AD724 RGB to NTSC/PAL Encoder

[gregmcc]

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



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

 

[Nigel Goodwin]

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.

 

[unclejed613]

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.

 

[rjenkinsgb]

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.

 

[gregmcc]

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.

 

[Nigel Goodwin]

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

I've spent decades repairing TV's, you soon find out that you need x10 probes for checking crystal oscillators

 

[Ian Rogers]

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!!