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 Post subject: Composite video in old TV
PostPosted: Apr Thu 15, 2021 4:36 pm 
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Joined: Apr Thu 15, 2021 4:32 pm
Posts: 49
Hi guys,

Looking to get my first vacuum tube based set. Obviously will need repair, I am an electrical engineer so I'm comfortable with schematics and repair of sorts. My only thing is I haven't necessarily worked with vacuum tube electronics before, but am comfortable around CRTs.

Anyway...does anyone have experience bypassing the RF input and going directly into the video amp and what kind of input I would need to build. I've seen a few vacuum tube amplifiers and some where its directly wired in. I would prefer a transistorized solution going into the video amp but up to building anything. Just to keep the rest of the TV original and least modified as possible.

Any help would be great!

-Tp4by3


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 Post subject: Re: Composite video in old TV
PostPosted: Apr Thu 15, 2021 6:45 pm 
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Joined: May Sun 18, 2008 3:18 pm
Posts: 115
Location: Sioux Falls, SD
Tp4by3,

I am interested to see what others have done to splice in composite directly. If the set is a not a transformer set you will want to isolate the inputs, the other concerns are around providing the correct signal levels that the video amp is expecting and will need to insure the input signal has the proper phase. Two points of reading material I have came across so far:

https://web.archive.org/web/20110422150207/http://www.ankythera.com/Projects/video_stage.htm
https://antiqueradio.org/A-V_AdapterForVintageTVs.htm

For what it is worth I did successfully directly splice in composite video directly ahead of the video amp tube on an Admiral 19A1 chassis, see the red X, this is a smaller 7jp4 set.
Image

My current project, a 1968 color hybrid set I am tempted to again splice just ahead of the video amp transistor at test point B
Image


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 Post subject: Re: Composite video in old TV
PostPosted: Apr Fri 16, 2021 1:29 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1570
Location: 07450, New Jersey
I have used the AD828 video op-amp to adjust the video input to the level expected by the particular circuit .

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"One of the advantages of being disorderly is that one is constantly making exciting discoveries." A. A. Milne


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 Post subject: Re: Composite video in old TV
PostPosted: Apr Fri 16, 2021 6:59 pm 
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Joined: Apr Thu 15, 2021 4:32 pm
Posts: 49
Alright, I found a TV i'm looking at. Quick look online its already been composite modded, so I will take a look and see what I can do.

Thanks guys, I'll report back if I have trouble.


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 Post subject: Re: Composite video in old TV
PostPosted: Apr Fri 16, 2021 8:45 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 39559
Location: SoCal, 91387
Why not just use a separate RF modulator? Composite video/audio in, RF channel 3/4 out.

I used to use them with pay-TV decoders I built.

Image

Another method would be to use a VCR with composite inputs, and an RF output.

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 Post subject: Re: Composite video in old TV
PostPosted: Apr Sat 17, 2021 11:52 pm 
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Posts: 1564
Location: Belrose, NSW, Australia
It's easy to get this working, more difficult to get it working properly in many cases. Unless the set has DC restoration somewhere following your input the results may be disappointing. And if the contrast control is done via the AGC you will have no contrast control.

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 Post subject: Re: Composite video in old TV
PostPosted: Apr Mon 19, 2021 12:38 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 896
Location: Rockaway , nj 07866
One advantage of feeding directly to the video output is you will get the best picture the tv can offer but as stated it might not be easy , some cases it is. I fed video one time to a set like in the above schematic and had no sync vert or horz


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