CT-100 B1001599 Failed Convergence Transformer
 

Hi All,

This weekend I began working on the CT100 again. The set was working fine through the weekend until the picture began to jump. It looked like a focus problem I cautiously powered the set. As I adnance the focus pot a dreading popping started and the higher the voltage the worse it would become.

Looks like the convergence transformer is open internally at the secondary brown lead from the focus pot. Between Blue and Yellow it measures 21kohm and the primary is okay.

I suspect a break internally due to the pitch potting. Might be lucky and be able to find the break at the connection to the brown lead.

The transformer looks like it has been replaced once in the life of the set from the look of the soldering: the replacement is a silver canned transformer.

I recall a couple of decades ago, John Folsom and others discussed rebuilding the transformer. Are any spares around?

(The transformer failed Sunday evening (December 17) which I recall was the day exactly 70 years ago the FCC adopted the NTSC standard!)

Any help or anecdotal experience regarding this problem will be greatly appreciated.

Hi to all,
Hi Penthode,

Sorry! to learn the failure of this high-stress part in your CT-100.

I searched on the ETF site in the "Classified" where John used to advertise his rewound transformer: long gone.
In the CT-100 section by Pete Deksnis, there is a photo of the transformer and John's address in Florida (see pix), maybe try to contact him & find out if he's got some spares or can rewind yours.

Alternate solution:
contact Ed Dinning in the UK, he's still currently rewinding transformers and can do with success EHT windings, Pre-War AC EHTs, etc.
Ed is in Newscastle-upon-Tyne, Northern UK.

ed_dinning AT yahoo DOT co DOT uk

Mail current as of Oct 2023,

Hope this helps,

Best Regards
jhalphen
Paris/France

Hi Jerome,

I recalled years ago John Folsom and the ETF looking at making replacements But that was about 20 years ago.

Thanks for the UK contact. I need to find out the characteristics of the old transformer. I plan to remove the potting and have a look at it.

It also occurred to me that perhaps a solid state electronic device could be designed and constructed as a substitute? I shall investigate that as well.

Cheers,

Terry

Given that the xfmr's secondary sits at focus voltage, making a ss replacement would be an interesting design challenge.

I'm watching with curiosity. My Westinghouse H840CK15 uses the same transformer and probably could benefit from a new one.

I've heard when these fail they often take the unobtainable focus pot with them, and that originals should not be used. At some point I'm going to dig out my Westy as get it the final mile from mostly working to turn it on anytime and watch status.

It would be an interesting challenge. I only wish I had the waveforms the circuit would deliver. It would make the problem somewhat simpler.

It looks like the circuit including the transformer is generating only the vertical parabola. The brown lead would be at sinal ground by virtue of the 0.01uF capacitor from the brown lead to groundwhich means the parabola would would have a slight application to the focus electrode, likely to modulate the focus perhaps? and a much greater amplitude which would be coupled through the 820k resistor to the convergence electrode. Which means a single amplifier fully sitting above ground by 4000v coupled with some voltage gain (by virtue of the step transformer) from the output of the 12AU7. Hi voltage coupling capacitors and some diode spike protection should make it work. Afterall, the dynamic convergence correction should only be slight. I am going to think on this.

I would take up Jerome’s suggestion and contact John Folsom. We obtained a replacement transformer from John 4 years ago for our Westy and more recently, I know of an H840CK15 under restoration with contacts with John about the transformer.

My understanding is John’s transformers were averaged to work with both the CT-100 and H840CK15. At the very least, John could share technical information you desire.

