Mechanical Television (3)

This section goes into quite a bit of detail on how I built my rotating disc television. Please note that I'm not responsible if you do or copy anything I describe and something goes wrong!




The motor must rotate the disc at 750 revolutions per minute (rpm), or 12.5 revolutions per second. This was the speed required by the original transmissions made by the BBC in the 1930's and is also the club standard for the Narrow Band TV (NBTV) Association. The picture shows an early motor from a Baird Televisor.

There are many kinds and sizes of modern motor available but taking into account the required speed, torque and smooth running characteristics the best choice is usually a small dc motor. The type used in a cassette players is known to work well.





The early method used to control the speed of the motor was to put a rheostat in series with it. This technique can still be used but making the disk run at precisely the right speed is difficult. I suspect the method may have worked better than expected because once the disk was very close to the right speed incidental coupling between the radio driving the neon light source and the motor supply (such as common ground connections) may have helped lock the motor to the correct speed. The modern method is to monitor the speed of the disc and use an electronic circuit to adjust it to match the signal to be displayed. More on this later.


Small DC electric motors, although well suited for this purpose, generally perform best at around 2000 to 3000 rpm. Some constructors take advantage of this by using a drive belt with a reduction ratio of typically between two or three to one. However for simplicity I designed my disk to be directly driven from the motor.




I did experiment with a few different motors reclaimed from various pieces of equipment such as a failed computer CD drive, but the most successful motor turned out to be a 9 volt cassette player motor I bought from the NBTV club shop.

This particular motor is type EG-500AD-9F (the club shop does not always have the same type available) and it rotates clockwise when looking down onto the spindle.





The problem with cassette player motors is they usually contain an electronic circuit to regulate the speed. The circuit inside this motor tries to keep the speed at 2400 rpm which is not helpful when you want it to run at 750rpm.

Apparently it is easy to disable the speed control circuit, but I've not come across any instructions how to do it, so here is how I did it with this motor.

First I used a screw driver to flip off the back plate. To make this easier I temporarily screwed it to the mounting bracket I would use to mount it in its final position. Levering it off seemed like a bad idea because this might damage the printed circuit board where it protrudes through the case.




Behind the back plate is a disk of rubber which can be lifted out to reveal the printed circuit board (PCB). This picture shows the PCB in its unmodified form. The connections to the motor brushes are the two large blobs of solder next to the hockey stick shaped slots. These are where direct connections can be made to the motor.








Before making direct connections I looked over the circuit and decided that the speed control circuit could be sufficiently isolated from the motor by making one track cut - this can be seen at the top left of this picture.






Here I have soldered wires to the points which connect directly to the brushes. Because there is a large enough gap with the back plate fitted to bring the wires out it was not necessary to rewire it to use the original power connections to the PCB.

The red and black wires are attached with the same polarity (red being positive) as the speed controller circuit would have applied the power.






The rubber disk is put back in position and the wires positioned to come out of the operning in the case.









The back plate is then pressed back into position, aligning the flat with the opening. This picture shows the motor with the modifications finished.






With the electronic speed control disabled you may think the motor can be run clockwise or anti-clockwise just by reversing the power. This is not wise because it is not just the electronic circuit which determines the power supply polarity but also the design of the brushes. This picture shows another motor disassembled with the brushes (which are springy metal blades) in their white plastic mounting on the right.

When turning in the correct direction the force on the brushes from the spinning commutator is away from their mounting points which keeps them straight and in good contact with the rotor.



If reversed the force is towards the mounting point which will distort the brushes, reduce the effectiveness of contact to the commutator and cause it to run less smoothly and ultimately shorten its life.

My modified motor is designed to turn clockwise. Therefore to make it turn the disc anti-clockwise, as required by the standard, I mounted the motor in front of the disc with the spindle pointing away from the viewer.

Onwards to the Nipkow Disc.........

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