If I recall correctly you could change the system speed with the keyboard ctrl-alt-num+/- keys.
You could try booting the system at low speed, then running a system diagnostics/benchmark switch to high speed.

I highly doubt it's the dallas mod. Hanging on floppy makes me wonder if it's the FDC or memory.

also, is that a typo on the motherboard photo description ?

Yes thanks I can try to change the speed by these key combinations, I read on the user manual it's possible. Tomorrow I'll try, hope to don't frezee like the last time because it wouldn't boot correctly after tried 12MHz till the day after ....don't know really why.

If the keyboard speed change don't work, it could be due to a keyboard driver being loaded in dos. if so, unload or remove the keyboard driver from being loaded and try again.

Did you try updating the BIOS to the latest version? There seems to be BIOS images available for download on this website

Also, did you try to change the system speed using the ATSPEED command? I'll copy and paste notes from the PC40-III user manual (the PC30-III indeed has a PC40-III mainboard inside 😀

SETTING THE MICROPROCESSOR CLOCK SPEED The 80286 microprocessor in the PC40-III is capable of running at three different clock ( […]
Show full quote

SETTING THE MICROPROCESSOR CLOCK SPEED
The 80286 microprocessor in the PC40-III is capable of running at three different clock (i.e., processor or CPU) speeds:
• Standard speed 6 Mhz
• Turbo speed = 8 MHz
• Double speed = 12 MHz
The PC40-III is preset to the standard 6 MHz speed. You can switch between the clock speeds by using special key combina-
tions or by using the MS-DOS ATSPEED command.
To set the clock speed from the keyboard, usc these key sequences:
• CTRL-ALT-S for standard speed (6 MHz)
• CTRL-ALT-T for turbo speed (8MHz)
• CTRL-ALT-D for double speed (12 MHz)
NOTE: Some software may require that you select standard or turbo speeds for normal operation.
To set the clock speed using the ATSPEED command, first make sure the MS-DOS prompt is showing on the screen. Then
type the word ATSPEED, followed by a space. a dash(-), and then a letter (S, T, or D) denoting the desired speed. For
instance, if you are in standard speed and you want to change to turbo speed (R '\1Hz), type the following and press Enter:
ATSPEED -T

Really thanks guys! Tomorrow I'll do some tests. hope to solve the problem with your help!
Actually I connected a common 5din keyboard but I have even its original commodore in garage...if it could be the keyboard driver problem...

I'll let you know..

weedeewee wrote:

also, is that a typo on the motherboard photo description ?

No, many or every PC30-III have inside a PC40-III motherboard without all the video (vga?) integrated chips that was added on the ufficially PC40-III.
There is even a red switch disconnected that may be a cga/vga output?

SETTING THE MICROPROCESSOR CLOCK To set the clock speed from the keyboard, usc these key sequences: • CTRL-ALT-S for standard s […]
Show full quote

SETTING THE MICROPROCESSOR CLOCK
To set the clock speed from the keyboard, usc these key sequences:
• CTRL-ALT-S for standard speed (6 MHz)
• CTRL-ALT-T for turbo speed (8MHz)
• CTRL-ALT-D for double speed (12 MHz)

I tried these commands and switching at 12mhz FREZEE immediately.

How could be possible??? Maybe the crystal?

Booting at 12mhz it crashs after the floppies check, just before the chechin of the COM LPT port (even disabilitated.) Never before. Should be useful to know the sequence it does booting.

could be Logic chip failing that divides crystal signal.

weedeewee wrote on 2023-02-05, 09:35:

could be Logic chip failing that divides crystal signal.

Yes, could be, thanks. But which chip is it? Maybe the FE3000A? I could try to serch a new or used one and replace it easisy.
Here attaced an extract of the PC40-III user manual with the moterboard component scheme, if someone could help.....

full manual can be downloaded here: https://ia902308.us.archive.org/0/items/pc-40 … 90_MAR_text.pdf

page 55
U410 or U403

I can't find the 36MHz crystal in the manual, and looking at the photos I can't find U410.
By logic chip I mean one of the 74 series chips that are located around the crystals.

weedeewee wrote on 2023-02-05, 11:45:

page 55
U410 or U403

thanks weedeewee, I found the U403, not yet the U410. If I can found a replacement I can unsolder these chips easily. If you are agree I'll do this work....

attached a zoom of a pc40-III motherboard with the U403 chip in red.

I do not know. I'm just pondering possibilities.
If you have access to an oscilloscope then you can measure if the output signal is 12 MHz when you switch it.
At the moment i'm just guessing it might stop there on boot or after switching to it.
The problem could easily be elsewhere.

At 8mhz works fine. If I switch at 12mhz by the bios it crashes after the floppyes check. If i switch at 12mhz by the keyboard command or with atspeed program it frezee immediately. If the crystal of 24mhz or 36mhz ,don't know, was damaged and runs at a faster speed then 12mhz and causes the crash?

I don't have the oscilloscope, only a couple of tester, solder an unsolder.

I could even don't try to fix it but changing the u403 it is an easy work and i could try if there aren t alternatives...

The cooprocessor speed of the pc30-iii is the same of the cpu speed while for the other 286 zenith z-286 lp I have, the cooprocessor speed is 2/3 of the cpu...

Ironically, Zenith runs at 16mhz instead of 12mhz like it should but never chashed.

What to say .... do how the hell you want!!! 😀

Not sure if you are still having the 12MHz crash issue on your Commodore PC30-III, but I have EXACTLY the same issue with my Commodore PC30-III.

No matter what I tried, I could not get a VGA card to work in the machine at 12MHz (0nly in 6Mhz/8MHz).
I even purchased an 8-bit VGA card, hoping that would solve my problem, but the PC still crashed, as soon as I switched it to 12MHz.

THE SOLUTION: Upgrade your BIOS from Rev. 2.01 to Rev. 2.03. See attached screenshot taken from "Commodore Techtopics #31" for reference.

LIMITATIONS: After the BIOS upgrade, an 8-bit VGA card will work in any of the four ISA slots at 12MHz. However, a 16-bit VGA card will ONLY work in the 8-bit ISA slot (otherwise it will still crash like before). Luckily most 16-bit VGA cards are backwards compatible to work in an 8-bit slot. I purchased a 16-bit Paradise PVGA1A, and that works just fine in the 8-bit slot.

Hope that helps.