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 Post subject: Re: Simplest TV a beginner could construct?
PostPosted: Sep Thu 22, 2022 4:04 am 
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Joined: Nov Thu 11, 2010 5:03 pm
Posts: 1883
Location: Pewaukee, WI
Location: Pewaukee, WI
You might want to look for a 1948 RCA 8TS30, Philco 48-1000, Motorola VK-101, Stromberg Carlson TV-12 table model or the radically different looking Dumont RA-103 doghouse (The S-C used that same Dumont chassis). The Dumont and RCA sets are well known for producing incredibly sharp detailed pictures that reach the limits of broadcast NTSC resolution.
They all are fairly common affordable (if you shop unrestored sets) Pre-War looking post war sets....If you wanted to spice up those factory made monochrome sets after restoring them to operation you could build a col-R-tel color wheel color converter that would sit on top/Infront of the screen of set and convert it to color. http://earlytelevision.org/col-r-tel.html

With modern electronics (you can ignore the original circuit) you probably wouldn't need to modify the TV to add a color wheel. You could probably implement it with an HDMI to 640x480 VGA (VGA is 3 Separate R,G, and B video signals, plus 2 sync channels, and some unimportant extra signals) adapter feeding V sync to an Arduino controlling a servo with a magnetic pickup for speed feedback. The Arduino could switch on and off 3 video OP amps gating each of the 3 color video signals one at a time to another amp that would combine the active video signal and sync then feed that signal into an RF modulator which would feed the sets antenna input.
A properly working RCA or Dumont set ran with a color wheel fed by VGA would theoretically have better color resolution than any color set fed by composite video... Granted a color wheel in a set running NTSC frame rates is going to have noticable color flicker...I've seen a col-R-tel in operation, and while flicker is noticable it does produce a watchable color picture.


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 Post subject: Re: Simplest TV a beginner could construct?
PostPosted: Sep Fri 23, 2022 3:06 am 
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Joined: Nov Fri 30, 2012 2:35 am
Posts: 693
Location: Phoenix, AZ
Looking once again at the simple TRF-based design from August 1941, and assuming input of an RF signal in NTSC format, where in the circuit - and how - would one tune and access the audio for the chosen channel used? A discriminator circuit of some sort? Slope detection? I'm trying to figure out a likely (but simple) circuit for the add-on audio section that was promised but never published. Thanks.
R/ John


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 Post subject: Re: Simplest TV a beginner could construct?
PostPosted: Sep Fri 23, 2022 4:39 am 
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Location: Ann Arbor, MI
Location: Ann Arbor, MI
With a NTSC signal there will be a 4.5 MHz signal at the video detector that is both FM modulated with the sound and AM modulated with the video. Most TVs use this for the sound. They have a 4.5 MHz IF amplifier and an FM detector for the sound. The FM detector will ignore the AM modulation.
You could extract the sound circuits from an old parts set TV.
Or you could cheat. If you are getting the NTSC RF signal from a converter box or VCR, just connect the audio output jacks from that box to any audio amp and speaker.

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Tom


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 Post subject: Re: Simplest TV a beginner could construct?
PostPosted: Sep Fri 23, 2022 5:26 am 
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Joined: Nov Fri 30, 2012 2:35 am
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Location: Phoenix, AZ
Thank you Tom, good info ... coming up with a circuit (or borrowing one from an old set) might not be too difficult after all.
R/ John


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 Post subject: Re: Simplest TV a beginner could construct?
PostPosted: Sep Sat 24, 2022 1:40 am 
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Posts: 23390
Location: Dayton Ohio
There is info of converting a BC-412 oscilloscope into a simple TV in this Military Surplus Conversion Manual.

https://radionerds.com/images/2/28/Surp ... lume_1.pdf

I remember mulling over the idea but I never found a BC-412 to do it.

-Steve

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Consoles and floor models, the bigger, the better!


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 Post subject: Re: Simplest TV a beginner could construct?
PostPosted: Sep Sat 24, 2022 1:07 pm 
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Joined: Jan Wed 15, 2020 11:29 pm
Posts: 2237
Location: AUSTRALIA
Location: Queensland.
A classic TV set appeared in Radio Television & Hobbies in AU that used the easy to get 5BP1 CRT, you could use many others like the 5UP1 very common in scopes:

https://www.cool386.com/5BP1/5%20inch%2 ... eiver.html

In the UK they were fond of using ex Radar CRT's, and Practical television made a whole project out of it, I built one called the Argus, these were interesting in that the Pentode tubes used in them were instrumental in the Brits winning WW2 and the invention of Radar. These tubes, the EF50 with a loctal base, suited to airborne work, are not electrically dissimilar to the American 6AC7 with the plain octal base:

http://www.worldphaco.com/uploads/ARGUS.pdf

(also if you look up the 1939 TV's the Andrea KTE-5 and the Meissner 5" TV set, you will see similar circuits for electrostatic TV sets)


Since analog TV transmissions have closed down, there is less of an incentive for a home constructor to make the TV's RF & IF stages. These of course were always the trickiest part of the design for a novice, as they required to be set up with a sweep generator and scope and by a technician with a reasonable knowledge of the properties of band-pass amplifiers and how to adjust them with these tools, which most home constructors lacked.

So if you are venturing down this road, you would be better to simplify things by essentially making a VDU (video monitor) which contains the sync separators, video amplifiers, H & V scan generators , CRT high voltage support circuitry and not bother building the radio frequency front end. Then you can drive it with a typical baseband monochrome composite video signal, rather than an RF source carrying the video signals & sound as modulation intelligence.

In the UK post WW2 they were still using AM sound in the 1950's. Many sets of this era (there) were magnetically deflected, with magnetic focus (often with PM magnets) and were TRF designs with separate tuned RF amplifiers for the video and sound channel, which was AM sound. America had already gone to FM sound post war.


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