Repairing a "Yoke 401-386SX-08" mainboard / 386 Split BIOS Question \ VOGONS

Repairing a "Yoke 401-386SX-08" mainboard / 386 Split BIOS Question

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First post, by majestyk

Posted on 2025-04-06, 13:46

majestyk Offline

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At the moment I´m repairing an old "Yoke 401-386SX-08" mainboard that has a Zymos/Intel chipset.
It refuses to output any BIOS codes, just hangs at "- - - -" and the "FRAME" LED constantly lit. Reset however seems to work.

There are two BIOS chips 256Kb =32KB each, all address lines (A00 - A14) are connected in parallel, so I thought the selection of the chip that is being read at a time is controlled with the "OE" input ot the "CE" input, bot both of them are in parallel also.
OE is connected to the "MEMR#" output of the "N82230" and "82336" of the chipset.

How does the system manage to read the complete BIOS code out of the "LO" and "HI" halves??

Last edited by majestyk on 2025-04-07, 10:53. Edited 1 time in total.

Reply 1 of 4, by Mov AX, 0xDEAD

Posted on 2025-04-06, 14:41

Mov AX, 0xDEAD Offline

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majestyk wrote on 2025-04-06, 13:46:

How does the system manage to read the complete BIOS code out of the "LO" and "HI" halves??

Probaby LO and HI always readed together as 16-bit WORD, then chipset/cpu swap HI->LO internaly if adress is odd

Reply 2 of 4, by dominusprog

Posted on 2025-04-06, 16:06

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Take a look at this.

The attachment 5170_u27_u47.jpg is no longer available

A-Trend ATC-1020 V1.1 ❇ Cyrix 6x86 150+ @ 120MHz ❇ 32MiB EDO RAM (8MiBx4) ❇ A-Trend S3 Trio64V2 2MiB
Creative AWE64 Value ❇ 8.4GiB Quantum Fireball ❇ Win95 OSR2 Plus!

Reply 3 of 4, by majestyk

Posted on 2025-04-07, 11:05

majestyk Offline

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This makes sense - at least as far as reading speed is concerned.

The leaking battry had "evaporated" a lot of traces and vias, each time I thought I had fixed them all I still found further ones.
The CPU, 82335 and 82331 had to be desoldered, traces underneath repaired and soldered back into place again.

The mainboard is now up and running again.

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When I tried to boot from floppy I was welcomed by the infamous "DISKETTE BOOT FAILURE" message:

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And indeed this is just another case of a broken "DRQ2" signal. The DRQ2 interface stage (pin 47 of the 82231) is dead, probably killed by -5V or -12V when someone pulled an ISA card while the system was running or PSU caps not yet discharged (this can take a while especially when you don´t have any cards plugged in that use -5V or -12V voltages).
Now I have to wait for a spare N82230-2 to arrive...

Reply 4 of 4, by majestyk

Posted on 2025-04-13, 06:12

majestyk Offline

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Yesterday I finally replaced the defective Intel N82231 and now floppy access is operational again.
The original AWARD BIOS that was made for "Young Keeen Enterprises" (YOKE) doesn´t support user defined harddrives or 8MB RAM, so I replaced it with a "Zymos Poach" Mr. BIOS, that is far superior here.

It has now become a fine 386SX mainboard:

The attachment 401_386sx_fin1.JPG is no longer available

It wasn´t a good idea, to move the Varta battery from the usual location near the keyboard controller / connector to a place at the center between the chipset components. The damage is even more severe here.

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