Hi to all,
Hi Penthode,

more info about CT-100's V convergence transformer on Phil Nelson's restoration chronicle + photo of Folsom's Xformer & EHT cage wiring diagram:

https://antiqueradio.org/RCACT-100Television.htm

about risk of Focus Pot failure & solutions, here's how Miniman82 (Nick W.) solved the issue when he restored his CT-100 in 2011:

https://antiqueradios.com/forums/vie...p?f=3&t=175030

Convergence (H & V) theory of operation - extracted from this 19 page CT-100 RCA document (page 18):
https://antiqueradio.org/art/THE%20C...20RECEIVER.pdf

"The dynamic convergence and focus modulation voltages, each having the proper waveform, amplitude, and synchronism with deflection, are produced by linear addition of a variable shape vertical parabola with a variable phase horizontal sine wave. The composite alternating output voltage is coupled to the kinescope convergence electrode and focus electrode through the respective output taps.
The vertical-deflection dynamic convergence amplifier circuit combines, shapes, and amplifies the parabola and sawtooth waveforms derived from the vertical-deflection circuit for application to the kinescope."

"Sine-wave horizontal dynamic waveform is derived from two cascaded tuned circuits excited by horizontal kickback."

+2 last photos:
V convergence waveform shape (no scope photo nor measurements values) from SAMS CT-100 SM on ETF site.

Convergence section schematic from CT-100 Service Clinic manual (page 31) on ETF web site.
Sorry! for tiny size, forum display limitation.
http://www.earlytelevision.org/tv_sc...ams_color.html

scroll down to RCA CT-100 section.

Best Regards
jhalphen
Paris/France

I think radio electronics or one of the TV magazines (probably on radio history site) has the convergence waveforms and explanation of the circuit... I'm just about sure I read the original paper about a decade ago.

You have a part number for this transformer?
Besides eft did you check Moyers and parts-link
Com
Doug

Thanks all for the advice and information.

I never gave Mouser or any other parts supplier a thought because I figure someone would have beaten me to it! But you never know. I will grab the part number and Google away...

I shall also follow up with Messrs Dinning and Folsom.

Thank you Jerome. The waveform depicted with the combined parabolas is what I would have anticipated. And the secondary is simply adding a lower coupled parabola to the focus electrode and higher level to the convergence electrodes. And as I thought, the parabolas will modulate the focus to ensure focus can be simultaneously optimized center screen to the screen edge.

The transformer appears simply the means of coupling the 12AU7 to the high voltage focus/convergence electrodes for the vertical parabola only. I remain interested in an electronic substitute eliminating the need for the transformer and will mull over a design.

Meanwhile I shall melt the pitch and see if the old transformer is salvageable. After all it is an open circuit and I may strike lucky if the break is at the winding termination to lead out.

Anyhow I will share what I find with photographs here.

HV "signal" transistors for focus and G2 control are around in the years of hi-end PC monitors, but in general, are for ~1200V, like the 2SC4632.
I noticed some MOSFET for 4500V some time ago, but I don't remember the code or maker. But it if for power switching and have considerable capacitance.

And about using a small H sweep tube for direct coupling? (well, this will probably overtaxes the heater supply and also needs a extra +++B)

Looking at that circuit I think that it could probably be made to work
with any audio transformer of the proper turns ratio and impedance by
coupling to the HV side with capacitors. Given the availability of
modern film caps of high capacitance it should work fine. You'd just need
some HV high value resistors and/or perhaps some Zener chains.

I use a Zener chain for the focus DC. The pot setting is quite stable. Thus, a 1 or 2 meg
pot and some fixed resistors or Zeners would surely work OK. The lower voltage across it would likely allow an available pot to work.

One of the common schematics has 27K for a plate resistor in the
convergence amp. Its supposed to be 270K, but 180K or 150K probably is better.

Doug McDonald

Removed the convergence transformer this afternoon and the laminations this evening.

My recipe was to bake the can in a foil pan with the transformer can opening facing downwards at 400 degrees F for 20 minutes. The pitch dripped in a pool at the bottom of the pan. Then lifted the transformer can and the transformer could be removed. Placed the transformer in a second pan and baked it another 10 minutes to remove excess pitch.

I then removed the transformer wiped it down when hot and let it sit until warm. Then washed it in varsol until clean.

After it was dry, the transformer when back into the oven at 400 degrees for 10 minutes to again heat the remaining pitch. Then while hot, sat the transformer on the open jaws of a vice and tapped out the laminations. The laminations came out quite easily when warm.

The open winding was the focus voltage secondary input. I found the wire to the open winding is the outer most winding which should raise the chances of success repairing it as it will only require the opening the attachment of the outer most winding.

Fingers crossed as they say.

My attempt at the transformer repair was unsuccessful. I was able to find the broken winding submerged within the transformer. The break unfortunately was between the primary and first secondary buried deep within.

So I am looking for alternatives. The transformer in my set was a light silver colored cadmium plated box which I understood to be a later replacement. When restoring the set, it was obvious that it had been replaced in the past.

I remain weary of replacing the transformer with an even later replacement and am now looking for alternatives. I was told John Folsom had a bunch of transformers rewound in 2005?. I looked at his previous posts and the last one was dated 2016. Is he still on this forum?

It is fortunate I live only about 15miles away from the Hammond Transformer Factory in Guelph Ontario. I have measured the primary and secondary wire with a micrometer and determined the primary is AWG #40 and the secondary AWG #42. From the winding resistances and with the help of a wire gauge table, estimated the number of primary and secondary winding turns. I have attached my finding and would welcome any questions about my calculations.

In addition, I have been examining an electronic alternative to the bulky transformer. I have made a conceptualized drawing which is attached. The basis of the design I shall layout below:

The vertical 60Hz parabola signal which has been generated by the 12AU7 I would estimate is 30 volts p-p. (I haven't measured it yet and if someone has put a scope probe to the plate connection of the 12AU7 feeding the transformer and can tell me what it is, I would be most grateful). The transformer with the estimated 11:1 step up ratio suggests the amplitude across the secondary winding total is 330v p-p. This is suggest is a maximum because I would expect the dynamic convergence correction amplitude to be less than this.

Because the B supply is 400 volts DC, it appears reasonable that with an amplifier with a voltage gain of 11 I should be able to linearly amplify the parabola. Instead of transformer coupling, it should be possible to capacitively couple to the high voltage convergence and focus feeds. The amplifier is driving a very high impedance load since the focus and convergence electrodes draw no current. Therefore instead of a split inductance, I can use a resistive divider to feed the convergence electrode with the higher amplitude parabola and the focus electrode the lower amplitude. (The parabola feed to the convergence electrode is the correct for dynamic convergence errors away from the center of the screen and the lower amplitude parabola is to compensate for edge of screen focus errors due to the sweep arc across a flat display panel within the CRT).

A high voltage capacitor will couple the output of the addition FET amplifier stage to the focus circuit high voltage path where the vertical parabola with added horizontal parabola effectively modulates slightly the focus voltage to improve the edge of screen or display focus. The vertical parabola is capacitively coupled to the Convergence electrode where too a horizontal parabola is added.

I am aiming to use a power MOSFET and believe I should be able to achieve the gain. Again comments welcome.

one guy that was a member on arf has
My friend John Folsom says, 'It's all about the spares.'"
-Bob the Antique TV Guy in 2021

so
good luck

Yes, it is all about spares. But as time marches, it it all about creativity and much of the time making spares!

I was looking at the high voltage MOSFET option to replace the transformer. It is a way lot cheaper (parts under $10) and maybe more efficient and less prone to breakdown.

Looking at the FETs these days, they have a very large transconductance so with such a light load, the gain should be achieveable with a single moderate power MOSFET.

I have thrown in some values for a generic MOSFET to achieve the bias voltages and quieesent point of around 225 vdc to allow for a wide swing. I anticipate average vertical convergence level will only be in the region of 50 to 100 volts p-p to the convergence plate. But that is just a guess.

Find updated my circuit with some component values. In the drawing where my circuit is inserted into the CT-100 circuit, I left a gate resistor with no value to provide the option to reduce gain.

