HP Vectra 386/33N graphics memory upgrade to 1MB

(Fig. 1: The S3 86C801 on the mainboard of the HP Vectra 386/33N, with four new ZIP memory chips next to the original ones)

My HP Vectra 386/33N is my machine for early DOS games: a 386SX with 33 MHz on the mainboard, 16 MB RAM, a Sound Blaster Pro 2 and a Roland RA-50 modded to an MT-32. (The sound and MIDI hardware moves between the Vectra and my other 386 from time to time.) The graphics chip is an S3 86C801, soldered directly onto the mainboard. In 2024, I already had to fight a bit to get the S3 drivers for Windows 3.11 running with 72Hz (see my thread on Vogons).

What I only noticed later: the onboard graphics has only 512KB of video memory. The HP hardware test shows it, too:

(Fig. 2: HP System Hardware Test: integrated VGA with 512KB)

What the Service Handbook Says

The HP PC Service Handbook (Volume 2, 1995) describes the video memory of the Vectra 386/33N like this:

• The PC comes with 512KB of video memory. That is enough for up to 1024x768 in 16 colors.
• An additional 512KB can be installed in four video sockets, for a total of 1MB. Then 1024x768 in 256 colors is possible.
• If additional video memory is added, all four sockets must be filled.

HP sold this upgrade as the "512 KB Video Memory Upgrade kit" (D2713A). The switch blocks SW-A and SW-B on the system board have no setting for the video memory - so there is no jumper or switch to change.

Empty Sockets on the Mainboard

Next to the graphics chip, there are four memory chips soldered on: Siemens HYB514256B-60. These are 256K x 4 DRAMs, so each chip holds 128KB - four of them make 512KB. Next to each of them is an empty socket for a chip in a ZIP package (zigzag in-line package).

So I needed four more 256K x 4 DRAMs in a ZIP package. On eBay, I found Fujitsu MB81C4256A-60PSZ from Switzerland, still in their original tube:

(Fig. 3: Fujitsu MB81C4256A-60PSZ: 256K x 4 DRAM in a ZIP package, 60 ns)

Installing the Chips

The chips only need to be pressed into the sockets. Pin 1 must point in the right direction.

(Fig. 4: The opened HP Vectra 386/33N)

On one of the Siemens chips, there is a resistor soldered onto the pins. It was already there from the factory. I don't know what it does.

Result

After switching it on, the HP hardware test shows 1MB - without changing any setting:

(Fig. 5: HP System Hardware Test: integrated VGA with 1MB)

With 1MB, the S3 86C801 can now display 1024x768 in 256 colors instead of only 16 colors.