Electronics > Beginners

Using schematic to calculate transformer windings.

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gkmaia:
I am trying to understand how the transformer T460 works and further down re-wind it and the only thing I could recover from the faulty one were the schematics and it's primary turns.

I know it may be a very long shot, but I would like to give it a go and see how much I could learn from it. I have winded quite complicated transformers before but I always had all the data I needed re-do the windings. In this case I have very little data and this goes beiond my understanding.

It’s primary 4-5 takes 120VDC unregulated from mains transformer and it has 30 turns.

I have 2 questions to start with:

1 - How many tuners will be required on the secondary 1-8 to output -2.7kv to the CRT given I only know the primary VDC before oscillation by Q548.

2 - I know 6-3 feedback is supposed to oscillate at 50khz. What part of the circuit makes it oscillate and forward bias Q458 and what part of the circuit defines it’s oscillation frequency?

T3sl4co1l:
It's a blocking oscillator, Q454 is actually going to control it (frequency modulation, sort of; more on that later).  The feedback winding will be sized to give ca. 1-5V.  Less and there's insufficient gain for hard switching, more and V_EB is exceeded, drawing bias current through that path, and Q458 literally beats itself to death (or until R458 fuses).

Primary peak voltage is normally about double the supply voltage, but you might design it for more just so that you can save some on secondary turns, since the voltage is high.  You can't go too far, because again, the feedback needs to be okay, and a large ratio of peak voltages (turn-on vs. turn-off) gives less feedback voltage while still avoiding E-B breakdown when off.

This is basically how things go, but the high voltage winding very likely has quite a lot of capacitance, and so rather than a nice square flyback pulse, you get a slow wumpy resonant (or quasi-resonant) waveform instead.

In a well damped blocking oscillator (and I will note R457-C457 will give some good damping, if the primary inductance is about what I would guess it to be), the transistor turns on, then turns off, and just stays off, even after the output rings down.  Because if it's well-damped, it doesn't ring down at all, it just kinda...and it's done.  Back to zero.  But if it's underdamped, after the flyback pulse the voltage keeps swinging, and about when it completes that first cycle, it can kick the transistor back into conduction.  And so on.  It runs for a burst of pulses, rather than just one.  This is called squegging.

How long the transistor runs for, depends on C455.  The feedback winding pushes on the transistor base, and pushes against C455, discharging it.  If it's fully discharged (i.e. by several volts) during a single pulse, it won't squeg; if Q454 is supplying enough current to keep its voltage up, it will run CW (continuous).

So, this may be a resonant blocking oscillator, in which case Q454's control kinda looks more like a variable on-time control, but frequency is changed a bit too, as it looks kinda quasi-resonant (class E).  It still rings down and does some kind of pulse-skipping or burst mode operation at really low currents (possibly at really low beam intensity??).

The resonant frequency is controlled by the capacitance of the secondaries (and the rectifier diodes), and the transformer's core gap (and thus winding inductance).  You'll need a gapped ferrite core here, but the gap probably won't need to be very much, since you'll want to use a relatively large core to keep the V/turn up, so you don't end up needing ten thousand goddamn turns or whatever.

Good luck winding the secondary -- it only needs to supply ~1mA so the wire can be very fine indeed.  If you don't have, or aren't comfortable winding, wire this fine (>40AWG?), you may find you need a far larger transformer than the original.  And, if this is a repair thing, you really ought to just find an original part; but if this is for just demonstrating the HV supply for its own sake (and presumably for some purpose, perhaps making a CRT glow?) it's not bad, but keep this in mind.  (Upside, I guess: you could use a much bigger transistor, and beefier primary, and kick some serious current out of the thing.)


TL;DR well, if you know the primary, assume peak about equal to supply and wind secondaries accordingly.  Mind core size and gap.  Hand-waving notes on operation and design of a blocking oscillator, not much for numbers or equations but more like hints at how to figure out what equations would apply.

Tim

Jwillis:
Won't the winding ratio be the same regardless of frequency. Since the feedback is oscillating at 50Khz so would the primary and secondary. Is that right? So if the primary is 30 turns then the ratio between 120V and 2.7kV would be 22.5:1 so the secondary would be around 675 turns.Or do I have the thinking wrong.

gkmaia:
Thanks for the answers. I really appreciate the time you put to it.

I could start by rewinding the primary and feedback and check if it oscillates. If it does I can move to the secondaries and do those after.

Based on what you said I did have a look at the T921 from tektronix SM as that one has the signal charts. My one which is T912 does not have the signal charts.

Both circuits are very similar on the primary side. Based on T921  chart #9 peak to peak voltage on the primary will be around 250v which matches what you said very close.

Also the peak to peak on the feedback would be about 4 volts. If you look at the winding chart attached and signal #8 attached seems the feedback will need just one turn to output 4 volts.

Winding 40 gauge is fine. I believe I do have still a lot of 42AWG at home which is also fine to wind. And yes i will need to do some magic winding that as the original one was machine winded and was pretty damn tight to fit the original coil former.

Would that be a good approach? 30 turns on the primary and one turn on the feedback and report back here?

andy3055:
Is this one of those ferrite core transformers? Not to scare you but take adequate precautions when you do the winding as it involves EHT. When you wind an ordinary transformer for say, one in a tube amp, since the voltages are in the 300-350 volts range, you may not pay much attention to the moisture in your fingers handling the wire. Also, you would bake it first (to rid of any moisture) and then immerse everything in shellac and bake it again. In this case, it is good to wear some cotton gloves so that the wire stays clean. I am not sure what can be used instead of shellac for this since the frequency will be quite high. I have not dismantled one of these but imagine it is immersed in some sort of resin. If that is the case, you have to be sure of the turns ratio before you finalize the thing unless you can leave several taps. Did you find the original easy to unwind or was it immersed in resin?

Just my thoughts.

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