The U177 clock input (pin 9) should always have an AC waveform on it, so your DC meter will be trying to read the average voltage. I can't say if 0.5V is normal or not, as of course you need an oscilloscope to actually see what's happening. An AC waveform with an average of 0.5V and 0.5V DC will measure the same on a multimeter, you won't be able to see the difference between the two. The meter should display some kind of AC voltage if there is a clock waveform present, though what's displayed will depend if the meter shows a true RMS reading or not. You really need the correct tools to diagnose this kind of problem at this level, it's like trying to explain how to change a wheel on a car using a pair of pliers because you don't have a wheel brace
I wrote this RTC repair guide a long time ago, you could follow the continuity testing parts:
http://amiga.serveftp.net/RTCrepair.htmlIf U891 (in the fast memory interface) has been subject to corrosion, it would be worth removing that to see if the system starts booting or not. Though there won't be any fast memory present.
Failing that, I do component level repairs if you want to send the main board to NZ. Contact me for an estimate:
http://amiga.serveftp.net
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