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?
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
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.
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?
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.
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.
Yes ferrite and embedded in EPOXY. Impossible to pull apart. That is the reason I am trying to reconstruct it based on what there is on this schematic. The only think I could retrieve was the primary. The other windings are 40AWG and too delicate to unwind.
I want to start with the primary & feedback then go to the lower voltages on secondary. If it all goes fine I will do the HV as the last thing.
Thanks for tips. I've done quite a few transformers but was always able to get the turns while disassembling. This one is just a nut case...
Yeah, that looks about right.
Yeah, use enough tape, and pot it if you can. Or at least transformer varnish (I'm not sure where you get small quantities of this, though?) Vacuum impregnation if possible.
You can adjust the ferrite core (gap) before potting, should be able to get the resonant frequency close, but a little high probably? Then after potting, the capacitance will rise some, and if you're lucky you can get it spot on.
Tim
Cool, thanks guys I really appreciate the knowledge sharing.
I will 3d model a new coil former to use with the same ferrite. That will take a couple days. On the man time I would like ask another question that could be pretty naive.
On the images bellow I can see the primary is being feed with +100vdc. When Q458 is open voltage at node 5 should be +100vdc. And when Q458 is closed node 5 gets grounded & voltage should drop to 0VDC as there is no resistor after the coil to provide a voltage division. So I should be seeing a sine wave with 100v at the top and 0v at the bottom.
But instead... when I look at the charts provided by Tektronix the signal they represent if of a wave with ~220v peak to peak with just ~+40V positive amplitude or peak. And low peak of the signal on node #9 all the way down to ~-180v?
I cannot understand how an input of +100vdc can become a wave with such amplitude... plus negative amplitude.
I suspect Q458 feedback plays a role on that, but that is beyond my comprehension.
Sorry this may be a silly question...
No, mean of 100V, lower peak of ~0V and upper peak of ~200V.
Oh, and just to fill out what I was discussing earlier: if the feedback and bias are relatively light, it can operate as a class C oscillator instead, where the transistor doesn't saturate hard (it doesn't have enough drive to do that), it just kicks in some more energy towards the bottom of the cycle. In this mode, the resonant frequency is mainly the characteristic frequency of the load (i.e., dominated by the secondary L and C in this case), and the amplitude varies with bias. A good model for this is an amplifier where the transconductance depends on base current.
Tim
"I cannot understand how an input of +100vdc can become a wave with such amplitude... plus negative amplitude."
This is a DC switching circuit. Just like in a TV flyback/EHT generation or an SMPS. Does that make any sense? I am not too conversant with it either.
When you are winding the High Voltage part of the secondary, you will have to think of how to isolate between the layers. I feel that winding all the turns over each other may be not the way to go simply because the potential difference between the layers will be too much and internal arching that can occur.
Just a thought .Have you attempted de-potting the transformer by heating either by boiling in water or in the oven ? Or by soaking in denatured alcohol.I thought I would try these methods but I don't have any transformers that are potted in epoxy. All I have are layered in tape.I know that boiling can separate the glue on the cores .
"I cannot understand how an input of +100vdc can become a wave with such amplitude... plus negative amplitude."
This is a DC switching circuit. Just like in a TV flyback/EHT generation or an SMPS. Does that make any sense? I am not too conversant with it either.
I understand a DC switching circuit uses ferrite at high frequencies and how having more turns on the secondary increases the voltage that flows throughout the magnetic field. This part of the concept is fine. Also how other types of flybacks work with optocouplers which is slightly different to this circuit.
What I cannot understand is the oscillation on the primary ITSELF. See the chart bellow I think it may explain what my doubt is better than words.
Just a thought .Have you attempted de-potting the transformer by heating either by boiling in water or in the oven ? Or by soaking in denatured alcohol.I thought I would try these methods but I don't have any transformers that are potted in epoxy. All I have are layered in tape.I know that boiling can separate the glue on the cores .
I tried all that... plus a heat gun. The only thing I did not try was thinner. Yes indeed I usually use hot water to separate laminates then re-coat them with clear spray. But this transformer is tinkering proof...
If you can get the core out, or are willing to sacrifice it, you could saw the bobbin and HT winding in half to get a cross-section and count turns from the exposed wire ends under a microscope. Some polishing may be required to get a good enough cross-section surface. If its tightly and neatly wound, you can probably settle for counting turns in one layer and multiplying it by the number of layers. It will also show you details of construction like margin tape that will be important to duplicate if you want it to stand up to the EHT without inter-layer arcing.
