So I managed to hunt down a 25mhz Harris 286 chip to plug into my 1992 vintage HT12A based motherboard. Next step is to source some appropriate oscillators but i've come up with little info as to what kinds are appropriate. I've searched the part numbers for the ones on the board and come up empty. One is marked "X-TAL OSC PH-1100 25.000Mhz PRE" and the other has a "T" inside a diamond with a date code and "X0-105BIC 32.000000MHz". I believe the first is the oscillator used for the "12 Mhz" setting for the CPU and is also used with different dividers for the coprocessor (likely 1/3, 2/5, and 1/2). I'll socket this one but I won't replace it. The second I suspect is the oscillator used for the 16 mhz setting for the CPU (1/2 divider most likely). This is the one I plan to socket and swap out. There are 2 other oscillators in line with the ISA bus: one full can @14.318180MHz marked otherwise identically to the 32MHz oscillator (likely for EGA and CGA cards), and a 2 pin oval can at 8mhz (I suspect for the ISA bus). There is also a small watch type crystal near the Hitachi RTC chip (obvious what it's for). The ones on the board are mostly full can DIP-14 style (already have the sockets), and I guess 5v TTL output is what I want because early 90's mobo not early '00 mobo. A search on Digikey got 5 hits: 2 @ 40 Mhz (different part numbers but the specs seem identical), 2 @ 50 MHz (same as the 40s) and one at 48 Mhz. :

40 Mhz #1: https://www.digikey.com/en/products/detail/ec … 00AX-400/827243
40 Mhz #2: https://www.digikey.com/en/products/detail/ec … -100A-400/20514
48 Mhz: https://www.digikey.com/en/products/detail/ec … 00AX-480/827251
50 Mhz #1: https://www.digikey.com/en/products/detail/ec … 00AX-500/827244
50 Mhz #2: https://www.digikey.com/en/products/detail/ec … -100A-500/20515

Do the specs for these oscillators look right for a late 286 mobo? If so, I'll try hunting down similar crystals @42 and 46 MHz as well. The case I bought for this build has a fan mount on it that is directly in line to the SIMM slots, so keeping the RAM cool might not be as much of a chore as I expected. Wanna get this thing as fast as I can while staying at zero wait states!