CED VideoDisc and Player Discussion

[SRSanford10]

RT9342 , in a post of 03/06/07, mentioned that in CED player models SJT and SKT the stylus arm motor drive in playback is "time based"(versus the stylus sensor control used on the SFT and SGT models). Would he or others have more information on how the servo derives the time information?
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Scotty

[RT9342]

I can check my service data to be sure, but if I'm not mistaken, one of the microprocessors simply fires a pulse to the motor every 2 seconds during playback on the SJT models (and I'm pretty sure the SKT models work the same). Quite different from the earlier models, which seem to use an analog balancing circuit that has to be properly calibrated and causes either "runaway" or "freezing" if not set correctly. I guess they were originally concerned about issues with the grooves not being the same width on all discs, but apparently the disc mastering was accurate enough that a constant, steady tonearm drive speed is sufficient for playing all discs.

[SRSanford10]

Thanks to RT9342 for the info on SJT and SKT arm action. I'm looking into using a modified CED player as a support for an objective lens and right angle mirror in order to expose photoresist on a metal substrate. The laser beam would be shaped like the end of a chisel and would need to be much wider than the production CED discs(perhaps 10 microns versus 2.7 microns) to compensate for horizontal runout in this crude "lathe".
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Scotty

[RT9342]

Sounds like an interesting project, and a way to make good use of a junk machine. I once worked on making a machine to drill holes in circuit boards by using the print mechanisms from 2 Apple II printers, with an old IBM XT serving as the controller. It was a real cheap way to do it, and probably would have worked, but I ended up scrapping the project. Anyway, if you're intending to use the existing arm drive circuit, that could be a problem if you're not using the player to play CEDs, because the pulses that drive the stepper motor every 2 seconds during playback originate in the player control microprocessor U5101 (the 42-pin IC). It would probably be difficult to get this IC to produce the pulses without detecting a DAXI signal from a disc. However, if this helps: the processor only generates the "B" pulse (pin 27) and the "D" pulse (pin 26), which are buffered by Q5104 and Q5107 respectively. Then these signals are fed into Q5105 and Q5106 to generate the "A" pulse, and Q5111 and Q5112 to generate the "C" pulse. Apparently the "A" and "C" pulses are just inverted forms of the "B" and "D" pulses. These pulses are sent to the motor through J1 (pin 1:"A", pin 2:"C", pin 3:"D", pin 4:"B"). I'm guessing the motor works similar to the ones I was using in my circuit board drilling machine, in which "A" goes on and off every 4 seconds and "C" goes on and off every 4 seconds, but with a 2 second time lapse in between (while "B" and "D" are simply opposite of "A" and "C" respecively). The direction of rotation would be determined by whether "A" chages before "C" or "C" changes before "A" if I'm not mistaken. This may sound complicated, but a 2-bit binary counter with a couple of logic gates (to produce the following sequence: 00 01 11 10) would probably work. This would be a way to make the motor run without needing the player's main board.

[SRSanford10]

Thanks to RT9342 for that design. I may be able to do that , or I may "borrow" the drive pulses from one of the two players I own(in PLAY mode) and feed them to my modified player(with an isolating amplifier). A difficult problem for a laser recorder is that the photoresist requires the use of a blue or violet source. These are quite expensive. The other way would be to cut holes in a thin metal film. This could be done with a red laser(less expensive) but requires perhaps a half watt or so of laser power.I'm researching this now.
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Scotty

[RT9342]

I was thinking, if you're planning on having one player play a CED while the other "borrows" the step commands, I'm wondering if you couldn't just remove the CPUs from the board in the "dummy" player and use the existing pulse amplifier that I mentioned to drive the motor in the "dummy" player. I'm pretty sure that the outputs from the CPU are probably strong enough to drive 2 sets of transistors, though you could probably use the outputs from the first transistors on the working player to drive the second and third stages of both players. But if you do that, make sure that you keep the players' power supply connections seperate, EXCEPT for ground - they should both have a common ground. Hope this helps, and good luck with your project!