Mac Classic II Random Hangs, Logic+Power board recapped

[howardc64]

I get random freezes and errors. Sometimes doesn't boot up properly. Some times have some single pixel vertical lines when it hangs. Seems to be able to make it occur by wiggling SCSI+Floppy ribbon cables or have some shock motions (like inserting a floppy or tapping on the case)

Logic+Power board are both recapped. Nice stable 5 and 12V on the external floppy connector. Touched up solder on Power + SCSI connector on the logic board.

The way logic board sits in the chassis does create a lot of flex whenever pulling and installing the 3 logic board cables (power, SCSI, floppy). I wonder if there are common solder weakening problems due to this flex?

Just wondering if anyone knows of any common problems beyond logic+power board recap.

 

[360alaska]

You recapped it, but did you wash that logic board? A lot of times you really have too. Could also be bad ram. Can you post some pics of your recap?

 

[howardc64]

Should have mentioned a bit more detail

- Logic board recap was over 10+ years ago with tantalums and worked fine afterwards. Had no major leaks and backup batteries didn't leak fortunately. I cleaned up residues. Did lift the pads on one of the cap for sound. Repaired it with small electrolytic cap with legs soldered to the traces (boot bong sound not heard without repairing this circuit)

- Power board recap was more recent. Just recapped around the typical 5+12V power supply generation area.

- Also rebuilt one end of the SCSI ribbon cable connector (disassembled the connector, moved to fresh ribbon cables and recrimped it) Didn't help or make it worse.

Just tried it again, I can reliably tug SCSI cable lightly at the logic board connection and make bad screen pixels appear and hang the machine. I can feel how easy the board flexes so will check for the recap (and perhaps reflow) on the logic board to make sure all solder joints are still solid.

Here are the pics. Will post better ones tomorrow under more natural lighting. https://drive.google.com/open?id=0Bwns7f1Ly3k1dDZqYnF4M0pNRGM

 

[apm]

It's probably at least as likely to be a (nearly) broken trace or via as a failed solder joint. The most likely cause of that is leftover cap goo continuing to slowly corrode the traces, so look for your culprit around the caps.

Given that you see bad screen pixels appear consistently, it probably has something to do with RAM or the circuits accessing it, rather than say SCSI or floppy. From your photo, I'd start with the caps near the RAM chips (C15 etc.), and also maybe the big cap below the floppy connector. Look for any corroded or questionable traces or vias. It won't be easy though, since it seems to only fail when flexed, and you probably don't want to spend too much time flexing the board.

 

[howardc64]

Just an update, Still haven't found the problem

- Reflowed all the logic board recaps.

- Removed the big cap just below floppy connector, cleaned up everything and resolder it

- Recapped C15 since thats the messiest one due to lost pads. Made sure I got the cap onto the right trace/via points and no shorts along the traces. I did use a tantalum for C15 recap 10 years ago until recent rework in that area. Just lost all pads so decided to use an electrolytic with longer legs to catch the traces further away.

Some gentle tugs on the SCSI cable will hang the system. Perhaps I'll wait and get another SCSI cable to test later.

Anyhow, just an update. Not going to be as to find this fault.

 

[apm]

It's probably not the caps themselves, nor the pads they connect to directly. More likely, the cap goo has rotted out a nearby trace or via which is now intermittent. You'll need to test all the traces near the caps for continuity. It won't be easy since it only happens when the board is flexed.