VOGONS


First post, by AndrewBSSC4

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Hi,

Hoping someone can give a lead on find the schematics for my old 386 motherboard I found. Unfortunately it looks like battery leakage took out some traces and there's a small blown ceramic capacitor I can't get a part number off of.

The motherboard info is:
Model: CH-386-25D/33A
P/N: 001-00386-B08

There's a layout diagram in the manual which identifies the diagram as: H1448M

I'm pretty sure I can follow and fix the broken traces but the capacitor I'm not sure what to replace it with.

Any help would be welcome.

Thanks.

Reply 1 of 16, by Horun

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AndrewBSSC4 wrote on 2020-11-20, 20:34:
Hi, Hoping someone can give a lead on find the schematics for my old 386 motherboard I found. Unfortunately it looks like batte […]
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Hi,
Hoping someone can give a lead on find the schematics for my old 386 motherboard I found. Unfortunately it looks like battery leakage took out some traces and there's a small blown ceramic capacitor I can't get a part number off of.
The motherboard info is:
Model: CH-386-25D/33A P/N: 001-00386-B08
There's a layout diagram in the manual which identifies the diagram as: H1448M

Hmmm CH-386-## is usually a Chicony motherboard.
Can you post a good picture of it ?
Finding a schematic will be nearly impossible as most board makers never released a true schematic.
Figuring out the capacitor can be easy if there are same others on the your board or by comparing to other 386 boards with same chipset.

Hate posting a reply and then have to edit it because it made no sense 😁 First computer was an IBM 3270 workstation with CGA monitor. https://archive.org/details/@horun

Reply 2 of 16, by AndrewBSSC4

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Hi,

See the attached pictures.

Chipset seems to an Opti 82C381 & Opti 82C382.

There are two other caps nearby. The one I'm leaning closest to a match is the C1 which is labelled 7K. While there is another C3 which is labelled 502.

Thanks.

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Reply 4 of 16, by quicknick

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Most likely 47 pF, the value isn't that critical. More important is to neutralize the battery spill in that area if you want to enjoy your board for the years to come. Use vinegar, old toothbrush, rinse with plenty of water and dry thoroughly (or just give the board a final bath in isopropyl alcohol which is non-conductive and dries fast).

Reply 5 of 16, by AndrewBSSC4

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Sounds good I'll track one down.

Definitely. I soaked it vinegar and some WD40 and got it pretty cleaned up.

The good news is it posts. But I have a short on the -12V rail and a "no keyboard" error. Pulled the -12V off the PSU and confirmed the board is alive. Checked the keyboard port and there's power and traced the data pin back to a pin on the nearby keyboard BIOS chip. So not sure if this capacitor might be playing a role in the "no keyboard" error.

But I'm psyched the board is alive and posts.

Thanks.

Reply 6 of 16, by quicknick

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Short on -12v is most likely caused by a tantalum capacitor, you'll have to check them one by one on that particular rail (shouldn't be too many, could be a single one).

From the KB connector you should have 2 pins connecting to the keyboard controller (bios) chip, on Pin 1 and Pin 39 (data and clock, I don't know if this is the proper order or the other way around). Probably a corroded trace near the KB connector, or maybe even a via (there are a few that don't look so good). The 47pF cap is not critical and I think it should work even without it.

Reply 11 of 16, by Deksor

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Does this look like your motherboard ? ^^
http://www.win3x.org/uh19/motherboard/show/1253

Trying to identify old hardware ? Visit The retro web - Project's thread The Retro Web project - a stason.org/TH99 alternative

Reply 12 of 16, by evasive

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Not a match JP3 is near the keyboard connector on this board, on the other side on the UH19 one.

Actually, we don't have this board yet.
CH-386-25D/33A

we have A/B/C/F/G/H versions in various speeds but no D or E yet.

Reply 13 of 16, by AndrewBSSC4

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Thanks all for the replies.

I actually have an update on this. After spending days and days tracing virtually all the traces from pretty much every component on the bottom half of this board end to end I could not find a problem. After my initial capacitor fix and trace fix everything traced fine and the computer would post but would have the "Keyboard IO" error. I had just about given up and was ready to put this board aside and move one to something else and maybe come back to it some time in the future. But before packaging it all up for storage I figured I'd replace the electrolytic I had used for a ceramic which had arrived. So I did that. So just before packaging I figured I'd hook it up and confirm the error is still the same and I had not created a new one to confuse me more later on. And - SURPRISE - Post and no error. Board seems to be working fine now. Keyboard works. Machine boots. I couldn't believe it. So not sure what the story was with that capacitor. Toning out the traces everything was fine.

Anyways moving on. Only problem now is I get a Floppy or HDD error. But I think that's the old I/O card that might be dead. So waiting on a new on to arrive. Oh and a dead SIMM - also on order. I am so pumped this board is working. Seeing that board boot up brought me back 30 years.

Reply 15 of 16, by AndrewBSSC4

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HI all,

Sorry for the delay.

Here's the full board picture and a scan of the manual. Unfortunately I don't have the reader to dump the BIOS - yet.

Thanks.

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