r44r1 wrote on 2025-03-09, 18:36:

Hello all! […]
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Hello all!

Trying to build a 386-based system now. I'm not a totally newbie with retro hardware but my experience lies in P1+ era, where things are much easier in general.

Motherboard is BEKtronic-3703 with AM386SX40: https://theretroweb.com/motherboards/s/bek-tronic-bek-3703 Mine has yellow mask, probably some later revision

Chipset

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BIOS is AMI 5.19 on DIP28 chip with custom markings. Not sure if it's EEPROM or factory OTP. BIOS string is X0-0100-000000-00101111-060692-386SX-H
As it's 386, there is 528MB limitation and ofc I have no such small disk. So I'm trying to install a Freedos on top of Ontrack DDO on 4gb IDE drive

After assembling system I've set up datetime, ensure all hw is detected and file away things for a couple of months. After powered up machine, got "CMOS battery low" and "CMOS settings incorrect" errors.
Checked pack voltage, it was about 4V. Replaced batteries, redo cmos setup, played with system for a day, disconnect it from a wall socket and file away things again for about a week. Getting "Low voltage" error again, but measured pack voltage was 4.7V this time. That looked suspicious to me so I've experimenting around for a bit. Seems like CMOS settings (except datetime) are cleared next day even with batteries attached. If I do not unplug system from a wall socket, everything is fine (+5VSB do the magic, I guess). Datetime is saved a bit more, maybe 2-4 days, but cleared as well.

Do 3xAAA not enough here or I missing something again?

Some (very few) required 6v (4 x AA or AAA batteries) for the external. Quick look at the picture I see 3 diodes, maybe two in series with external ? that could drop 4.5 v to less than 3.0v.
With psu unplugged and the battery connected check voltage on pin 24 of w85c168 rtc (should be the VCC/VDD pin)..

BIOS settings? if the AT bus divider is set wrong, it could be overclocking the ISA bus, maybe the controller can't take it... needs to be CPUCLK/5

Horun wrote on 2025-03-09, 18:59:

check voltage on pin 24 of w85c168 rtc (should be the VCC/VDD pin)..

Thanks. Will check tomorrow before cold boot.

Matth79 wrote on 2025-03-09, 19:38:

BIOS settings? if the AT bus divider is set wrong, it could be overclocking the ISA bus, maybe the controller can't take it... needs to be CPUCLK/5

Doesn't seems to present in Setup. Remember that option was on my P1 board with AMI.

The only looks somehow related is "ATBus Stepping". But it only accepts enable/disable values, enabled by default. Playing around value gives no result.

Floppy issue probably isn't this, but it could very well be FreeDOS. If you're worried about that try a clean SYS of DOS 5 or 6 to a blank floppy. If that doesn't work you know you've really got issues.

Test a legit MSDOS 6.22 boot disk with MBR (master boot record) set on the disk.

EG. use Rufus to make an MSDOS boot disk, then copy MSDOS 6.22 files over.

Another way to create an MBR is from an MSDOS 6.22 prompt: "format a: /q /s". /s creates the MBR and copies command.com.

If OnTrack picks up the disk, as well as installing it onto the disk, look for the cyls/heads/sec information, and enter the closest numbers into BIOS for HDD setup. Otherwise the HDD won't be found.

I have the same problem with CMOS settings..

Horun wrote on 2025-03-09, 18:59:

With psu unplugged and the battery connected check voltage on pin 24 of w85c168 rtc (should be the VCC/VDD pin)..

It turns out I have KS82C6818A RTC, but pinout looks the same.
Voltage between p24 and pack gnd is 4V with 4.5 directly measured. Ordered 2x2032 housing and will try them instead.

keenmaster486 wrote on 2025-03-09, 22:27:

If you're worried about that try a clean SYS of DOS 5 or 6 to a blank floppy

MikeSG wrote on 2025-03-10, 11:43:

Test a legit MSDOS 6.22 boot disk with MBR (master boot record) set on the disk.

Unfortunately I'm out of clean floppies now, only 2 out of 10 passed test. Ordered Gotek clone, but this will take a while.

Get back with some progress.

Boot issue appears to be Freedos (or disk) issue. Bought a pair of brand new floppies and succed with MS-DOS installation.

For CMOS, things just getting more obscure.

I replaced holder with 2x2032 and get 6V from pack directly and 4.8V on RTC VCC. Power system on and get "cmos battery low" again.
But I forgot to disconnect multimeter from chip and was very suprpised when it drops to "0" when I power up machine. Doesn't RTC power should always be connected to battery?

Any ideas?