Compact Drum Monitor

Forum for discussion of narrow-bandwidth mechanical television

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Postby DrZarkov » Mon Apr 04, 2011 1:52 am

The result is very convincing! Great little monitor, congratulations. :)
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phonic wheel magnetizm

Postby holtzman » Mon Apr 04, 2011 6:24 am

I made some tests with electromagnet for sync. Seems like my initial plan to make phonic wheel with C-shaped core and bolts closing its gap needs revision.

The problem is that the bolts would be attracted either to one end of this C or to other, the system is not stable in the direction parallel to drum's axle.

Anybody has an idea how this obstacle was solved in this Italian tv? There are 3 coils, and a core of strange shape in it's electromagnetic synchro set. How do small steel pieces stay more or less in the middle between 2 electromagnets?

I know there was written a lot about phonic wheel theory, but I didn't come across anything about sync coils set like these historical one (which did work I believe). Many thanks for the feedback...
Attachments
phonic.jpg
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nipkow5.jpg
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nipkow9.jpg
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Postby AncientBrit » Mon Apr 04, 2011 5:36 pm

@ Holtzman,

Could you please post a close up photo of the stepper motor and the attached shutter?

I'm still a bit unclear how you get such clearly defined vertical scan lines with no oblique cut-off to the left and right of image.


Kind regards,


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Postby Andrew Davie » Tue Apr 05, 2011 1:07 am

AncientBrit wrote:I'm still a bit unclear how you get such clearly defined vertical scan lines with no oblique cut-off to the left and right of image.


Looks to me like the inner (small) rotating mask is actually large enough so that the image area is quite a bit smaller than the apparent diameter of the wheel. So there's minimal curvature between the center and edges of the image.
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Postby kareno » Tue Apr 05, 2011 1:53 am

Wonderful work!

On the phonic wheel: Could you bring the coils closer together and place the pole faces just below the bolts? ...so that each end of each bolt passes by one of the pole faces? That would keep the forces acting outward at all times.

(see attachment)
Attachments
phonic.jpg
Pole faces below bolt
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Postby DrZarkov » Tue Apr 05, 2011 5:35 am

How about using permanent magnets as bolts?
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small drum

Postby holtzman » Tue Apr 05, 2011 7:15 am

Yes, Andrew explained it right. There are 6 slits on the small drum, so the image area is narrow compared to drum diameter.

It's possible to make only 4 slits on the small drum, 20 slits on the main drum, and to rotate the main drum at 3 times higher-than-normal rpm. This way a 60 line raster is made.
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IMG_0083.jpg
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Phonic wheel

Postby holtzman » Tue Apr 05, 2011 7:38 am

Karen, thank you so much for your idea, it's very clever. This way it should work most efficiently because "magnetic cirquit" is being closed yet no sideway forces appear.
But before winding coils by myself I tried to use something ready-made, a small solenoid converted to electromagnet. I removed the core it and attached a U core made from some bolts and brackets. With this core magnetism was much weaker than with the solenoid's original core.
So I'll try first this strange E core from original solenoid parts, it's central part attracts bolts quite strongly.
Permanent magnets are an option, but they should be mounted tangentially, perpendicular to the drum rotation axis.
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IMG_0085.jpg
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Postby Jean-luc » Tue Apr 05, 2011 7:43 am

Great work once again, Holtzman !The régularity and the linearity of the vertcal scaning lines are perfect,, it’s realy amazing.

About the Fracarro sync, I think that a coil of one side of the aluminim disk is used as an electric brake , like a Barlow weel for a fine adjust of the rotating speed

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Postby AncientBrit » Tue Apr 05, 2011 5:17 pm

>Holtzman,

Thanks for the photos and the explanation,

Regards,

Graham
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Postby kareno » Thu Apr 07, 2011 11:10 pm

Holtzman,

I have to say, that is one of the best NBTV images I have ever seen. Certainly one of the largest!

I'm particularly impressed by the precision of your vertical timing. Even with the greatest care, results often have uneven horizontal detail due to tiny timing errors.
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Thanks

Postby holtzman » Sat Apr 09, 2011 11:37 pm

Many thanks for the comments!

Yet the sync is not solved at all... I tried the attached circuit, already published on this forum, for inverting the video. This is because my sync circuit needs inverted signal and if connected to the same power supply with the video, it's short circuited. The inverter circuit does not work properly, and I decided to make 2 power supplies, like in my previous tv. Just curious why it didn't work for me...I replaced transistor, resistors, caps... It started to give an inverted signal when R4 was reduced to just 120 Ohms, but there was strange noise mixed with the signal. I tried this with batteries, disconnected from tv circuits, still the noise was present.

Actually, there's going to be 3 trannies in this project: one for video, one for sync, one for audio. I know it might be better, but the audio and small drum circuits need 9V and there is no choice.

Many thanks for your feedback, hope to proceed as time allows...
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inverter01_1442.jpg
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Sync story

Postby holtzman » Sat Apr 16, 2011 11:39 pm

I spent some hours over phonic wheel experiments, and I have to say it turned out to be a dead end for me. Humming noise produced by the wheel was terrible and it gave no enough power.

So I proceeded with my "alternative synchronisation", but tried to reduce it using only one main motor without sync motor. Instead of driving the sync motor by short pulses, I used pulses to cut power from the main motor. This did not work stable from the start, but after adding some enhancements the sync works! I'll publish the circuitry as time allows.
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YouTube clip

Postby holtzman » Sat May 21, 2011 5:29 am

Dear friends, you are welcome to watch my drum device synchronized and working:
http://www.youtube.com/watch?v=EXpA9HNiyHg
Sorry for a poor footage but it works.
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Postby DrZarkov » Sat May 21, 2011 4:44 pm

It works very good! Very good picture, great work!
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