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I do need help, big time, with 486 boards

K

k2x4b524[

Veteran Member
Joined
Jun 13, 2009
Messages
1,521
Location
Tacoma, Washington, USA. Zip code 98404
Well, my grid box went south, as did several of my mainboards for 486, including a known good 486 dx-2 66, i think this may be to my bad jumpering of the boards, for a 486 board this is highly confusing. But my 486/66 board doesn't need much jumperage and can sense the chip. The sensor is the UC4913 Rev 1.0, this had my 486 66 in it. I think the problem can be traced to my weitek power 9000 VLB video board, as after plugging it in and removing it after figuring out it was bad, is only displays in mono. The motherboard just turns on and the cpu fan spins. nothing else, this is the same of all my 486 boards now.

Anyway, the 2 i want to get jumpered right are the UC4913 Rev 1.0 and my AL486 Rev 1.1
How can i test to see if the 486/66 itself didn't crap out? if it did, i've got a 486-dx 50 that in theory should be faster than the 66 because of the 50mhz bus speed as opposed to the 33mhx bus speed.

Any advice on fixing these would be awesome, i want a 486 for my dos games as it's slow enough to play doom efficiently, and fast enough to run Win95 without a hitch.
 
I'd probably first start by getting one of those cheap Chinese (on eBay) POST cards that have both ISA and PCI connectors. At least you'll be able to see if your CPU is doing anything at all. Short of a 'scope or logic analyzer, you stand a pretty good chance at figuring out if anything is happening at all.
 
i suspect the weitek video card as a source of trouble, being it only displayed monochrome then the UC4913 stopped working after that one, short of replacing the crystal on my full size 386 board, i will look into the post card option, but i am also trying to find a cheap replacement or someone who just simply wants their 486 box gone in the seattle / tacoma area
 
I just received a POST card as mentioned by Chuck. I'm looking at a 486 board with similar issues.

So far, the POST card in the ISA bus shows power good, but no action on bootup.

The board's "turbo" LED responds to the turbo switch, so I am guessing that the CPU may be working unless that turbo parameter works independent of the CPU. Until I get a known-good board, I can't be sure. Crystals have previously tested OK.

I'm thinking that the problem may lie in the chips that drive the ISA bus - without a working bus, the POST card can't tell me anything more. Next step will be to check the bus socket lines with a scope, till I get access to known-good components to compare and substitute.

Still playing Blind Man's Buff here...

Rick
 
Rick & k2,

I feel foolish for pointing this out, but have you checked the P/S. Also, hope you haven't be feeding 5 volts to a 3.3(4) volt CPU.
 
RickNel said:
The board's "turbo" LED responds to the turbo switch, so I am guessing that the CPU may be working unless that turbo parameter works independent of the CPU.
Click to expand...

For all intents and purposes, it most often is. Remove the CPU from its socket. The Turbo LED will probably still function. I have at least one case here where it functions without a motherboard present...
 
that's right, GRiD. It's a grid MFP/420s, looks like a Tandy 2500HD 386-sx 25

but as for my 486's, all my boards are toast. except one, and unfortunately that one has a custom bios for telebit corp, which is limited to 16mb ram, forced network boot, disabled VLB slots and the 504 mb limit, but it works with my 486/33. I can't even put another bios into the telebit board, the other bios chip gets burning hot, so i think i have some bad bioses too. My 66, my Cyrix S40 and my everex p75-133 are toast, least the other boards wont work with any of them in. The chips don't even get warm to the touch. So i definitely need to find a 486 mobo that works. But why would they all go south at the same time? And now 2 of my full size 386 boards are exhibiting the same thing? I think i have some serious bad luck here..

So i need a 486mobo that is either pci or vlb, pci preferred, i have my 486dx-50 chip i will populate it with, and a 386-board Preferably AM386-DX40.
I have looked at the various sites and am not willing to pay 100 for a board, and e-schmay is absurd with their prices.
 
@k2x4b524[

If your bios chips are getting hot , something is extremely wrong. I have at least 10 486 machines that I have managed to get working over the years, and have had similar problems a few times. First place I start you already have, the pci/isa tester. If it is not showing post, next step be to make sure the cpu is getting juice. Check rails on the psu as well as check the mosfets on the motherboard with a multimeter. If you do see 3.3v or 5v power, then proceed to checking voltage on the bios chip. If either these are out of spec, meaning over or under by 15%, then check for dried out caps. If you have NO juice, then start tracing back the circuit. Most times its just dried out caps or a bad psu. Most boards I have mishandled still work over the years, for these old boards are pretty resilient! Also, make sure the pc speaker you have is good! IF the speaker is dead then you wouldn't get any error code beeps. So I swap out the speaker next for a known working, as well as swap the ram too...

