5" black and white tube TV PK-4184 - looking for composite input

Discussion in 'General Electronics Chat' started by bbiandov, Mar 6, 2016.

  1. bbiandov

    Thread Starter Active Member

    Nov 24, 2007
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    Hi everyone,

    I've got 5" black and white tube TV model PK-4184 and I'm looking for composite input? The TV doesn't have one, it only has an old style analog tuner which is useless now days. The main and only IC running the TV is D2915CP. The PCB has markings TV502PCB. It's a Chinese thing built in early 2000s.

    I've looked at the data sheet of that IC and many schematics of similar but NOT identical TVs and I'm still unsure whether it's possible to capture a trace or something like that so I can feed it composite signal, which is my ultimate objective.

    It seems that pin 5 is the holly grail but the TVs that do have composite inputs also have bunch of other parts sitting in between pin 5 and the tube driver and those things aren't present in my TV. Any advice would be appreciated!

    Thank you
     
  2. crutschow

    Expert

    Mar 14, 2008
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    You need an old TV analog modulator circuit such as one of these. It converts the composite input to a channel 3 or 4 RF output that you connect to the antenna input.
    You can likely also use the modulator in an old VCR for that same purpose, if you have one stored somewhere.
     
  3. bbiandov

    Thread Starter Active Member

    Nov 24, 2007
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    Thanks for the reply crutschow , I do have one but I don't like the RF artifacts which are introduced by the up-converting and then down-converting it back to composite. I'm not feeding video but rather text and that's when one can observe all the RF defects while video is so much more forgivable...
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Do you have an oscilloscope?
    You might be able to find the composite video signal coming out of the tuner and then tap into that for your input.
    To identify the signal it helps to input a composite RF signal from a modulator through the tuner while probing with the oscilloscope.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  6. RichardO

    Well-Known Member

    May 4, 2013
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    I once did this. I think I did a DC restore of the video to some place in the circuit near the CRT cathode...

    Just be careful that you do not have a hot chassis TV when doing any modifications of this sort! An isolated power supply is a must.
     
  7. bbiandov

    Thread Starter Active Member

    Nov 24, 2007
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  8. Dodgydave

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    Jun 22, 2012
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    Pin 5 looks like its the modulation signal with video and audio subcarriers, the video signal should spilt into Audio and Video separately, through ceramic filters then processed each.
     
  9. bbiandov

    Thread Starter Active Member

    Nov 24, 2007
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    Strange, if I shunt pin 5 only video blanks out while the audio continues to play just fine...?
     
  10. Dodgydave

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    Jun 22, 2012
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  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Datasheet here...pin 5 is definitely the video out, it doesn't have an AV pin, you have to use the emitter follower that drives the cathode.

    http://archive.esfile30089.html 1068/file30089.html
     
  12. bbiandov

    Thread Starter Active Member

    Nov 24, 2007
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    Thanks Dodgydave, I did find that schematic a week ago and it seemed like a close match, hence my idea to capture pin 5 just as it is shown on the schematic with the AV IN jack. However their tube appears to be simplified or I am missing something? My crt has 8 wires plus the high voltage one. 4 wires for deflection control (side of tube windings) and 4 for gun control - top of cone. Does that make this schematic inapplicable in my case? Of course real life testing had revealed that cutting pin 5 and feeding composite video downstream the circuit doesn't produce the result I want -- crt remains dark.

    Here's what I did -- cut at the "X" and feed composite at the arrow -- still no go?
     
    Last edited: Mar 7, 2016
  13. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Looks like their circuit feeds into pin 6 as well as the transistor driver, to separate the sync pulses, and black level clamp, maybe your circuit is different?
     
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