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Choosing 386 motherboard and CPU

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Reply 20 of 26, by lalakobe

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Thanks everybody for your help! I see now that DX motherboards are indeed rare. Will look for 'classic' mbs that are available (will be assembling 486-era PC some time later, so additional versatility would be an overkill, but thanks for detailed explanation!)

As for CPU/FPU, I have one restriction (kinda like a challenge 😀 ) - it must be the fastest Intel-manufactured CPU and/or FPU, so that narrows down my search even more (no AMD DX-40 motherboards).

Were all 386 motherboards designed with barrel batteries? I found a picture of EVEREX STEP 386 REV. E (presumably) motherboard, and it has only CR-2032 slot attached (possibly later modification by owner?)

Reply 21 of 26, by lalakobe

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I ended up bying M321 rev 2.6 motherboard (https://stason.org/TULARC/pc/motherboards/I/I … -386-M-321.html)

Strange thing, it had no keylock (+power led), turbo switch, turbo led, reset switch and speaker connectors (J10 to J14 on scheme), looks like they were pulled off (why?).

I Installed speaker, power led and reset connectors, and everything works except of power led.

By looking at motherboard images, I saw that mine is missing also some other components - C47 and R38, also located near J10-J14. Can someone, who owns the same MB, tell me C47 and R38 nominals?

I think that R38 might be the ballast resistor for +5v for power led, am I right?

Reply 22 of 26, by Horun

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R38 appears to go to pin 1 of J13 so yes is probably for power LED. From the pictures it is a 150 ohm (brown-green-brown), cannot read the Tantalum C47.
https://www.ultimateretro.net/en/motherboards/3059

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.

Reply 24 of 26, by lalakobe

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Horun wrote on 2022-05-01, 17:09:

R38 appears to go to pin 1 of J13 so yes is probably for power LED. From the pictures it is a 150 ohm (brown-green-brown), cannot read the Tantalum C47.

Wattage of R38 should be 0,25 watt or more?

Reply 25 of 26, by Horun

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lalakobe wrote on 2022-05-01, 18:14:
Horun wrote on 2022-05-01, 17:09:

R38 appears to go to pin 1 of J13 so yes is probably for power LED. From the pictures it is a 150 ohm (brown-green-brown), cannot read the Tantalum C47.

Wattage of R38 should be 0,25 watt or more?

yes 0.25 watt should be fine. actual current be about 25mA so about 125mW

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.