Wish you luck!!!!! If by any reason you have 0 luck, let me know , perhaps I can help troubleshoot more. I do have a few VLB 486 motherboards that I might be open to trade with as a last case resort, or burn a copy of the bios too if I have a board with a similar chipset. I have a special feeling in these cockles for 286/386/486 systems. >.<


@RickNel

BTW the turbo switch working is a good sign, it shows at least part of the motherboard is getting power. Unfortunately that is all it shows. :(
In your situation since you get 0 POST response, I clean all exposed connectors (cpu socket, ram, slots etc) or run that board thru a dishwasher try again... Also if you have a spare semi soft toothbrush, brush all surface mounted chips rigorously to remove any tin whisker buildup or oxidation. Sometimes that's all it is believe it or not! Usually i'll dip the brush in some electrical cleaner or wd-40 then clean up with qtips and alcohol. The cleaner i use isnt the easiest to find but is the best I ever used! CRC 5-50 No. 05003. I squirt the smallest amt on a qtip and spread it that way, very thin. This stuff is like magic on old boards! Cleans, removes oxidation, rust, etc.. and safe to leave on if you wanted. :thumbsup:
 
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Twolazy, i don't have the gear to test voltages and such, but i can tell you the cpu's get power in the telebit motherboard, they get warm, the others they do not. one of the boards, the bios chips get hot, even though they are the same 27512 or 27c512 chips. None of the boards beep to life or anything. All this happened after i started tinkering with my 2mb Weitek 9000 VLB Video card, which i think itself to be verschniked, as it displays only in mono. The telebit board is crippled, but will not accept any other bios accept the telebit one, i think it's an early asus board, but can't find ANY documentation. Twolazy, i may take you up on your offer, send me a pm, if it's full, let me know and i'll clear it out.
 
I can't even put another bios into the telebit board, the other bios chip gets burning hot
Click to expand...
AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!!!!~~~~~
That is REALLY, REALLY bad!!! This means you are shorting the chip out. If you are quick about it, you can save yourself, otherwise you nuke the EPROM and ruin the chip.
I did this a long time ago with a Packard Bell system. If you EVER have to move BIOS chips around, make sure that: A) the chips are identical and B) you are inserting the chips the right direction.
 
oh they were inserted the right way so maybe the chip went south. I've got one more board i can try, it's a phoenix bios based board, a second AL486 Rev 1.1, i found a AL486 1.0 on stason but it's not even close. This board has 2 72 pin and 4 30 pin slots with the ali 1429 and 1431 chipsets on the board, this is kinda last ditch unless i can find out who made the telebit bios and find a rom image for it. As for the bad boards, anyone got any methods for bulk de-soldering components?
 
Perhaps I missed something, but the way I read it is you're putting board after board in a box and they all end up fried. If that is correct, I'd stop putting boards in that box. There could be a common electrical denominator related to the situation. Are you sure you're not using the same power supply every time? :)

PS: In my personal experience over the years, when something didn't make sense, it has turned out that I was making an assumption.
 
Oh no, this is over 3 different chassis, grounded motherboard stand offs, hell, i even found some plastic replacements for those brass deals, ya know, the hex ones that screw in the bottom? Yep, even put felt pads on the bottom of them, now it IS possible i've got 3 bad cases or 3 bad at psu's but the odds of that are pretty slim, as all the psu's turn on and power everything up. This one is a mid side project right now, as i've got a model 30 8086 variety that may need a hard drive. See other thread in the vintage pc section.
 
Before you proceed any further, I get ahold of a meter...

http://www.ebay.com/itm/DT830B-Digi...000?pt=LH_DefaultDomain_0&hash=item335e60a888

4 bux shipped. Invaluable here.


When I used to demount chips on pcb's to turn them into clipboards, I used to use cheap heat guns... heat up the board till the solder is runny and whack the board against the edge on a table or something. The chips just fall out! Once everything is removed, spray with clearcoat a few times... I never let old boards goto waste. ;)
 
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i have many working 486 boards for sale om my website pcequip.com all come with cpu,memory and are working all are at motherboars some has ps2 keyboard plug instead of large at i do have vlb boards and vlb video cards i even have pci/isa slot board even onbe with a vlb slot in the search box put in 486
 
Your prices are a bit above my price range, and maybe the price range of everyone else here. Are you a spammer?

RBARDY said:
i have many working 486 boards for sale om my website pcequip.com all come with cpu,memory and are working all are at motherboars some has ps2 keyboard plug instead of large at i do have vlb boards and vlb video cards i even have pci/isa slot board even onbe with a vlb slot in the search box put in 486
Click to expand...
 
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