Last part of my post didn't come thru
https://antiquetvguy.com/

Your second images doesn't work

Doug

@Penthode, are you sure the amplifier output will be the right polarity?

Haha. I have thought about that. I should find out shortly.
My guess is that perhaps the parabola inverts depending upon the position of the CT100 convergence control? Eg the midpoint there is no correction and one control extreme the parabola is inverted from the other extreme? I have nowhere read of a reported phase error when wiring the primary of the earlier replacement transformers or if any attention was paid to phase. It would appear logical to have a plus/ minus correction assuming when the 15GP22 was constructed, the convergence was close to begin with and that the dynamic correction was to correct for minor physical discrepancies.

If it does need to be inverted I could add an inverting amplifier stage.

Maybe I will be lucky? Unfortunately there are no sample waveforms to reference. It will be interesting to find out and dig into this more deeply.

The customer control is DC only, so it would be set for proper convergence at the center.

The H and V waveforms will have a specific polarity with variable amplitude because the beams must converge at a sharper angle at picture center than at the edges due to the geometry of scanning a flat surface.

I also note that the transformer wires have different colors on each end of the winding, which would indicate polarity was important.

So yes, it appears you have a 50/50 chance of needing an extra inverting stage.

Following your progress with baited breath!

Yes I have thought about that too. I found that the later color faceplates which have a curvature, I believe the deposition of the phosphor was aligned through the faceplate itself. This would mean that the dynamic correction would only need to be minimal. I thought the flat faceplate in the 15GP22 would have been manufactured similarly so as to minimize dynamic correction suggesting only a plus/minus minor correction would be needed.

I am curious now to find out. I plan to pick up a MOSFET tomorrow and externally breadboard it to measure waveforms. I can also try the chassis with a plate load resistor on the 12AU7 to examine the waveform and the dynamic vertical control range waveforms to see what will be input.

Fingers crossed!

Only the DC convergence of a CRT needs to be plus/minus. Because the radius of curvature of the screen is/was always larger than the yoke to screen distance, there is always a positive polarity dynamic waveform required. In other words, the screen edges are always farther from the yoke deflection point than the screen center is. Much later designs that simplified the convergence were based on deflection yokes with a center of deflection that varied with the direction of the beam.

Somewhere I have the winding data for the John Folsom replacement parts. The details seem to match closer to the Westinghouse 15" transformer, but the basic operation is the same: 1:10 step up ratio with a secondary having a tap around 1/3. The winding resistances are not critical here because both windings carry very little current. The problem with the original parts is the same as all the vertical output transformers going bad now; the older materials have not held up and are breaking down under voltage stress.

Here is the results on my first test with the single MOSFET. Yesterday I bought an IRF830-1 500v 1A MOSFET locally. It has a forward transconductance of minimum 2.5 Siemans which is 2,500,000 umho. That is a whopping lot of gain available espectially when you are used to dealing with only a few thousand umhos with vacuum tubes!

With the single FET, there was too much gain. I added a 330k resistor on the input side to brring the gain closer to 11 times. I was able to achieve 350 v p-p on the output with the input only 20 v p-p. (My HP 204 Sine Generator maximum output).

I found non-linearity (no clipping) became noticable at 250 v p-p. You will see the output trace slight distortion in the photo.

I found I could slightly reduce the non linear distortion by increasing the drain current slightly, hence the reduction of the source resistor from 5.6 kohm to 4.7 kohm.

Note the phase shift. Testing the circuit at 600Hz the phase inversion is exactly 180 degrees. At 60Hz as depicted in the photo there is a shift. I tried bridging the coupling capacitors which did not make much difference. I believe the shift may be due to one of my scope probes. I am investigating this.

the circuit was tested as per the diagram including the 27k 12AU7 plate load and the 560k and 330k load resistors.

Next step is to put the CTC2 chassis on the bench and perform some measurements.

A couple of comments:

Phase shift at low frequencies could occur if your scope is set to AC coupling. If you have been using AC, switch to DC to double-check.

