mt777 wrote on 2021-10-04, 00:05:

I desoldered keyboard controller and put socket to replace with other chip. Not helped. Pin of 8042#22 - is not connected anywhere so problem must be somewhere else.
Do you think that replacing F82c206 is good idea?

206 doesn't control A20, it's just a single-chip solution that integrates DMA, interrrupt controller, timers and RTC. If the mobo is cached the A20 will be usually controlled by chipset, internally.
How do you know it's A20 issue? Is BIOS saying that? It could also be cache related problem.

it boots with strong delay and shows message about a20 gate.
when I swapped with other keyboard controller then boots immediately but stop before calculating memory and endless reset loop.

I can try desolder cache firstly...

Before you do that, replace the RAM sticks with some other ones. A faulty RAM can also give such errors. Also, try using reset button, see if that problem is only on power-on or always.

SIMM sticks will be too easy issue. I cleaned contacts. Sometimes in the past I had broken simm connector. Will check carefully using magnify, but don't believe that it is issue.

Desoldered cache chips (ok) and replaced Opti 295 chipset from the second alaris. Still problem with A20.
will desolder logic chips below simms. Maybe one is faulty.

I had to replace a few TTLs on my Alaris Leopard Rev. C to fix an issue where the cache would randomly not be detected.
While a dying/dead TTL is definitely possible, I'd check for broken traces and lifted pins first. (I also found some lifted pins on the 82C206, which I promptly reflowed, not entirely convinced that was part of the original fault though.)