Need help fixing A4000

[Hanzu]

I get a black screen regardless of whether or not chip RAM is installed

Try plugging mouse to mouse port and then to joystick port and see if you get any video signal. If possible also try plugging mouse and joystick simultaneously or even better two mouses.

This may sound like a joke but helped me to repair one A4000 Rev B after first trying to find the faulty part myself and with the great help from Castellen.

My A4000 Rev B symptoms:
Since I only got black screen with no video signal. I never bothered to plug in mouse or joystick, but when I did they closed the circuit somehow and gave classic "Insert disk image of Kickstart 3.1". Then I held both mouse buttons to get to Early Startup Menu and notices mouse buttons worked but mouse pointer did not. This gave me boots to change U975 and U976 which I had not changed yet, because they looked ok with no leakage around them.

After changing U975 and U976 A4000 Rev B started to operate normally! Either one of these troublemakers managed to not only prevent boot but also to kill video signal too which fooled me to think there was some bigger problems so before finding the fault I had already PLCC socketed and changed both CIAs, Paula, Super Buster and U460 (video DAC?).

Changing CIAs is very often suggested but believe when Castellen suggested is that that they require someone using Parallel or Serial ports to accidentally break them. Some decades ago I broke A500 CIA by hot unplugging laplink or was it null modem cable. After seeing broken U975 and U976 I am not than eager to hot plugging or unplugging mouse or joysticks either. People plug and unplug them like USB ports for some 10 years which may cause damage eventually. If you disagree feel free to pass on your arguments.

[Hanzu]

I tried plugging a mouse into the mouse port and attempting to bring up the early boot menu. That didn't work. I did monitor the serial data out of U975/U976 and I believe they are working correctly given the set of inputs they have going to them, so I don't think that's the problem in my case. I am tempted to throw the CIAs on there, just because I have them. I have a couple of PLCC sockets to fit as well, so it's not a big deal for me to do. Couldn't hurt to try.

Sorry I didn't read your fist post carefully enough at first and only noticed now that you have video signal of some sort which means it is not the same problem I had.

When socketing PLCCs you may notice that sockets and motherboard pads are not aligned perfectly with either one not following the standard properly. So it requires very careful insertion and careful first soldering to make it freeze correctly without making a short circuit by PLCC pin touching 2 pads instead of one. Also if not paste soldering PLCC sockets then the tricky part is to hand solder through PLCC holes with minimum size solder tip. Especially corners are tricky. I didn't want to use paste soldering because I didn't want to heat the motherboard and its components all around. But anyway I believe you know these things well too.

Make sure those Kickstart 3.0 chips are not accidentally mixed, reversed, pins bent under or scratched motherboard traces under them. This is a place where previous owners often touch with their tools. In a hurry plug valueless Kickstart 3.0 chips so they can sell more valuable Kickstart 3.1 chips separately.

I would start by trying with another ROMs and another CPU card. Seeing some high resolution pictures of the motherboards might rule out something too.