Ok, got the video memory OKI chip, but I also got me 2x512KB zip memory for a total of 2MB.. hopefully.

I tried adding one of the zip memories, and VRAM jumped to 1MB. Yay! Then I added the other zip memory but VRAM remained at 1MB.. so I thought maybe it's not a supported configuration (1.5MB).

So, removed the zip memory, and soldered in the OKI chip, and I immediatelly got 1MB VRAM reported, and testing the video modes that need 1MB worked successfully (e.g. 640x480 16Mil colors).

Next, I installed the two zip memories, which should jump total VRAM to 2MB. Alas, only 1MB was still being recognized.. big bummer...

I suspected that a jumper might be needed to enable 2MB, so I focused on jumper JP I, which is next to the VRAM sockets and the Cirrus Logic chip. The jumper documentation only mentions JP I as used for enabling/disabling the on-board VGA chip.. but the jumper block is much larger than just the enable/disable setting, so I thought it's most probably under-documented. After some careful testing, I found that the odd row is used for enable/disable, while the even row of the jumper block is actually for VRAM size control. Indeed, bridging pins 4 and 6 results in 2MB VRAM. It gets recognized in WHATVGA and SpeedSys. I now have access to 1024x768 hi-color (64K), and even 1280x1024 256 colors.
VESA extensions also are able to find those 2MB modes and use them correctly.

So yeah, I'd say it's a success. I couldn't stand a DX4 with just 512KB VRAM.. 1024x768 at 64K colors is very nice for Photoshop and CorelDRAW 4 or 5 on Win3.1 🙂

Woohoo congrats. Glad you got 2MB I would have guessed the zip and SOJ to be in parallel, alternative parts. I think I'm only going to bother with 1MB though, don't need it to have great windows performance at higher res.

Indeed great to read your success, good work.