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AD724 RGB to NTSC/PAL Encoder

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gregmcc

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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:
https://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.
https://trastero.speccy.org/cosas/JL/Harlequin/Documentacion/harlequin_rev_g.pdf
 
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.
 
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.
 
that's not the way to use a crystal for a clock
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.
 
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.

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