If you can get the core out, or are willing to sacrifice it, you could saw the bobbin and HT winding in half to get a cross-section and count turns from the exposed wire ends under a microscope. Some polishing may be required to get a good enough cross-section surface. If its tightly and neatly wound, you can probably settle for counting turns in one layer and multiplying it by the number of layers. It will also show you details of construction like margin tape that will be important to duplicate if you want it to stand up to the EHT without inter-layer arcing.
I already destroyed the core. Isolating the HV inner layers must be fine to do. But looking at the old one there is no isolation between HV layers, just between windings.
FWIW, you can count on enamel handling, eh, a hundred volts should be fine, a few hundred volts would be questionable without extra e.g. triple insulated wire. If the width of a layer is such that the layer-to-layer voltages are modest like this, you don't need to worry about tape within a winding.
Tim
First test with a new coil former. Just the primary and feedback to start with.
Ratio seems to be working as predicted by our calculations at 1:~30/33 ration between the primary and feedback.
In circuit there is oscillation now. But quite low at 100hz instead of 50khz and for that reason I believe, by being an HF transformer, my feedback output is quite under at 1:~90 ratio.
Does it look like I should leave the transformer quiet for a while and look at Q454/C455 block and see if that is the cause of the low frequency?
Also did some scans with 2,3 & 4 turns at the feedback winding.
I don't see any oscillation at 100Hz. I do see AM; I take it, your power supply unfiltered?
Tim
By the way, what is the Tek part number of your transformer?
By the way, what is the Tek part number of your transformer?
Is it 120-0997-000.
I don't see any oscillation at 100Hz. I do see AM; I take it, your power supply unfiltered?
Yes you are right... this is nothing more than a ripple that come from the mains transformer as it is a +120DC unregulated. My bad not seeing this.
Any tips on why it is not oscillating?
It was oscillating at 4 turns?
Tim
Did you wind the feedback the right direction?
Sorry for my late reply. I got stuck with another issue on the vertical board. A shorted tantalum brought the -8 rail up and messed with all the other rails. Toke a while to find the bastard. Just pulled it off for a while.
Back to the feedback winding. I managed to get the secondary to oscillate please see the attache image.
The start of the windings are 3 and 4. Clockwise both ending at 6 & 5 respectively. It only oscillates in one direction which is the direction the schematics mention.
R457 is fusing. With one turn it gets hot and will fuse after a while. Two turns or more it fuses immediately. Weird… I would expect R458 or R459 to fuse as per what T3sl4co1l said. Should most of the current flow over R457 and not R459?
Also the sine of the feedback (probe #1) is not correct as well as frequency. It is at 200khz instead of 50khz.
All components before Q454 have been pulled out of the board and tested on a BK curve tracer. That block is fine.
Q458 has been taken to the curve tracer and is fine.
+8v, -8v & +120v rail are spot on. The +120 unregulated is a bit above at 140v but the DS796 neon is solid and not accusing an issue as before so it is within margin for the rail. I also believe when the HV transformer is operative the unregulated voltage may get down a bit.
So that is all I got so far… any tips?
Is the core gapped?
If you can, try to reduce the gap, this will reduce the frequency. Evidently 4x is needed, so 16x inductance, which will be ballpark 16x thinner gap.
The higher frequency causes R457 to absorb more power. The RC time constant is 3us, so a 5us period is in the damping range. The intended 20us is just outside. (It still seems suspiciously aggressive for an RC damper, though. Eh, it's what they intended I guess..)
Tim
Evidently 4x is needed, so 16x inductance, which will be ballpark 16x thinner gap.
Yes the core is gapped at 0.5mm. Core diameter is 12.5mm. See photos for more details. I am a bit afraid to modify the core as it was supposed to work with it originally. And if I modify it there is no way back.
If we park the frequency issue for a moment is there anything to look at to try to get a proper sine and increase the primary sine pkpk? Or that is dependable of the frequency?
Oh, using original core. Implies the turns (or wire dia. or spacing) is different. By a fair factor I suppose. Which, *shrug*, Idunno.
What was feedback turns at? Did you try more? How about more bias?
The RC can be decreased, probably keeping the R about the same but reducing C by the frequency ratio. So, around 1nF I guess, or a bit less.
Tim