Thans for all the answers !
Horun wrote on 2021-08-28, 01:41:
Yep Mine is v1.1, yours is v1.0 (printed between the ISA slots.)
So it runs up to the KB controller after going thru logic up by the 85c206 ? Saw your drawing and it of makes no sense (not your fault) but to run a clock circuit that far to logic then back to the KBC just seems odd.Am probably mis-reading what you meant but that just seems out of spec and not a normal way to do things.
Can you take a good picture of the back of your board ?
Sorry, english is my second language so....
That seems odd to me too, but I've tracked connections from the PCB .
snufkin wrote on 2021-08-28, 09:16:
Well the datasheet for the P8042 gives the min/max clock period as 80 & 613 ns. Which I think works out as 12.5MHz down to 1.63MHz. So that narrows it down a bit. Looks like they might be using the second clock input circuit from the datasheet:P8042_Clock.jpg
Exactly my point - that's why I've asked.... but I'm a 'hothead' so the storry continues fast.... 😉
This is the bag of oscillators that I got with the mainboard. The guy I got it from is a ham radio enthusiast... he took out oscillators 25years ago and (Thank God) forgot to trash the rest...
He is 100% positive that this is the set from this motherboard (they turned out not so useful after all...).
Batch dates on them seem to confirm his words.
233691527_858937911667704_8928073650607012373_n.jpg
I've done some soldering...
240661644_870333280528167_1392049088788469858_n.jpg
and...
1) The mainboard 'gave signs of life' 😉
It showed BIOS version and stopped on memory test (sometimes counted 64k..... of RAM after power on but in most cases nothing at all) and made a reset (in infinite loop)
2) To be sure I've mounted 100% verified RAM and CPU and removed FPU (same behavior).
3) I took out keyboard controller (8042AHP) and without it, the mainboard starts, tests all installed RAM and freezes with constant beep where it should display 'Press ....'(quite normal right ?).
So... I've returned to this 'enigmatic' oscillator.
Signal on the output (capacitance of the probe makes it look like sine).
IMG_1072.jpg
This is the signal on the input of the buffer that is connected to the XTAL2 of 8042AHP chip (pin 9 of the 7407 IC)
IMG_1073.jpg
And on the output of the buffer (this is connected directly to XTAL2 of 8042AHP) ...
IMG_1074.jpg
[EDIT]
It was late.... and I have to repeat last measurement (trigger is not set correctly...).