Scope innards
Here are some photos of the insides of the scope. The first photo shows the main circuit board.
This scope was still fully functional, so I looked at the waveforms to see how the motors were driven. I knew I was going to have to build new electronics to control the scope the way I wanted. This circuit board was going to have to go, but I needed to see how it worked.
The motor drive was very clever, driving the DC motors with 12V pulses. Each strong pulse was enough to overcome stiction and get the motor moving. The speed of the motor was set by how many pulses per second were sent. The oscilloscope scale here was about 25 milliseconds full scale, and the vertical deflections went from zero to +12 volts. The pulse duration was about 3 milliseconds and the pulse repetition rate (PRR) was from zero to several hundred Hz. So I needed to design circuitry to generate these waveforms, and make those circuits controllable by an onboard computer.
Under the circuit board was the central nut of the azimuth axis. The nut remained stationary, attached to the black and orange base, and the circular surface rotated to point the telescope to the desired azimuth. Perfect place for an encoder!
Under the altitude axis cover I could see the nut that defined the altitude axis. Here the nut rotated as the altitude changed while the housing around it remained fixed. Another perfect place for an encoder.
For the encoders, I selected these from U.S. Digital:
These encoders, I used three of them, have a fundamental resolution of 10,000 counts/rev, which is about two arc minutes. However, this company also sold an interface device that could multiply the effective resolution x4 to 40,000 counts/rev, about one half an arc minute. However, they were incremental encoders, not absolute encoders. Fortunately they had an internal index pulse which permitted an absolute rotation reference point to be established. With that, and careful tracking of the total number of pulses emitted, an absolute encoder could be mimicked. They cost about $100 apiece.
Here is the interface device. It cost about $400.
Here are images of the encoders after installation on the scope. I had a small circular cups machined to cover, and grip, the exposed axis nuts. Brackets were hand made to permit attachment to the scope body. Here is the altitude encoder mounting:
And here is the azimuth encoder mounting:
