Hi all,

I wanted to document the work I have done on a Zenith Data Systems 386SX notebook from 1992. From what I can tell, nobody on Vogons seems to own a 620N, and I can barely find any trace of one anywhere online. So this might be one of the only documented examples.

Apologies in advance for the number of pictures. Given how little information exists online about this model, I felt it was worth documenting it properly.

System information
Model: Zenith Data Systems 620N
BIOS: Phoenix 3.32 (04/19/1990)
CPU: 386SX-20
FPU: Intel i387SX-25 (new old stock, installed recently)
RAM: 4 MB extended memory
Video: VGA, GD610/620 Rev C
Storage: IBM 40 GB IDE drive limited via HTA + BIOS geometry Type 42
Floppy: Slim proprietary 3.5 inch drive with flat ribbon cable

What has been done

1. i387SX installation
I installed a NewOldStock Intel i387SX-25.
It detects the 387 correctly and Windows runs in 386 Enhanced mode without issue.

2. Windows 3.11 and memory behavior
Machine has 4 MB extended memory.
In 386 Enhanced mode, Windows reports about 7 MB free. This is virtual memory, not physical RAM.

3. Hard drive replacement
Original drive was 60 MB. ( defect )
Replaced with an IBM 40 GB IDE drive.

BIOS has no user defined type and no LBA.
Type 42 is the largest predefined geometries supported. ( 127MB )
=> Cyl 1023 Hd 15 Pre -1 LZ 1024 Sec 17 Size 127

The drive was HTA limited in another machine to match BIOS geometry.
System works correctly in DOS and Windows.

4. BIOS access
Phoenix Setup Utility.
Ctrl + Alt + Esc works at POST and even from DOS. This is intercepted by BIOS directly.
If a configuration error occurs, it shows F1 to continue, F2 to enter setup.

Confirmed using original Zenith 620N manual.

5. OEM software packs
Installed: Ms-DOS 6.22 and Windows 3.11 from usual medias but I still own the original sealed MS-DOS 6 Zenith OEM pack and Windows for Workgroups 3.1 Zenith OEM pack. Yes both sealed.

6. USB-C powered dummy battery setup

The original Ni-Cd main battery pack is long dead. I rebuilt a new NiMH pack, but the original external power supply burned out two weeks ago. Without schematics, it is not safe to reverse engineer the pinout blindly.

So I designed and 3D printed a dummy battery pack powered externally from USB-C !

Setup:
USB-C PD trigger cable
Buck converter set to about 5.8 V
Two 1500 µF capacitors on the output to smooth transients

This feeds the battery terminals directly.

The system boots and runs stable on USB-C power.
It is quite satisfying to power a 1990 386SX from modern USB-C.

7. Floppy drive condition
Slim proprietary floppy drive with ultra flat ribbon cable.
Not standard 34 pin interface.

Eject mechanism is weak.
Likely sticky grease issue. I'll check later.

8. LCD panel
Passive matrix screen has a small vertical dead zone.
Possibly ribbon contact or column driver issue. Still investigating.

9. Games tested, all working with PC Speaker
Prince of Persia runs correctly in VGA.
4D Sports Driving (Stunts) works fine.
Xenon2 runs well

Current status

Need to investigate on Floppy drive and CMOS Battery ( it's a not regular CR2032 but a small 4.8v Ni-Cd battery, probably 200-300 mAh that's supposed to be charged with the power supply )

If anyone else actually owns a 620N, I would love to hear about it. At this point it feels like this machine barely existed.

Some pictures attached !
Posted other pictures below.