The 100 microfarad source capacitor is a 27 ohm impedance. It's no wonder you have too much gain, and the circuit is non-linear. I'd suggest bridging the 330k, removing the 100 microfarad, and changing the source resistor from 4.7k to about 2.7k. This may require adjusting your gate bias, which I have not calculated.
This will reduce the gain dependence on the FET transconductance and linearize the circuit. Of course, eliminating the 330k will mean more loading on the preceding tube circuit, so the gain there may be reduced, requiring some increase of your FET amplifier gain. I have not gone back to the original circuit to calculate the loading of your amplifier compared to the original circuit loading.

Thanks. Excellent. I will make your suggested changes and repost.

I removed the source resistor bypass 100uF. I then chaned the bias resistors to raise the input resistance and keep Id at 5mA.

The generator was in series with the 27k simulating the 12AU7 plate impedance. The 330 kohm resistor was removed.

The gain is now just a hair under 10 times which should be close enough. The phase error was due to the source bypasss capacitor which is now gone and the remaining is the AC scope coupling: switching to DC coupling on the input reduced the phase error by about 1/2. I cannot remove the output AC coupling because the drain is at 200 VDC. So I believe the phase error has been acounted for and is not in the circuit.

I have insufficient input signal (it was only 15 v p-p) so I cannot check linearity at the higher output voltage. Although removing the 27kohm input resistor pushed the output to 200 VDC and the sine was perfect hence the non linearity appears to be no longer there.

I had made a minor error in my second implentation. The Drain resistor in the diagram states 47k and I had inserted a 33k resistor. As drawn for the second test with the correct value of drain resistor, the output voltage is 200 v p-p with no evidence of any nonlinear distortion. That is a gain of 13 times which should be more than ample. Now on to the chassis. The attached photo shows the input vs the output.

There was a concern that the polarity of the applied parabola after the MOSFET amplifier may be inverted. I went back to the schematic and reasoned that the polarity after the inverting MOSFET amplier should be correct. I have attached a pictorial reasoning for my assertion.

This evening connected my breadboard circuit to the CT100. The first thing I found was the waveform on the plate of the 12AU7 inverted compared to the waveform printed in the service manual. This means I will likely have to invert it with another stage. Nevertheless I connected it up and was measuring voltages and looking at waveforms. I powered down and up again and found the MOSFET gate to source had shorted. I tried installing clamping diodes on the gate and replaced the MOSFET only to find it shorted. I found that the B+ shot up to just over 500 volts before the tubes warmed and as they were 500v fets that spelled their demise.

Anyhow, without the vertica dynamic convergence correction, with resistor feeding the convergence and focus electrodes only, the focus and convergence did not look bad. The worst was at the top which I feel touching up the vertical linearity will address. The two photos attached is the set running with no vertical convergence transformer. Only the 560k and 330k resistors substituting for the convergence transformer secondary.

I have buttoned the set back together while I think this thing through.

Post thought: Because the vertical convergence transformer has only a minimal effect on the convergence, it is conceivable that a replacement may be installed with a reversed phase primary which may not be noticable. Certainly I saw no impairment in focus and the convergence except for the very top is remarkably good.

Mulling over the failure of my circuit yesterday. Before the MOSFET died, Adjusting the Vertical Convergence Phase and Vertical Convergence Amplitude controls did react on the screen. Unfortunately I was unable to proceed further. I plan to purchase higher voltage MOSFET (800v) and look at presenting some gate protection.

Yesterday I managed to measure the peak to peak parabola at the plate of the 12AU7. It measured about 25 volt peak-to-peak with a simulated MOSFET input load. I emphasize simulated because before I had a chance to measure it, the MOSFET had already been destroyed.

Yesterday, I discovered that the 12AU7 plate waveform was inverted which will require an extra stage to invert the signal before presentingt it to the convergence/focus electrodes.

