So I picked up an LCIII for repair. The TDK PSU was toast - caps are good but the optocoupler is open-circuit and I'm awaiting a replacement. So I bodged into service a spare Astec Beige G3 PSU and the machine booted-up fine on its aged caps.
I quickly recapped with tantalums (as you do) but the machine then refused to run .. the PSU shut down after a second or so. I fiddled around and discovered that the -5V rail was the problem and the machine booted with this disconnected. Now, looking at 68kMLA, I find that the polarity of C22 is mis-marked https://68kmla.org/bb/index.php?threads/lciii-recap-apple-design-fault-47uf-reversed.27834/. Evidently the original aluminum caps were wrongly installed, didn't work, but didn't really care; but tantalums behave badly with their polarity reversed (and may emit magic smoke).
I flipped C22 around and all's well
I quickly recapped with tantalums (as you do) but the machine then refused to run .. the PSU shut down after a second or so. I fiddled around and discovered that the -5V rail was the problem and the machine booted with this disconnected. Now, looking at 68kMLA, I find that the polarity of C22 is mis-marked https://68kmla.org/bb/index.php?threads/lciii-recap-apple-design-fault-47uf-reversed.27834/. Evidently the original aluminum caps were wrongly installed, didn't work, but didn't really care; but tantalums behave badly with their polarity reversed (and may emit magic smoke).
I flipped C22 around and all's well