socket_42 wrote on 2026-05-09, 11:34:

It did this both with a closed and open turbo jumper.

weird

socket_42 wrote on 2026-05-09, 11:34:

Is it possible, that I bricked the CMOS? (i assumed, that CMOS settings were volatile without a battery)

Not possible. The only explanation I have right now is board was marginal to begin with.

socket_42 wrote on 2026-05-09, 11:34:

Could it possibly be fixed with re-flashing the CMOS chips?

most likely no

socket_42 wrote on 2026-05-09, 11:34:

If so, could one of you with the same board maybe offer me a copy of their BIOS file, if you can read it? I have board rev. 2.3 .

You might be the only one at the moment with this board 🙁 but good news is Im pretty sure your bios is ok. Would be great if you dumped it 😀

socket_42 wrote on 2026-05-09, 11:34:

Is there another way to reset the BIOS, like attaching an external battery and closing a CLRCMOS jumper? (there is no obvious one on the board)

nothing to reset without the battery keeping it saved, cmos contents simply evaporate

Do you have an oscilloscope by any chance? slow running and dying on clock source conveniently points at clock problems, specifically 14MHz crystal in the middle of the board. Try smacking it a little (not to hard) or just soldering new crystal, also try new main clock gen (66MHz Can)

MagefromAntares wrote on 2026-05-09, 11:55:

Hi, […]
Show full quote

Hi,

I cannot find the manual for this exact motherboard, but I have heard that it has the same jumper layout as this:
https://theretroweb.com/motherboard/manual/30422.pdf

It might worth a try.

EDIT:
And I have found this motherboards manual too, I didn't know why I haven't found it before:
https://theretroweb.com/motherboard/manual/32 … 25549109648.pdf

Thanks you for the manuals. Sadly the jumper’s description and naming don't match, even on the Modular Circuit Technology one, but some are similar positioned. I attached a schematic of the MCT C386-33 that I made myself. J1, J8 and J18 are explained on the board, J101 has no explanation given whatsoever, it’s trace connection is unclear at the moment. I am not sure, was J14 "mono/col" does, could it be for monochrome/color screens? Was I right, to assume the J13"clk" jumper to be the turbo button?
Jumper 7 "discharge" connects the barrel battery to ground via resistors.

Thank’s a lot for your hints and help!

rasz_pl wrote on 2026-05-09, 12:33:

Not possible. The only explanation I have right now is board was marginal to begin with.

What do you mean with "marginal" here? (I’m not a niative english speaker - I read it as "a little"/"few") Do you mean that it had a latent issue/defect, that just showed after some runtime?

rasz_pl wrote on 2026-05-09, 12:33:

most likely no

That is good to hear. I think I also mixed up, the concept of the volatile CMOS chip and the EPROM conatining the BIOS whaen I asked for reflashing.

rasz_pl wrote on 2026-05-09, 12:33:

You might be the only one at the moment with this board 🙁 but good news is Im pretty sure your bios is ok. Would be great if you dumped it 😀

I will try to do it. Do I need to merge the even and odd BIOS halfs into one or can I leave them seperate? I think the latter would be the most usefull.

rasz_pl wrote on 2026-05-09, 12:33:

Do you have an oscilloscope by any chance? slow running and dying on clock source conveniently points at clock problems, specifically 14MHz crystal in the middle of the board. Try smacking it a little (not to hard) or just soldering new crystal, also try new main clock gen (66MHz Can)

I’ve got an oscilloscope, I will probe around a little and see if I find the clocks.

Whatever happened, the board is running and posting again 😀

When I connected it to power to measure the clocks, It happily booted into DOS in a few seconds. The CMOS capacitors on this board seem to be really strong, a few hours of grid don’t seem to bee enough, I had unplugged it for roughly a day now.
Oddly enough, the BIOS had the floppy set to 5.2" (I set it to 3.5" the last time it started) and the clock showed 14 hours and some minutes. (no battery and no PSU were connected the whole time)

I measured the crystal oscillators and they had their nominal frequencies.

I am so happy, that it came back to life and is fast now, I hope it stays that way 😀
I will be sure to try and dump the BIOS and add the board to theretroweb. Is their a good tool or template to draw the schematics "the professional way" with all the right socket and slot pinouts?

Yay for good news 😀

This might still be marginal board. Very old tired/mechanically damaged (like from a shock from big hit or being thrown around) crystal oscillators either die or sometimes start behaving erratically by randomly glitching out/running at wrong speeds. For example It might run fine for a while/until moved again.
Another possibility is oxidized jumper on Turbo pin, or cracked solder joints that work depending on temperature, humidity and the way board was put down/bend 😐
I would still try some percussive maintenance - smack it a little while looking at clocks on scope.
And definitely would try sysclock*2 CMOS setting again 😀

Best would be high quality picture. McBierle uploaded some great closeups of the repair job, but the overall one is below 1megapixel so jumper names are unreadable.

https://theretroweb.com/motherboards/s/auva-c … puter-tam-33-p2 "MCT-C386-33/40" is a distinct model just like "MCT-386-M" https://dosreloaded.de/forum/thread/330-modul … e-at-mainboard/
Sadly retroweb is missing entries for both "MCT-386-M" and your "MCT-C386-33"