Quote:
Originally Posted by John Folsom
I have wound 2 color flybacks so far, one for a 15" Motorola color set, and one for a CT100. The Motorola flyback works, but it has lower impedances than the original, and has a big "dip" in the top of the flyback pulse, which is causing some issues with the gated circuits. So just having the winding turns count recipe is not all there is. More experimentation will be required to attempt to determine what parameters need to change.
|
I'd like to put in 2 cents worth of speculation here.
1) I have no idea why the primary inductance is less than expected if the turns count and pattern is the same, except maybe the core gap has changed in the process of disassembly/reassembly. Don't know what kind of gap material was used in these units. When I worked on flybacks in the 60's, we used hard paper gap material in the lab. Later, a material containing beads of a specific diameter would be painted on the face of a mating part for production. This was bright yellow stuff you see on some parts.
There is a danger of upping the inductance by decreasing the gap, in that you will decrease the current at which the core saturates (a bad thing) -but if the gap currently is too wide, it is worth trying.
2) The dip in the top of the flyback pulse is because there is too strong a 3rd harmonic resonance in the HV winding.
a) I don't know if these old flybacks were supposed to use 3rd harmonic tuning - it was common practice by the 60s, especially for solid state sets, because the right amount would flatten out the plate/collector pulse and reduce stress on the horizontal output device.
b) I am beating myself on the side of the head, but it's not helping me recall what to do to modify the strength of the 3rd harmonic - probably involves choice of the dielectric material, but since you generally don't have options, changing the HV winding parameters - more turns and less layers or vice versa. However, as I think about it, it seems to me those kinds of modifications would change the frequency of the tuning more than the amplitude. Right now I feel really dumb, 'cause I used to do this for a living, and the one thing I'm sure of is I'm not remembering it straight.
c) just FYI, some later designs used 5th harmonic tuning to flatten the collector pulse even more - approximating a square wave by a combination of fundamental, 3rd, and 5th.
Edit: there is always the possibility that if you get the correct primary inductance, the ratio of 3rd harmonic amplitude will turn out to be correct -- fix the first obvious thing, and it may improve two problems.