NBSC Colour televisor construction diary.

[Rydepier]

I used a 1N4005 as in your circuit. I will get the counter tomorrow and look at the VCXO board, I remember doing some testing a year ago when I built the circuit. I looked at the unused vburst output and it is a copy of the frame pulse but the amplitude varies, does that sound about right.

Chris

[dominicbeesley]

Yes that sounds right.

It sounds like everything is close we could just do with measuring the frequency limits of the oscillator. This is likely to be the part of the system that is most sensitive to changes in build. I did mine dead-bug so low-capacitances round the crystal will/should give a better chance of the varicap doing its job. We may need to substitute something different for the diode like and LED but it needs tailoring to the environment to get roughly the right frequency pull range - too much and you'll end up getting side locks occasionally.

If you've not got a frequency counter the other option is a dual-trace scope and a good frequency standard (that could be a 15kHz sine-wave played back from a computer. Trigger the scope from the "known good" source and then you can tune the by playing with the inductor and offset pot.

With the burst signal disconnected you can adjust to get the second trace (say SC clock U or SC clock V) to go faster or slower or steady with the first trace...

Even a single trace scope should be capable if you can trigger from the 15kHz computer and trace U or V.

Looks like you're using a fancy PC scope so should be possible.

D