All my 286 CPUs get ridiculously hot (enough to burn fingers), I always glue a heatsink on top of them.

I have the exact same problem as you had "Gate A20 ERROR, 8042" on a AMD 386SXL25 motherboard. It's battery badly leaked, so I am still in search of eaten via's, but for now everything seems ok.
Been searching on the internet how the KBC (intel p8042ahp) and buffer connect (SN7407N) but not a slight chance of finding that info. Can somebody help?

I have this motherboard https://theretroweb.com/motherboards/s/mg-8517-rev:2.2 or is the closest to this at least visually.

Martin85 wrote on 2022-11-22, 17:06:

I have the exact same problem as you had "Gate A20 ERROR, 8042" on a AMD 386SXL25 motherboard. It's battery badly leaked, so I am still in search of eaten via's, but for now everything seems ok.
Been searching on the internet how the KBC (intel p8042ahp) and buffer connect (SN7407N) but not a slight chance of finding that info. Can somebody help?

I have this motherboard https://theretroweb.com/motherboards/s/mg-8517-rev:2.2 or is the closest to this at least visually.

Keep in mind it 8042 is just an universal microcontroller. It has to be programmed as KBC to function as one. Your best bet is to source this part from some broken motherboard (it doesn't have to be exactly Intel P8042AHP - pretty much any 386 era KBC will work). 7407 is standard 74 series logic chip which you can buy in pretty much any electronics store online (mouser, tme and so on) or not.

Martin85 wrote on 2022-11-22, 17:06:

I have the exact same problem as you had "Gate A20 ERROR, 8042" on a AMD 386SXL25 motherboard. It's battery badly leaked, so I am still in search of eaten via's, but for now everything seems ok.
Been searching on the internet how the KBC (intel p8042ahp) and buffer connect (SN7407N) but not a slight chance of finding that info. Can somebody help?

I have this motherboard https://theretroweb.com/motherboards/s/mg-8517-rev:2.2 or is the closest to this at least visually.

If there's damage to the board around the KBC, pin 22 would be the place to check since it's the A20 gate...

Checked... but cannot figure where does pin 22 of the KBC go... The pin of the socket on the back side of the board is a dead end... that means it should be on the top of the board, but also cannot see it go anywhere...

Now... or is the path hidden under the socket, or is the PCB a multilayer board, which would make this near to impossible to fix (but this is rather improbable for a pcb of that era and price).
So I guess I will have to try and take out the socket... 🙁 That was something I wanted really to avoid... Will try tomorrow with another KBC from another 386 motherboard (as far as I know they are all the same, and if the datasheet confirms the two chips are electrically identical then I see no reason why it wouldn't work right?).

Martin85 wrote on 2022-11-22, 20:17:

Checked... but cannot figure where does pin 22 of the KBC go... The pin of the socket on the back side of the board is a dead end... that means it should be on the top of the board, but also cannot see it go anywhere...

Now... or is the path hidden under the socket, or is the PCB a multilayer board, which would make this near to impossible to fix (but this is rather improbable for a pcb of that era and price).
So I guess I will have to try and take out the socket... 🙁 That was something I wanted really to avoid... Will try tomorrow with another KBC from another 386 motherboard (as far as I know they are all the same, and if the datasheet confirms the two chips are electrically identical then I see no reason why it wouldn't work right?).

I don't think 386SX board would be multi-layer. If you can trace the path on one side of the board it should be pretty easy to spot where it comes out on the other side (there should be a via). If it's multi layer board there will be just a via on the other side without any traces (there are no vias that go just through the some layer - they alway go all the way through). Also you can shine some strong light through the laminate (like 500W halogen or LED equivalent strong) which will reveal it's secrets. Perhaps it simply goes under some component that's why you can't spot it... To be honest I gave up wasting my time on repairing boards like this some time ago. Not really worth it while you can still easely source them in working condition (simple math - I don't charge my clients less than 40 EUR for 1 hour of my work, and I can buy 286 and 386 working boards just for this amount, so unless it's some super rare board or the fix is not obvious and won't take less than an hour... then sorry motherboard, you are not worth the effort).