I've been a proud owner of the original Amiga 4000 (040) for the past 15 years or so. Several years ago I stopped using it and stored in a basement. Soon after that I realized the risk of battery leakage and removed the leaking battery from the motherboard. Unfortunately, it had caused some corrosion to the motherboard near the battery location. Namely the shift-registers related to RTC, joystick and that mysterious jumper. I have since removed the shift-register chip related to RTC. Right after I removed the battery I checked that the device booted at least to kickstart. That was about six years ago.
A couple years ago I noticed that the Amiga no longer boots to the kickstart but rather ends up in yellow or green screen, no sure any more. This time I replaced the original PSU with ATX PSU and was able to boot to the kickstart a couple of times but mostly it ended up in yellow or green screen. I've also replaced some capacitors of the processor board. It didn't help.
Today the screen stays black and I would appreciate some advice what to try. I have some basic troubleshooting skills and have access to an oscilloscope etc.
When powering it on with only PSU, keyboard, CPU and memory connected, this is the behavior:
- Black screen with no visible colour changes (no white, gray or anything)
- Keyboard caps-lock flashes during startup and I can switch it on/off repeatedly but it freezes after 6-7. Some people have mentioned that this is a test for CPU. Somehow I believe CPU card is fine. What is the mechanism for this test?
- Lisa generates SCLK to read the shift-registers mentioned earlier (measured with a scope)
- All the other reset seems to be high but _IORST is not stable 5V or 0V but rather ~4V with visible (scope) ripple in the voltage. What could cause this? The signal from the reset circuitry is fine, the ripple is generated elsewhere. Dunno whether this is relevant.
- Resistance between GND and 5V is about 180 ohm without PSU connected.
I've also tried it with another processor card (rev 3.1) with same results. Also, whether I connect the riser card or not does not seem to have any effect.
I would appreciate any measurement-based (voltage, signals) troubleshooting tips. I've tried most mechanical ("push chips down") operations already. I have the schematics available as well. For example, how to test that the CIAs are working etc? Most chips are soldered down and I don't want to replaces them without good reasons.
Feel free to ask any details. I appreciate any help or advice.