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PC Chips M209 has trouble POSTing

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First post, by quicknick

Posted on 
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Hello,
Got this in a lot of 3 motherboards and after two succesful POSTs it started behaving like this:
https://vimeo.com/354858564
The first code that appears on POST card is 81, then goes up to 9A only skipping a few numbers in this interval. I can provide the full list if it may help. Then it will stay at code 9A and click forever. Tried multiple RAM configurations (512 and 1024KB in SIPPs, then in DIPs, with and without parity.
It does this without a video card, and of the 3 video cards that I've tested with, still does the same with a Cirrus Logic and a Headland, but a Trident makes it stop at code 99, speaker remaining silent.
Also tried without RAM. Goes through codes 81 to 8C, no sound from speaker.
External battery for the CMOS doesn't make any difference.
Tried two other KBC chips, still the same.

The board had slight damage from the leaky battery, so after an initial round of unsuccessful troubleshooting I proceeded to remove all the chips near the battery area. These include the CPU, KBC and BIOS together with their sockets, and five additional smaller chips (which tested OK on my TL866). Checked all the traces and vias in the area and found no problem, so any suggestion is welcome.

Here I have a picture of the board.

And I'm attaching the BIOS dumped with my above mentioned programmer, someone was looking for it a few months back:

Filename
PCChips M209 (original).rar
File size
40.44 KiB
Downloads
35 downloads
File license
Fair use/fair dealing exception

Reply 1 of 1, by quicknick

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Progress (?) report:

Desoldered the rest of the chips from the motherboard, all tested good on the TL866. Put them back using sockets, for easy removal (I plan on stripping the board and using it as an ISA backplane if I cannot repair it). With a bit of struggle, I swapped the integrated peripherals controller (originally a SiS 85C206, swapped to CHiPS 82C206).

After all this effort, the board behaved exactly like before, absolutely no change. But after fiddling things a bit, trying again with other RAM configurations,KBC chips, things...evolved. The continuous clicking from the speaker started having breaks, and after a series of these it gave a single beep. Each power cycle or reset gave different results, different codes on the POST card, I even got a series of beeps of two different pitches. (Watching "Stranger Things" on Netflix these days, and I have to say this board seems a bit... upside down 😁 )

Anyway, after a few minutes the board seems now stable in a new state. Video here.

Tried with multiple ISA VGA cards, and they all either display some weird pattern, or just initialize the monitor (LED turns green) but display nothing at all.

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