Over the past few years, I have continued to amass a collection of the 1975 Bell & Howell IMD-202 digital multimeters with nixie tube displays. With some patience, I've been able to buy them on Ebay for less than $50 each. Given the case, power supply, nixie tubes, and 7441 decoder/drivers, that's a pretty good deal I haven't been able to pass up. However, now I have a bunch of these. So, I decided to revise the original audible dice tower design to simplify resto-modding these.

To achieve this goal, I've implemented the following changes.

1. Rotary encoders to select dice count and dice type - These encoders utilize the original faceplate holes that mounted the original mutli-way rotary switches. Therefore there are no modifications to the faceplate required. I can just clean off the original faceplate graphics with some acetone, apply new graphics (printed water slide or embossed labels), and install the encoders, button, led, and power/volume pot. The only modification to the case required is drilling holes to mount the speaker.

2. Simple class D amplifier - These little boards can be bought very reasonably from several sources and they simplify the audio amplifier circuit that was originally implemented with a LM386 op amp. It can be seen mounted upside down on the left of the protoboard in the picture below. Mounting this way lines up the polarity of power and ground correctly.

3. Larger protoboard that simplifies wiring and interconnect with original PCB - Using a larger protoboard that spans the dip sockets required for all the I/O and provides room for the processor and amplifier.

This video demonstrates how these revised pieces operate.