Heat sensitivity?

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Heat sensitivity?

Postby Andrew Davie » Sun May 20, 2007 3:07 pm

As I play with my now-working mechanical televisor, I have noticed that there appears to be some 'heat sensitivity' to the setup. If I startup 'cold', the setting I have for the motor speed control pot of the motor circuit isn't enough. I have to turn it up. After a while, I have to turn it down, as it runs too fast and loses synch.

Likewise, with the 'double speed' testing, sometimes I notice that I can't turn the pot far enough to get the motor to go fast enough -- and other times, it's plenty, and the motor will run OK.

I'm suspecting the motor speed circuit because when I remove the IR led signal, the motor literally takes off -- roaring into way past the needed speed. It seems to me that there's some characteristic of the circuit -- the PLL chip, perhaps, that is rather sensitive to temperature (?).

This annoys me a bit, because I can't set that speed control pot to a set position and leave it. In any case, I'm considering moving that pot to my front panel as a user-control, to allow the playback of out-of-standard frame rates. Another interesting observation, the speed control can be used for vertical centering of the frame -- plus or minus, say, a quarter-frame in size. I imagine this would normally be done by adjusting the position of the IR pair.
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Temperature sensitivity.

Postby Steve Anderson » Mon May 21, 2007 12:30 pm


There are two possible causes for your observed rotational temperature sensitivity...not yours, the disc.

First is the 'output' part of the configuration you're using. The nominal gate to source voltage will change as the device warms up, silicon in all its incarnations is horrendously temperature sensitive. This should be designed out, but frequently isn't. Hence you having to 'tweak' the pot.

The PLL is applying a small correction voltage to the major bias provided by the pot. It simply runs out of range and loses lock.

Second factor could be the motor. As it warms up it too could change its characteristics. Oil or grease will become thinner, clearances in bearings (friction) could become larger or smaller.

To find out which one is the major factor I would apply a fixed and well regulated Voltage to the motor, measure its RPM when cold and re-measure some time later. This should give you a clue as to where to point your finger.

Steve A.
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Postby Klaas Robers » Tue May 22, 2007 9:13 pm

Andrew, I can report the same. When getting warm the motor tends to run faster and I have to turn the manual speed control back to get the same speed. I fear that this is because of the brushes and the commutator in the motor. When running they get cleaned and make better contact. It may be that it is not really temperature, but that the commutator gets oxidized during standing still and this has to be removed again. After realigning the speed and resynchronisation, so after say a quarter of an hour, the monitor will keep its synchronisation for hours.
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