using triplers in old color sets

[DaveWM]

I was pondering the idea of rewinding a fly, where only the HV section failed.

Seems the issues would be working with the very fine gauge wire needed to get the number of turns up to the working voltage AND having to deal with insulation to prevent arc over between windings.

IF the lower voltage windings are ok (or even if not an you need to rewind the whole fly) why not just use thick better insulated wire and only step up to about 10kv then use a commercial tripler to get get the HV, no more 3a3 and have access to a focus supply as well.

I did try directly connecting a tripler to the plate of a horz out once on a working set and got 11-13kv (IIRC).

I know some of the older sets (pre CTC-12) have flys with a lot more taps for various purposes.

[miniman82]

Originally Posted by DaveWM

Seems the issues would be working with the very fine gauge wire needed to get the number of turns up to the working voltage AND having to deal with insulation to prevent arc over between windings.

These are the least of your worries- it's striking a balance between inductance, the resistance of the winding, and distributed capacitance. These influence the resonant frequency of the whole transformer, get it wrong and all your work will be for nothing. Never mind the fact that you need the special winding rig...

[DaveWM]

that is why I was thinking of just replacing the HV winding leaving the primary alone. At least all the inductance matching to the yoke and the aux drives hopefully would not be adversely effected.

[hi_volt]

The resonant frequency of the flyback transformer is highly dependent on the interwinding capacitance of the secondary. If you remove half of the secondary windings, your resonant frequency will increase and the transformer will no longer be properly tuned.

[DaveWM]

Originally Posted by hi_volt

The resonant frequency of the flyback transformer is highly dependent on the interwinding capacitance of the secondary. If you remove half of the secondary windings, your resonant frequency will increase and the transformer will no longer be properly tuned.

Is the resonant freq of the flyback independent of the resonant freq of the eff coil inductance? if not perhaps compensation for change in the fly could be offset by a change in the LC of the eff coil.

[old_coot88]

Here's an excellent treatise on the arcana of flybacks in general. It addresses the impracticality of trying to rewind them.
http://www.repairfaq.org/sam/flytest.htm#flydis

[old_tv_nut]

There are two resonances of interest in a flyback. The first is that of the primary and yoke with a discrete capacitor, which sets the retrace time. Reflectance of the secondary into this circuit probably affects it, but I can't say how much, because when I was designing flybacks we always went from an existing design, never tested a primary only by itself.

The second resonance of interest is the secondary. It is formed by the secondary inductance and distributed capacitance (not a discrete capacitor). While the total turns sets the output voltage, the number of layers/turns per layer plus the spacing between layers plus the characteristics of the potting compound determine the distributed capacitance. This second resonance is tuned by adjusting all these things that affect distributed capacitance. In the usual designs I worked on, the secondary was tuned to the third harmonic of the retrace. Some later ones were tuned to the fifth harmonic. The idea was to have the third harmonic in the secondary out of phase with the retrace pulse at the high voltage winding top, and have just the right amplitude so that the third harmonic tended to flatten the top of the transformed flyback pulse. This meant that the high voltage rectifier had a broader flatter voltage pulse input, making the high voltage output more stable with varying amounts of beam current. In other words, the impedance of the high voltage supply was reduced so that it was "stiffer" and there would be less variation with beam current, meaning less blooming. It is this improvement that you will lose if you wind a new secondary without redoing it by trial and error to get the tuning right.

[timmy]

Originally Posted by old_tv_nut

There are two resonances of interest in a flyback. The first is that of the primary and yoke with a discrete capacitor, which sets the retrace time. Reflectance of the secondary into this circuit probably affects it, but I can't say how much, because when I was designing flybacks we always went from an existing design, never tested a primary only by itself.

The second resonance of interest is the secondary. It is formed by the secondary inductance and distributed capacitance (not a discrete capacitor). While the total turns sets the output voltage, the number of layers/turns per layer plus the spacing between layers plus the characteristics of the potting compound determine the distributed capacitance. This second resonance is tuned by adjusting all these things that affect distributed capacitance. In the usual designs I worked on, the secondary was tuned to the third harmonic of the retrace. Some later ones were tuned to the fifth harmonic. The idea was to have the third harmonic in the secondary out of phase with the retrace pulse at the high voltage winding top, and have just the right amplitude so that the third harmonic tended to flatten the top of the transformed flyback pulse. This meant that the high voltage rectifier had a broader flatter voltage pulse input, making the high voltage output more stable with varying amounts of beam current. In other words, the impedance of the high voltage supply was reduced so that it was "stiffer" and there would be less variation with beam current, meaning less blooming. It is this improvement that you will lose if you wind a new secondary without redoing it by trial and error to get the tuning right.

just an out there question about the hv donut of a flyback. when the hv part, donut, is wound it starts off with a single wire that is like hair which i understand but so then its wound into what they are to produce the hv needed so that wire in the beginning where does this wire attach to because the other end is the wire for the hv rectifier. in wanting to switch donuts because its bad in doing this how is one to know where this wire goes. would it go to ground or some other place on the lower voltage coil. i have asked this befor but no one seems to know where the beginning of this donut wire starts out. there has to be a beginning and end of this hv coil in order to produce the hv and the end being the rectifier cap.

[DaveWM]

if its an autoformer its all one long winding.

[wa2ise]

Originally Posted by DaveWM

if its an autoformer its all one long winding.

Yes, but the wire will be thicker near ground. usually the HV winding starts at the point where the horiz output tube cap connection connects.

[old_coot88]

Originally Posted by old_tv_nut

...In the usual designs I worked on, the secondary was tuned to the third harmonic of the retrace. Some later ones were tuned to the fifth harmonic. The idea was to have the third harmonic in the secondary out of phase with the retrace pulse at the high voltage winding top, and have just the right amplitude so that the third harmonic tended to flatten the top of the transformed flyback pulse. This meant that the high voltage rectifier had a broader flatter voltage pulse input, making the high voltage output more stable with varying amounts of beam current. In other words, the impedance of the high voltage supply was reduced so that it was "stiffer" and there would be less variation with beam current, meaning less blooming. It is this improvement that you will lose if you wind a new secondary without redoing it by trial and error to get the tuning right.

Interesting about the impedance-lowering effect. Just learnt something new.
I always assumed the HV 'tire' was sharply resonant somewhere around 70-80 khz (or kilocycles as we called it in those days). Would that be somewhat correct?

[Zenith6S321]

Originally Posted by old_tv_nut

It is this improvement that you will lose if you wind a new secondary without redoing it by trial and error to get the tuning right.

Assuming you could remove a bad HV winding from an otherwise good flyback and had some other source providing the HV, would any additional components be needed (to replace the HV winding tuning) to get the flyback to perform its normal functions while the other HV source provided the focus and anode HV? That would open the possibility to re-use otherwise bad flybacks buy providing another HV source.

[Alastair E]

I recently replaced a overwind with a tripler in a 1963 Bush TV125 monochrome set.

The principle is much the same for a color set.

I found that I had low width and low EHT. I did not need to make a small overwind but sourced the tripler input from the anode of the Line-Output Valve, The boost-diode being too low in voltage.

Adding capacitance to the scan circuit to 'replace' that lost in the missing overwind cured both issues.
--It also reduced the apparent heating effect of the primary of the LOPT.

I used 470pF additional capacitance in the scan circuit.

[old_coot88]

Interesting. What points did you connect the 470pf to?

[Steve McVoy]

I have successfully used a tripler to replace the HV winding of a flyback in our Hoffman 19 inch color set. I know of others in England who have done the same with color sets:

http://www.earlytelevision.org/hoffm...storation.html