that I was given to troubleshoot. Initially the system powered on to a garbled display. I broke out the oscilloscope and started checking the normal things. The first issue I observed was the reset wasn't functioning correctly. It appeared UF17 was not functioning correctly so I replaced it. The system still exhibited the same behavior but the reset was now working correctly. So I continued on. UD19 also appeared to exhibit unexpected behavior and so it was also replaced. At this point I got the expected beep and display on power on. Great!

Next I proceeded to run the built-in memory diagnostics. This is where things became confusing. Sometimes the test would pass, other times it would not. The indicated faulty chip was never consistent, randomly changing from chip to chip. A memory chip with a faulty memory location(s) is difficult to test without a chip tester (which I do not have). Since there are only four chips I decided to replace them with new old stock. Doing so didn't change the behavior.

After having watched a YouTube video where someone ended up diagnosing their problem as a dirty socket I decided to pull all the socketed chips, clean them with an eraser and isopropyl alcohol and spray each socket with electrical contact cleaner the system came up and worked fine. Every time I ran the built-in memory diagnostics it passed. Running the Apple II Diagnostics repeatedly resulted in passing results every time. Running APEX ][ also resulted in passing results. It appeared cleaning the socketed chips was the answer. So I put it aside to troubleshoot the next Apple IIc Plus board.

Since I was experiencing odd behavior with the second IIc Plus board I decided to check some singles on the fixed board. When I powered it up it exhibited the same problems as when I initially started my troubleshooting. I checked the usual suspects (clock, reset, etc) and all look good. It appears all the previous work I did was all for naught, that some random thing resulted in a working board.

Question: What ideas does everyone have that could explain this sporadic behavior? What should I be looking for? I do have a third, fully functional IIc Plus that I can probe but honestly I am at a loss as to what to check.

 

[mattstock]

I don't know how prone the IIc's are to this, but I'd examine the board for bad solder joints. It's possible that the contact cleaner and the pressure of pushing the ICs into the sockets fixed it temporarily, but there's an underlying issue on one of the data bus lines.

 

[retrogear]

Since the problem cleared up after working on the sockets, I would say it is still a socket connection. As it's running, try flexing each chip to see if it clears up. You may have to replace the socket or solder the chip directly.

Larry G

 

[george]

The PSU seems to be common between the two motherboards. It would be my primary suspect. Anyway, these are highly commercial computers lately, many of us don't have them, hence no experience with the exact hardware.

 

[oldpcguy]

Thanks for the suggestions, I'll give them a try over the weekend.

@george I did suspect the power supply but it's the supply out of the working system. Unless there is something about the other board which may be causing stability issues. The IIc Plus is difficult to get information on. One thing that is making troubleshooting challenging is the only schematics I have for it aren't clear. I've been searching for an original reference manual so that I could create some clearer scans but finding a hardcopy has been elusive so far.

 

[george]

How come you have so many //c pluses? And most of them for repair? I purchased one probably faulty //c motherboard (otherwise why was it separated from the rest of the computer) but it will arrive in the late summer earliest. This will be my first "live" experience with a //c or whatever left of it. Anyway, there are plenty of other apple2s around that do the same if not better job.

 

[oldpcguy]

This afternoon I tried pushing down on the socketed (and non socketed) chips and did not observe any change in behavior by doing so. However while doing so the screen would occasionally change what was being displayed. Unfortunately it was doing so absent any pressure on the chips / board.

Any other suggestions?

 

[oldpcguy]

How come you have so many //c pluses? And most of them for repair? I purchased one probably faulty //c motherboard (otherwise why was it separated from the rest of the computer) but it will arrive in the late summer earliest. This will be my first "live" experience with a //c or whatever left of it. Anyway, there are plenty of other apple2s around that do the same if not better job.

I have a friend who runs an Apple rescue store and she has all kinds of Apple stuff (from the early Apple 8-bit to modern systems). People clearing out their basements, estate clearances, etc. Sometimes it has issues so she asks me to take a look at it.

The board I was initially troubleshooting appears to be an early version of the IIc Plus called the "Pizza Board". The only difference I can tell, aside from the silk screen, is the 74F00 is hand wired onto the board (and the pinouts of this wiring do not match the schematic but they are functionally equivalent) and there are a couple of pull up resistors (R60 and R61) are missing.

While this board is exhibiting odd behavior it is not the board which caused me to post in this forum, that is a different, production board.