486 motherboard repair \ VOGONS

486 motherboard repair

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First post, by kevmif

Posted on 2022-10-22, 04:01

kevmif Offline

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Rank: Newbie
Posts: 42
Joined: 2021-06-09, 08:06

Hi all. Hoping for some help repairing a 486 board.
It is one of these: https://theretroweb.com/motherboards/s/suk-ju … -3vls#downloads (well very similar, looks like an earlier revision).
The traces to the keyboard controller for clock, data and ground are destroyed. 5v is fine according to the multimeter. No other apparent damage besides a spot just below the battery patched with some silver paste. Board does indeed POST.

20221022_143715.jpg?raw=1

I plan to run patch wires from the keyboard port to points L3 and L4 (top right of image) as attempting to repair the traces is beyond my skills.

Point L2, which I am reasonably sure is ground based on a continuity test had one of those black components which I removed to assess the traces better (same as L3 and L4).
My question is, what are these components and why is one necessary for a connection to GND?
Does that component have polarity or can I reinsert it any way? The black bit seems to move freely on the wire - is this normal or did I break it de-soldering it?

Thanks

Reply 1 of 3, by DOSDays

Posted on 2022-10-22, 05:07

DOSDays Offline

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Posts: 14
Joined: 2022-08-03, 06:25

Point L2, which I am reasonably sure is ground based on a continuity test had one of those black components which I removed to assess the traces better (same as L3 and L4).
My question is, what are these components and why is one necessary for a connection to GND?

Those black components are inductors (also sometimes called coils), and they are not polarity-sensitive so you can orient it any way when you put it back in.
I'm not sure I understand the second part of your question... do you mean why is one side of the inductor connected to ground? The simple answer is to close the circuit.

Reply 2 of 3, by kevmif

Posted on 2022-10-22, 05:17

kevmif Offline

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Rank: Newbie
Posts: 42
Joined: 2021-06-09, 08:06

DOSDays wrote on 2022-10-22, 05:07:

Point L2, which I am reasonably sure is ground based on a continuity test had one of those black components which I removed to assess the traces better (same as L3 and L4).
My question is, what are these components and why is one necessary for a connection to GND?

Those black components are inductors (also sometimes called coils), and they are not polarity-sensitive so you can orient it any way when you put it back in.
I'm not sure I understand the second part of your question... do you mean why is one side of the inductor connected to ground? The simple answer is to close the circuit.

Inductor coil - right. Causes current to build up gradually instead of all at once. Out of curiosity, what benefit does this have in the keyboard circuit?

I'm guessing the fact it moves around on the wire is probably fine and I wont need to replace the component? (it did get hot when being de-soldered).

Reply 3 of 3, by majestyk

Posted on 2022-10-22, 05:37

majestyk Offline

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Rank: Oldbie
Posts: 1356
Joined: 2020-12-04, 09:13

These are ferrite beads and their main purpose here is EMC related.

As long as the ferrite is not broken you can safely reuse them.

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