486 motherboard repair \ VOGONS

486 motherboard repair

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First post, by Jim5589

Posted on 2021-07-12, 20:04

Jim5589 Offline

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I am currently working my way though a joblot of old motherboards testing and repairing as I go. Took a look at this 486 board today which doesn't post currently. While looking over it I noticed the 12v trace from the power connector had been purposely cut and a resistor (correction diode) put in. I wondered if someone could tell me why someone might have done this? I doubt it's related to why it doesn't work now but I haven't come across this before so wondered what it's about.

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Last edited by Jim5589 on 2021-07-12, 22:10. Edited 1 time in total.

Reply 1 of 3, by weedeewee

Posted on 2021-07-12, 20:36

weedeewee Offline

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That's no resistor.

Right to repair is fundamental. You own it, you're allowed to fix it.
How To Ask Questions The Smart Way
Do not ask Why !
https://www.vogonswiki.com/index.php/Serial_port

Reply 2 of 3, by PC@LIVE

Posted on 2021-07-12, 21:22

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I think it is a diode, I don't know the reason for the cut of the track, but I think if he put a diode, he wanted to prevent the current from flowing in both directions, with the diode the current only passes on one side.
I too am working on various 486 cards and others of the Pentium generation, if they show any sign of operation you can understand from the POST CARD where the problem is, with those that do not show any sign of operation, you need to understand if there are voltages and if these are right.

AMD 286-16 287-10 4MB HD 45MB VGA 256KB
AMD 386DX-40 Intel 387 8MB HD 81MB VGA 256KB
Cyrix 486DLC-40 IIT387-40 8MB VGA 512KB
AMD 5X86-133 16MB VGA VLB CL5428 2MB and many others
AMD K62+ 550 SOYO 5EMA+ and many others
AST Pentium Pro 200 MHz L2 256KB

Reply 3 of 3, by Charleston

Posted on 2021-07-12, 22:11

Charleston Offline

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weedeewee wrote on 2021-07-12, 20:36:

That's no resistor.

Are there ever batteries charged on the 12V rail? I thought the vartas were charged only on the 5V but this looks like a replaced battery.

If you remove the diode and jumper over the two traces again (as they've been removed) maybe something cool will happen 🤣.

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