I agree, very unlikely that the ROMs are at fault. If there was a data corruption problem within them, this would be picked up by the checksum test during early boot.
Unfortunately I don't have a reference machine to test the Chip RAM and the A4000 crashes as soon as I want to open any folder or program on the workbench... But I have plenty of other SIMMs which I'll try after work.?
As others have pointed out, this behavour is consistent with memory issues. If you're able to boot without startup, run
Memcheck from floppy, you may get results before it crashes due to memory issues.
Another problem which can cause this is clock synchronisation, which can happen if the A4000 is clocked from its own onboard oscillator while the external CPU board is running from its separate reference. Though often this causes total boot failure, or failure during early boot. With the A3640, both clock source jumpers on the A4000 main board must be set to EXT.
If there has been corrosion damage around U177 (RTC latch) this part should be replaced, else there can be issues with the latch causing corruption on the CPU address bus. Likewise, if U178 is damaged, this can cause CPU data bus issues. Though usually in this case, the machine just won't boot at all. If in doubt you can remove U177 and U178, the machine will boot fine without them. Use caution; if you had problems when removing F175 (below), then you're unlikely to remove these parts without causing more damage.
You're right, possibly the lines under the Chip-RAM sockets are dodgy. I'll solder out that socket to have a better look at them. But honestly said, the Amiga worked more or less stable before I began to change the IC's and caps - except it sometimes didn't identified all of the 16mb Fast-RAM - that was initially the reason why I began to replace all the elements... Which chips are involved in memory management?
In the A4000, half of U250/Bridgette forms the bidirectional bridge between chip memory and other devices on the chip memory bus. But it's unlikely this is the problem. Has there really been damage to the chip memory tracks? This is the U261 socket. There's nothing common in this area to cause much damage, aside from major leaking of C190, which is fairly rare, or worse than usual battery damage. If there is visible damage to any tracks, these should be checked and repaired accordingly. Most damaged tracks in this area will be picked up by the early chip memory test in ROM, which shows a green screen for chip memory failure. Though this test isn't very thorough, it only does a simple write/read check at 16kB intervals.
There won't be soldering issues on the chip memory socket, but broken retaining clips or very dirty socket contacts are a possibility. As others have said, you _MUST_ use a known good/working SIMM for chip memory, else you're chasing the unknown at this point.
The machine should run OK without fast memory. But any fast memory access problems are usually caused by open circuit _R_W to pin 11 of the fast memory sockets and/or corrosion damage to U891 and/or damaged tracks/vias around U891.
Btw: When I replaced the Poly-Switch (F175) near the parallel-/serial-interface, I pulled out both VIA-connections :-/ Do you know if these are connected to an intermediate level of the multi-layered mainboard? I successfully bridged the lines on the upper- and lower-side, but I'll have a big problem if there are more connections inbetween... :-o
Why did you remove this for? Anyway, one side of this connects to the internal +5V plane - specifically the side nearest U350. The A4000D is a four layer board, the internal two layers mostly carry GND and +5V. So measure it, and if the U350 side of F175 doesn't connect to +5V, then just run a small jumper on the bottom side of the board to the nearest convenient +5V source, which is practically everywhere. If this connection is damaged, you'll just have no +5V_USR, so essentially you'll lose mouse movement. The computer will boot fine without it.
If you can't resolve the issues yourself, feel free to contact me for a repair estimate.
http://amiga.serveftp.net
