dataino.it wrote on 2020-11-01, 13:17:

I would need some help to understand which motherboard is the one in the photos and then if someone helps me to understand the […]
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I would need some help to understand which motherboard is the one in the photos and then if someone helps me to understand the VGA output circuit, I would have identified a problem in the lm339D but I don't understand the circuit.

Color Palett IC
RED ----- m339D
GREEN -- m339D
BLUE ---- m339D

The LM339D is a quad comparator, so it contains four parts that each compare a voltage to a reference voltage. In you case, I guess the purpose of the LM339D is to find out whether a monitor is connected. A monitor pulls down R, G and B via its 75 Ohm termination resistor. If no monitor is connected, the R/G/B outputs rise above 1V. Grayscale VGA monitors possibly only connect G, so if "G" is OK, but R and G are open, you most likely have a B/W monitor.

The motherboard is missing the video rom as well as the 2 bios chips. Can I assume you've found these chips and installed them? because the board isnt going to do jack without it. (to say nothing of the missing CPU)

I did find a similar motherboard, this one: https://stason.org/TULARC/pc/motherboards/W/W … -WD286-LPM.html LOOKS like a slightly earlier model compared to yours. The parts are mostly the same, just slightly moved around a bit.

luckybob wrote on 2020-11-02, 07:46:

The motherboard is missing the video rom as well as the 2 bios chips. Can I assume you've found these chips and installed them? because the board isnt going to do jack without it. (to say nothing of the missing CPU)

I did find a similar motherboard, this one: https://stason.org/TULARC/pc/motherboards/W/W … -WD286-LPM.html LOOKS like a slightly earlier model compared to yours. The parts are mostly the same, just slightly moved around a bit.

Hi

The photos are during the test of the power supplies I had removed what was possible.

I also came to the conclusion that that motherboard was very similar to
WD286-LPM12

AT&T 286 -> https://forum.winworldpc.com/discussion/11026 … 286-take-a-look
PACKARD BELL 286-12 -> https://www.ebay.co.uk/itm/PACKARD-BELL-286-1 … 3-/303237070314

In a few hours I will be able to dump the bios and see if there is any more information

Bios dump say

"Setup (c) Copyright 1988 by Phoenix,WDC [WDC 11/30/89 2.10] Setup Ver 2.10 (c) Phoenix Technologies Ltd.,WDC
For Memorex Telex Intelligent Systems : : ,

Diskette A:
Diskette B:
MB Cyl Hd Sec LZn Pcmp
Hard Disk C:
Hard Disk D:

At least the riser board looks simple enough to duplicate with just that ebay photo.

luckybob wrote on 2020-11-03, 01:08:

At least the riser board looks simple enough to duplicate with just that ebay photo.

Is a good idea !!

Now that I managed to turn on the video, I just need to make it recognize the hard drives (some problems on the interface).

And I miss to understand why it only starts if it has 2 floppies connected (and the disk inserted in B)