08-02-2018, 07:27 PM
Hi.
Well the issue is not with the oscillator. Flyback time is as it should be when driving the line output.
The issue is on the output stage. From what I can deduce it is a mismatch between the scan coils and the Jellypot, fine on the lower standards but the faster the line rate the worse the problem.
I introduced a bit of damping across the scan coils, this reduced the width but the foldover disappeared, increasing the HT full width returned and the picture was perfect although the damping resistor was getting really hot.
I then thought of adding flywheel sync and adding in some DC component to the discriminator for phasing, this works too as I cobbled up the Thorn 1500 circuit that uses a transistor as a DC amp, on 405 I can phase the scan to give a full sized picture with no cutoff either side. This is the way to go to keep dissipation at a minimum and have a very good result.
I'll build the discriminator and DC amp on some veroboard.
Well the issue is not with the oscillator. Flyback time is as it should be when driving the line output.
The issue is on the output stage. From what I can deduce it is a mismatch between the scan coils and the Jellypot, fine on the lower standards but the faster the line rate the worse the problem.
I introduced a bit of damping across the scan coils, this reduced the width but the foldover disappeared, increasing the HT full width returned and the picture was perfect although the damping resistor was getting really hot.
I then thought of adding flywheel sync and adding in some DC component to the discriminator for phasing, this works too as I cobbled up the Thorn 1500 circuit that uses a transistor as a DC amp, on 405 I can phase the scan to give a full sized picture with no cutoff either side. This is the way to go to keep dissipation at a minimum and have a very good result.
I'll build the discriminator and DC amp on some veroboard.
