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486 Machine woes

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NaokiS

Member
Joined
Jul 14, 2011
Messages
33
Location
Cheltenham, Gloucestershire
Thank you windows for making me retype this...

I have a 486DX2 machine here I got from a friend a while back. It seems to work fine at first glance, but then I'm constantly mistaken. Installed inside is:
  • PKM-0038S Mobo
  • Intel i486DX2 @66Mhz
  • 128KB CPU ram
  • 16MB SIMM (one stick)
  • 3GB Seagate harddisk (Jumpered down to 2GB)
  • Multi I/O card (2xIDE, floppy, serial, parallel, joystick (ISA+VESA))
  • Trident VGA (ISA+VESA)

Problems are normally loading games and some programs
DOOM - Either restarts during loading or crashes witha DOS4wg error
Wolfenstein 3D - Hangs or shows corrupted graphics on a hanged "One moment" hardware check screen
MPXPLAY - Crashes or hangs before loading

I'm not aware of other problems, if any. I'm really at the end of my teather with this. Everything I've tried has had no difference. I've tested what hardware I can on a spare PC and even used other parts for this machine and still nothing. I'm only inches away currently from breaking this purposely just because it's frustrating me that much...

Before i go genocidal on all electronics in my room, any suggestions to try..?
 
Some things to check are:
1) The cache memory may have gone bad. I don't see any way to turn it off from jumpers but maybe your BIOS will let you.
2) There could be a conflict between the two VESA cards. If you have a ISA I/O card or set of cards (one for ports, another for IDE), use that instead of the VESA I/O card.
3) Run memtest-86 or similar memory tester. I expect memory to give different problems but its about 20 years so prudent to test.
4) Double check that the VESA bus speed and the CPU jumpers are all set correctly. Possibly try changing the settings for a 50 MHz DX2 (which should slow the bus to 25 MHz) with a wait state on the VESA bus. The system especially with multiple VESA cards may not handle higher bus speeds well.
 
krebizfan said:
Some things to check are:
1) The cache memory may have gone bad. I don't see any way to turn it off from jumpers but maybe your BIOS will let you.
2) There could be a conflict between the two VESA cards. If you have a ISA I/O card or set of cards (one for ports, another for IDE), use that instead of the VESA I/O card.
3) Run memtest-86 or similar memory tester. I expect memory to give different problems but its about 20 years so prudent to test.
4) Double check that the VESA bus speed and the CPU jumpers are all set correctly. Possibly try changing the settings for a 50 MHz DX2 (which should slow the bus to 25 MHz) with a wait state on the VESA bus. The system especially with multiple VESA cards may not handle higher bus speeds well.
Click to expand...

Turns out the cache has probably gone bad. Turned off the external cache in the BIOS, DOOM, Wolf and MPX all load fine now.
 
NaokiS said:
Turns out the cache has probably gone bad. Turned off the external cache in the BIOS, DOOM, Wolf and MPX all load fine now.
Click to expand...

Could be one of the millions of "fake cache" 486 motherboards sold in the early-mid 90's, LOTS cheap boards of the era used fake chips and modded BIOS's that reported having cache installed even when there was none. Tell tale signs to look for are "UMC" chipset and "WRITE BACK" written on the "cache" chips.
 
A quick glance says nothing like that. I fouund this photo from when I first got the board. I was told the socketed RAM besides the SIMMs is the second level cache. Anything unnusual about it? It's a SIS chipset.

vlcsnap2011062014h58m53.png
 
The 9 chips left to the 4 simm sockets is the cache. Could be that one is broken. You can run the machine without cache but it will decrease performance. You could remove the chips and reconnect it, sometimes this helps.

As you said it is 128k cache, are you sure the jumpers are correct?
 
NaokiS said:
A quick glance says nothing like that. I fouund this photo from when I first got the board. I was told the socketed RAM besides the SIMMs is the second level cache. Anything unnusual about it? It's a SIS chipset.
Click to expand...
Yeah, its likely not one of the fakes if its socketed like that, likely mis-configured or there's a bad chip in the mix.
 
NaokiS said:
On some programs such as Lemmings 2, I get "Incorrect opcode: Killing process". Help?

EDIT: The jumpers were set up right as far as I can see
Click to expand...

Invalid opcode most often shows up if the system has a memory problem, either RAM or with the motherboard itself or if the CPU is overclocked. Test the RAM if you can and examine the motherboard to see if some traces got damaged. If the RAM passes and nothing looks obviously damaged (like a gouge across several motherboard traces), then you face the problem that older systems will be less than stable and it will take a lot of work to track down the correct failing component.
 
RWallmow said:
Yeah, its likely not one of the fakes if its socketed like that, likely mis-configured or there's a bad chip in the mix.
Click to expand...

Probably a bad IC. Jumpers have been configured correct

krebizfan said:
Invalid opcode most often shows up if the system has a memory problem, either RAM or with the motherboard itself or if the CPU is overclocked. Test the RAM if you can and examine the motherboard to see if some traces got damaged. If the RAM passes and nothing looks obviously damaged (like a gouge across several motherboard traces), then you face the problem that older systems will be less than stable and it will take a lot of work to track down the correct failing component.
Click to expand...

A RAM testerI used failed, so I took out all but one stick, sure enough it boots fine. Only have to figure out the bad one ;)

Thanks for all your help guys BTW.
 
another thing I keep getting a problem is that recently the SoundBlaster 16 I have will sometime s not output anything. The card isn't going bad as through trial and error I can get it to output sound, a but a power off later and it stops.
 
Could be the psu.. Hear me out. Old psu's are known to harbor dried out caps, making voltages unstable. Perhaps one of the voltages is below spec, causing these errors, I'll guess 12v/3.3v. Would explain strange errors, the soundblaster working funny etc.

Could also be bad ram still. Have you tried running memtest x86 on the machine yet? If you get any errors, try popping out the ram and cleaning the connectors with an eraser...
 
Well, 2003 - 2005 was the height of bad capacitors leading to frequent early failures of power supplies. If you have access to a power supply tester, test the output. Check the splices to make sure they haven't seperated. Also, check the various motherboard sockets/slots that none are loose. With the number of problems showing up, I am doubtful that RAM and cache and expansion cards all fail at the same time without some other central cause.
 
krebizfan said:
Well, 2003 - 2005 was the height of bad capacitors leading to frequent early failures of power supplies. If you have access to a power supply tester, test the output. Check the splices to make sure they haven't seperated. Also, check the various motherboard sockets/slots that none are loose. With the number of problems showing up, I am doubtful that RAM and cache and expansion cards all fail at the same time without some other central cause.
Click to expand...
I've seen numerous PSU's of that era having bulging and even leaking caps inside (usually one, sometimes 2).

Before I try out an old PSU I always open it up first to check if there are any obvious signs of damage (and to give it a good clean if it's dirty ;) ).
 
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