The 21/31 calculators use a Sperry SP-332 and SP-333 seven-segment planar gas-discharge displays (datasheet). The display segments glow red-orange-red and a red filter makes the display appear a deep red. These display modules wear out over time. They are both rare and expensive.

The museum's Tek 31 exhibit suffers from display wear out. Our volunteers looked into replacing them with 7-segment LEDs due to the scarcity and high cost of the display modules.

The display board is mounted on the front of the calculator and contains a lot of discrete logic.

The display modules have a lot of pins which will make removal quite difficult. The SP-332 has 19 pins and the SP-333 has 28 pins.

The board has  been dipped into some type of clear sealant likely to minimize humidity issues. This sealant can be seen across the top of the PCB in this photo of the rear.

Our board appears to have been turned upright before completely solidified as there is a large run in the center as highlighted by the yellow outline.

A side view shows the sealant completely closes the gap between the display module and the PCB.

This closeup shows the sealant effectively bonding the display module to the PCB. Click on the image to view a higher resolution PDF of the above image.

Due to the high cost and likely inability to remove the displays, our volunteers decided to replace the entire board and use a microprocessor to implement all of the discrete logic. Our LED displays with the red filter have a very similar appearance. The incandescent bulbs have also been replaced with LEDs. As you can see, the board is much more sparsely populated.

Tek 31 Installation Manual

This photo shows our new display board with the red filter overlay not yet mounted in the calculator.

The new Tek 21/31 Display Board is now available on our eBay store.