Amiga 4000 restoration and problems

[NeilBP]

Hi All, I really could do with some help here! I retrieved my A4000/030 from the loft a couple of weeks ago. It had been up there for over 25 years. When I opened it up there was signs of battery leakage and capacitor leaks. On powering it up I got a black screen unsurprisingly. Anyway, I identified broken traces and repaired them and it booted. The issue seems to have been broken traces from U177, flip flop IC. I’ve been gradually fixing other issues like no fast ram (now fixed), and I was trying to get the RTC going when I think I had a short between a couple of the traces wires that I had used to repair U177. It now won’t boot, black screen again. I replaced U177 as it was pretty corroded, and replaced all the trace wires, but it still won’t boot. I’ve checked the voltage on all of the pins, which look fine apart from the clock pin which is only reading 0.5V. Unfortunately, I don’t have an oscilloscope so can’t look at the wave pattern to see whether it looks right however 0.5V does seem quite low for a clock signal. I’ve had a look on PCB explorer and it appears that another flip flop IC (U354), and when I measure the voltage on its Clock pin it is also 0.5V. I’m really not sure what this means. Is U354 not working properly or is 0.5V normal for a clock signal? I don’t particularly want to pull and replace chips unless I am pretty sure that they are faulty.

[Castellen]

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.html

If 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