First post, by philipb2
I've been tinkering with an old 486 I rescued from work. It's an XT case, with a Chicony 491E motherboard --
https://theretroweb.com/motherboards/s/chicony-ch-491e
Currently it runs a 486sx 33. The AMI BIOS is dated April 1993. I have attempted to upgrade this to an 83mhz Pentium Overdrive, and alternately, a Kingston Turbochip 133 (AMD 5x86), rev G. No luck.
Once I've set the jumpers for DX mode (see manual available above), this is how it typically goes with either upgrade chip:
(1) I clear CMOS (JP1)
(2) Boot computer. It completes the memory test then, interestingly, says “CMOS battery low.” I have the 4mb RAM that the motherboard came with.
(3) Configure CMOS. It reboots then I usually get “CMOS memory mismatch.” I cannot get into the BIOS at this point. Keyboard is not responsive and often beeps when I press a key.
Notes -
-This motherboard does not have a clock multiplier jumper. Either of these upgrade chips should automatically handle this.
- I have a PS2 keyboard connected to the AT keyboard port via adapter. I've tried a pure AT keyboard with the POD - no difference.
- I haven't tried JP13, 14 for VLBus (0 vs 1 Wait State, CPU speed > 33mhz respectively) since I don't have any VLB cards.
- The power supply might be the original XT. I currently have a 12" VGA monitor plugged directly into the power supply. I have not tried my above steps with the monitor plugged in elsewhere. I should.
What should I check next? Power supply? Or try to upgrade the BIOS?
