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391WB/H motherboard how to set 40 mhz cpu?

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First post, by Nemo1985

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Hello, I recently bought this motherboard, which has been repaired from a great tech guy:
https://theretroweb.com/motherboards/s/dataexpert-391wb-h

Today I finally had the chance to try it but I wasn't able to find the jumper to set the cpu and fpu to 40 mhz.
The manual seems quite incomplete: https://stason.org/TULARC/pc/motherboards/U/U … 86-391WB-H.html
Also having the fpu with jp9 installed or removed seems to change nothing from the fpu performance, while if the jumper is off it seems to give better general performance.

Any idea how can I use the 40 mhz amd cpu?
Thanks

Edit: apparently the jp14 is soldered in 2-3 position, could it be the right jumper to move despite the manual says it's for cache?

Reply 1 of 4, by weedeewee

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there aren't any jumpers to change the clock speed.
You change the crystal on those boards.
An 80MHz one to replace the 66MHz one will do.
It might not work due to other parts of the mainboard flaking out at that clockspeed.

edit: on closer view, stuffing an 80MHz crystal into location OSC1 might do

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Reply 2 of 4, by Nemo1985

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weedeewee wrote on 2023-04-27, 21:01:
there aren't any jumpers to change the clock speed. You change the crystal on those boards. An 80MHz one to replace the 66MHz o […]
Show full quote

there aren't any jumpers to change the clock speed.
You change the crystal on those boards.
An 80MHz one to replace the 66MHz one will do.
It might not work due to other parts of the mainboard flaking out at that clockspeed.

edit: on closer view, stuffing an 80MHz crystal into location OSC1 might do

That's bad, I was hoping for some workaround, I have another motherboard which the online manual was wrong with jumpers settings and it indeed was able to use the 40-33 and lower bus speeds.

Edit watching the picture: https://theretroweb.com/motherboard/image/386 … 47961804024.jpg
There is an empty crystal oscillator socket next to the latest isa slot, could I add a 80mhz crystal oscillator here? The other is soldered.

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