Thanks for the additional info! In the meantime I also took out my trusty Tektronix T912 and did some test on the voltages:
- Configuration 1: LB out, rear case off. Connected the oscilloscope to the fan connectors on the AB. Powering on the clicking noise returned (at least some consistency). Looking at the osci the 12V was anything but constant 12V and instead "wobbled" between 8V and 12V, perfectly in sync with the clicking noise. Yikes! Measuring the 5V rail on the HDD connector yields similar behavior between roughly 3V and 5V.
- Configuration 2: LB in, rear case off. The clicking noise is gone (as before) and the 12V and 5V rail are definitely more stable. The machine crashed anyway after less than 2 minutes :/
- Configuration 3: LB in, rear case on. To measure at the HDD power connector I put in a piece of wire to connect to the Tektronix. Voltage now looks perfectly normal, switching to AC a bit of 50Hz is detectable but with scales at 20mV. But, more importantly, the CC now runs... and... doesn't crash? Huh? And no changes in the fan's RPM? Weird, but I'll take it.
I'm definitely not an electronics expert, but I can't recall any experience where adding consumers to a power supply made it run more stable. Anyway, this is not to say that everything works fine now, far from it. I'll definitely have to run more/longer tests (to check if the PSU *really* doesn't crash with the CC somewhat fully assembled) and fix some other stuff. For instance, the brightness control at the front now only darkens but no longer brightens the screen (something that worked last week) and, more importantly:
The Mac doesn't want to read something from the Floppy drive. At all. It just goes into the crossed-out flashing floppy disk icon. Putting a disk in does absolutely nothing, even when doing so before cold starting. This behavior seems really weird to me as, to the best of my knowledge, the Mac would at least eject the disk if it doesn't want to boot from it. And before it does that it should at least try to read something.
I know this doesn't have anything to do anymore with the initial problem I opened this thread with - so should I open a new one?
Edit/Update: Regarding Configuration 3: I now had the CC powered on/booted in the "no-boot-media" status for over an hour - far longer than ever in the last years. Still strange to me that more power consumption results in a more stable system.
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