Folks;
I have been working on bringing an old Macintosh II back to life. I'm not new to the process, but this one is stumping me.
- replaced both batteries
- replaced all the 47uF SMT caps
- replaced all the radial electrolytics for good measure
- repaired a failed trace in the power circuit (somehow I've had more than a few IIs and IIxes with this fault)
- cleaned the board with flux cleaner, then again with 99% isopropyl
- tested the assorted PSU rails
Symptoms are, power switch works to turn the machine on and off, ADB works to turn the machine on. No bong, no crash sound. Even if I remove the memory, I don't get a crash bong. RESET seems to be held low, so the CPU never comes out of reset.
I'm not sure if this is meaningful, but watching the RESET line relative to logic ground, I see 0V at power off, then it slowly climbs to 0.175V when the machine is switched on.
Clues, other tests to run? Thanks!
Answering my own question - before they were replaced, some of the leaky capacitors leaked electrolyte under the Sony sound chip at UI16, corroding a trace between the + side of C14 and pin 10 of the other Sony sound chip (at UI14). I repaired that, and the II is working again.
BTW if somebody else ends up troubleshooting a similar problem with similar symptoms but it ISN'T this trace, you might check some of the other nearby ones. UI14 is connected to both RESET and CLOCK and seems to have some role to play in bringing RESET high at the correct time, and possibly also suppressing CLOCK while the external reset switch is held. Using a logic analyzer I watched the clock stop and start itself at intervals, and if the reset switch was pressed, the clock would stop and never start again, so I suspected right away some kind of defect very close to this area of the board.
With the power on the clock should run constantly, as long as the reset switch is not held, and it should start immediately when it is released. The Sony chip at UI14 definitely appears to play some active role in this process.