I swapped VT83C41 to AMIKEY-2 I salvaged from dead 4DPS board and it almost works.
- Planting 74LS06 renders old VT83C41 completely unusable either with pull-up resistor pack (no key presses detected) or without it (some keys stuck forever).
- Replacing VT83C41 with AMIKEY-2 brings everything to life again.
- You have to update your BIOS to enable PS/2 mouse support
- You have to replace JP17 soldered jumper wires with fancy pin headers and jumper caps (see attached image) in order to be able to update BIOS in-circuit.
- With updated BIOS PS/2 mouse is detected and driver loads ok.
- UNFORTUNATELY every little move of mouse freezes the KBC
It is either some software problems or missing some pull-up resistors like R51.
R51 is a pull-up resistor on open collector output of 74LS06. This output is connected to Pin3 of 83C42 (AMIKEY-2, HT6542B, VT83C42, whatever) that is OSCO. Corresponding input is connected to SYSCLK (ISA B20). OSCI (Pin 2) in turn is tied to the ground. This is one of typical clock connections for 83C42. So improper clocking may lead to freezing of whole KBC.
Will check this evening.
UPD: I also noticed open JP18 and JP19. First one ties Pin 33 (GPIO P16) of 83C42 to ground. Second one connects Pin 36 (MS-OBFO) to some mobo internals, probably the interrupt controller. Seems I have to close JP19 to make mouse work.
Virus check complete. All viruses are working properly.