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AD724 RGB to NTSC/PAL Encoder

Discussion in 'General Electronics Chat' started by gregmcc, Aug 11, 2018 at 7:53 AM.

  1. gregmcc

    gregmcc Member

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    I'm having a go at building a ZX Spectrum which has been designed to use the AD724 chip.
    I think my AD724 chip might be faulty as I'm not getting any video but I would like some advise from someone who mas maybe used this chip before, before I replace it.

    AD724 datasheet:
    http://www.analog.com/media/en/technical-documentation/data-sheets/AD724.pdf

    Capture.PNG

    I've checked all the correct voltages are on the chip, pin 1 is low for PAL, no shorts. The problem is I'm not seeing a clock on FIN pin. Its a steady 1.9V. Should there not be a clock there from the crystal? (I've replaced 4.433mhz xtal/10pf cap/4M7 resistor). Or am I not understanding how it works? (Quite possible!)


    Full schematic for Spectrum if it helps.
    http://trastero.speccy.org/cosas/JL/Harlequin/Documentacion/harlequin_rev_g.pdf
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I've never used that chip, so can't comment on it specifically, but do you have a x10 probe on your scope? - using a x1 probe will almost cetainly stop the clock when you apply it, where a x10 one is usually OK.
     
    • Agree Agree x 1
  3. unclejed613

    unclejed613 Well-Known Member

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    that's not the way to use a crystal for a clock. you should at least use an inverter like a 4069 to make a clock oscillator. with the circuit you have now, the crystal does nothing.
    this application note: https://www.fairchildsemi.com/application-notes/AN/AN-118.pdf should make things a bit clearer.
     
  4. dave miyares

    Dave New Member

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  5. rjenkinsgb

    rjenkinsgb Active Member

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    For that specific device, it is.

    That's what it shows in the data sheet - see page 11 (though without the resistor, just a trimcap).
    It says you only need an external oscillator if you also need the osc signal for other parts of the design.
     
  6. gregmcc

    gregmcc Member

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    lol. That was it thanks. Setting to 10x I can now see the clock.
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    I've spent decades repairing TV's, you soon find out that you need x10 probes for checking crystal oscillators :D
     
  8. dave miyares

    Dave New Member

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  9. Ian Rogers

    Ian Rogers User Extraordinaire Forum Supporter Most Helpful Member

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    The original zx used the ULA to decode the video... Is this a Russian clone??? You may be better integrating the TEA2000 ( used in the 128 ) gives a better video output!!
     

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