Solo761 wrote on 2022-05-05, 11:42:
Finally got LT1085CT LDO voltage regulators to try on my ECS UM4980 v1.1 board and it works. Finally I can get other voltages to CPU beside 5V direct from PSU.
And now I have another issue. I put AMD 486DX2-66 on it, set jumpers correctly (checked multiple times, both against jumper pdf from ultimate retro, motherboard silkscreen and scan of the manual), but for some reason I get 486DX2 100 MHz on post if it's set to 33 MHz FSB. Regardless of if I set jumper to X2 or X3 multiplier... I get 75 MHz if I set FSB jumper to 25 MHz FSB.
So I'm currently WFTing...
Welp, I WTFed it 😅. Tried 5V Intel 486DX2-66 and it worked as advertised, 66 MHz...
This CPU (A80486DX2-66 V8T) has CLKMUL on pin B13, while others have internal cache write through / write back select on this pin. DX4 CPUs have CLKMUL on R17 and X2 vs X3 jumper on motherboard is connected to R17 which doesn't to squat on this CPU .
So in the end I stuck a wire into B13 and checked all unused pins and luckily found it's pin. It's used for writeback 486 configs which motherboard supports, it's just that this CPU model seems to be orphaned... Used jumper wire and connected this pin to GND and voila, CPU booted as 66 MHz instead of 100 😁.
Also found site with pin difference by socket 3 CPU which might be interesting to some 😁
http://ps-2.kev009.com/eprmhtml/eprmx/h12203.htm