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ZX81 Composite Video Mod?

chaoticjellychaoticjelly
edited August 2007 in Hardware
Hello.

Does anyone know how to modify a ZX81 to output composite video as opposed to a modulated RF signal?

I believe it can be done with one transistor (which?), and a resistor (value?).

If someone has a diagram, that would be great. Many thanks in advance..
Post edited by chaoticjelly on

Comments

  • beanzbeanz
    edited August 2007
    I'd also be interested in this. If there is a Dummy version (photographs if possible!) that would be great.

    Also can you do this on a spectrum easily for a novice?
  • GodsGods
    edited August 2007
    chaoticjelly wrote: »
    Hello.

    Does anyone know how to modify a ZX81 to output composite video as opposed to a modulated RF signal?

    I believe it can be done with one transistor (which?), and a resistor (value?).

    If someone has a diagram, that would be great. Many thanks in advance..

    http://www.zx81.de/english/video_e.htm

    Any NPN transistor (BC547,BC548,2N2222,...)
    For the resistor 75R to 100R...
    Simply connect at UHF modulator pins (+5V,GND and Video IN)
  • PhuPhu
    edited August 2007
    Hi,

    You may find you don't even need the transistor and resistor.

    I found my TV was quite happy to accept the video signal present at the input of the modulator. The connections are identified as follows; The casing of the modulator (including the pins which are soldered to the board) is the ground connection. The +5V is the wire that appears to be soldered onto the side (in actual fact there is a subtle insulating ring between the solder blob and the casing). The remaining wire that pokes in through a white insulator is the video input.

    You should be able to try connecting it momentarily to see if the TV will accept the input.

    -- Richard
  • PilsenerPilsener
    edited August 2007
    Gods wrote: »
    http://www.zx81.de/english/video_e.htm

    This circuit is not mandatory to get a signal at all. But when I tried both with/without it on my Speccy, I found that this buffer-circuit gave me a picture with less noise in it.

    The backside of this buffer, is that you do lose a tiny little bit of the signal due to the voltage drop between base-emitter on the transistor. However, it's not very significant.
  • chaoticjellychaoticjelly
    edited August 2007
    Gods wrote: »
    http://www.zx81.de/english/video_e.htm

    Any NPN transistor (BC547,BC548,2N2222,...)
    For the resistor 75R to 100R...
    Simply connect at UHF modulator pins (+5V,GND and Video IN)

    Many thanks for this link, Gods.

    An excellent circuit diagram and picture, as trivial to implement as I had imagined :)

    Thanks again!
  • PengwinPengwin
    edited August 2007
    I bypassed the modulator on my spectrum which, whilst worked, gave me a slightly washed out picture compared to the standard signal. I would advise implementing the circuitry.
  • chaoticjellychaoticjelly
    edited August 2007
    I did the mod today, it works beautifully.

    I made a small circuit using stripboard, I used a BC547B NPN Transistor and 100 ohm resistor.

    Disconnected the resistor attached to the phono socket, put a piece of heatshrink over the end of it. Tapped ground from the thick trace next to the RF modulator box, tapped the original composite signal from the left hand of the two wires going into modulator box, tapped +5v from the right hand of the two wires going into modulator box.

    Then I have the output from the transistors emitter going through the unused hole which feeds into the RF modulator, and then attached to the phono socket..

    A crisp interference-free picture and very easy to implement..

    Does this work on a rubber key Speccy too?
  • DEATHDEATH
    edited August 2007
    chaoticjelly wrote: »
    I did the mod today, it works beautifully.

    I made a small circuit using stripboard, I used a BC547B NPN Transistor and 100 ohm resistor.

    Disconnected the resistor attached to the phono socket, put a piece of heatshrink over the end of it. Tapped ground from the thick trace next to the RF modulator box, tapped the original composite signal from the left hand of the two wires going into modulator box, tapped +5v from the right hand of the two wires going into modulator box.

    Then I have the output from the transistors emitter going through the unused hole which feeds into the RF modulator, and then attached to the phono socket..

    A crisp interference-free picture and very easy to implement..

    Does this work on a rubber key Speccy too?


    Yes

    http://www.roygarland.co.uk/spectrum/composite%20mod.jpg
    http://www.roygarland.co.uk/spectrum/composite%20mod.2jpg.jpg
    Oh bugger!<br>
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