486 running slow

[dongfeng]

A strange problem occured today with my 486.

75MHz 486DX processor
16MB (4x 4MB 30-pin SIMM)
Western Digital hard drive 425MB
1MB graphics (VESA)

It's been working in the above configuration for about 3 months.

I picked up some throw-out computers from work last week, Pentium 1-2 systems that were in AT cases that looked rather retro. So I took my 486 apart and transferred it to one of these "new" cases. Works fine with no problems. LL formatted the drive and put on a fresh install of DOS 6.22, CD driver, SoundBlaster 16 drivers and installed Descent. Played 5 levels of Descent and all was well. The case is all wired up properly, with the 75MHz dropping to ~25MHz when the turbo is off. All good.

I turned the computer on again today, no problem on boot, but when I play Descent the speed is really slow, as if the Turbo is off. However, when I unpress the Turbo it gets even slower! A CPU speed detection software shows that the speeds of the processor are indeed correct when the turbo is on/off.

I've swapped the video card for two other ISA 1MB cards, also swapped the hard drive for another identical one (I have another I will try tomorrow). I tried changing the processor for a 66MHz (and adjusted the mainboard jumpers to suit), still nothing.

The only things left are the mainboard and memory. Unfortunately I don't have any more 4MB 30-pin-simms, only 1MB, and I don't think swapping 16MB for 4MB would do any good! I used a version of CheckIt to test the memory and mainboard and it doesn't report any problems :tellme:

Any thoughs on things to further diagnose?

 

[nige the hippy]

it's not wait funny states etc on your simms is it, did you lose the config when you swapped cases, and have you gone back to default?

The other thing is... is the bios shadowed, as it should run out of ram faster.

 

[Unknown_K]

Is the battery running low by chance? I think maybe the motherboard might have lost the settings telling it to use the built in cpu cache or mainboard cache (which would kill performance).

Get into the bios and see what those settings are.

 

[alexkerhead]

Maybe driver issue with sound card, maybe the sound card's IRQ/COM settings are off. I'd double check that.

 

[dongfeng]

Thanks for the replies

The cache seemed to be set correctly, and resetting the BIOS to the default values didn't help. Both the drives I tried earlier were Western Digital 425MB, I swapped it for a Conner Peripherals 425MB instead - a fresh FDISK/FORMAT/reinstall and all seems well. How strange, especially considering the second WD drive still didn't play fair even after a low-level format!

 

[IBMMuseum]

...The case is all wired up properly, with the 75MHz dropping to ~25MHz when the turbo is off. All good.

I turned the computer on again today, no problem on boot, but when I play Descent the speed is really slow, as if the Turbo is off. However, when I unpress the Turbo it gets even slower! A CPU speed detection software shows that the speeds of the processor are indeed correct when the turbo is on/off...

Just this particular section brings up a trouble call I had about a 486 whitebox some years ago. The secretary complained that the system was running slow all the time. When I booted it up from scratch it was fine for me.

So I asked her to take a session on it, to show me where it was slow. She sat down in the chair, and without further thought that I was there, toggled the Turbo button off because the LED was shining in her eyes! A couple of pieces of masking tape laid over each other fixed that call...