In addition, I do not see any noticable focus degradation not including the vertical parabola to the focus electrode. The vertical sweep angle of deflection is less. It is only the focus voltage which is applied the convergence transformer secondary since the vertical parabolais picked up capacitively by the Convergence electrode. However there is a degree of intreaction between Convergence and Focus Controls so it must have been designed that way for a reason.

I have attached my first stab at a two stage FET amplifier for the Convergence Transformer solid state replacement. I have also included screenshots from off-air signals this evening showing the misconvergence with out the Convergence transformer present. The greatest degree of misconvergence is the top of screen.

I received my components delivery from Newark this week. Thye past two evenings thinking over the electronic transformer component with the MOSFETs.

I tried initially with the single power MOSFET inverting amplifier. The results show is that the single inverting amplifier does pull convergence in the correct direction and does offer improvement.

The problem I am running into is insufficient amplitude. Using the CT100 400 volt power supply and a 900volt max. drain to source mosfet, the maximum linear amplitude is 350v p-p. I need a bit more!

I had to reduce the 12AU7 plate resistor to 10k to curb it's output amplitude feeding the mosfet It looks as if the 12AU7 typically delivers up to 50v p-p to the original transformer. I cannot use any more than about 35v p-p before the mosfet clips.

My next thought is to use a small step up transformer. It could be an audio transformer and I would only need less an a 2:1 ratio. Further, why not try a standard audio transformer and capacitively couple to the high voltage convergence electrode? The transformer would then be spared the stress of the high voltage and a 10:1 ratio should be easily achieveable.

Close!
A transformer with capactive coupling seems like a good thing to try. I would then be on the lookout for any phase distortion that would make a difference between the top and bottom convergence but it may not be a significant problem.

I am now looking for an audio transformer with 2:1 ratio. Checking my junkbox right now.

Here is the proposed circuit. The capacitve coupling seemed to work okay with anticpated waveform. And the circuit almost achieves the convergence required with insufficient at 350v p-p. I suspect if I can get 450v to 500 v p-p that will do it.

I like the capacitive coupling with the resistive division. Focus is good across the screen. And the capacitive coupling means the transformer does not have to stand up to the brutal 3 to 4 kV.

And is the mosfet needed at all? Maybe the transformer only?

“Is the mosfet needed at all? Maybe the transformer only?”

Almost back to the original design but with the transformer isolated from the hv with a capacitor and a resistor divider.... why not? :scratch2:
What was the ratio of the original transformer?

jr

The original transformer was 10:1

.



Can't you find a 10:1 transformer of the same physical size no matter what it's
original purpose was, and stick it in there.... (Insulation should be compatible
with the voltage in the circuit of course...) Maybe you have to add a cap
across pri.or sec. to raise, or lower output as needed by tuning it with
the cap..... Might even be a good idea to get a 11:1, or 12:1 that
way you can attenuate output down as needed, easier to do than
ending up with a signal too small from a poor match with 10:1.

For that matter an LC circuit in your mosfet circuit output might just
get it to "ring' a little higher p-p and do the job.....


It's a nice circuit you made, I was wondering what you were going to do
about the phase shift in your circuit when you first posted it....

Anyway - Interesting poop.....



.

After a couple of months hiatus busy with other projects. I am returning to the CT100.

If you recall. my circuit worked well but there was insufficient amplitude. I gave up looking for a suitable audio transformer to step up the signal.

Looking for a more efficient and less expensive approach, I have decided to increase the B plus to the FET in order to get a greater voltage swing on the drain. There was more than enough gain as the convergence controls would drive the FET output to clip the convergence waveform parabola.

Looking at Digikey and Newark, there is a large array of 1000v plus power FETs which exhibit the linearity and provide the signal gain. To boost the drain supply I will experiment with using a standard 300mA filament transformer in reverse. I will tap into the 6.3 vac filament line to step up to 230vac and use a simple bridge rectifier. The current demand is very low which means only very simple filtering is required. A bridge rectifier will yield about 320volts DC which I will stack on top of the +400v supply.

The new diagram with the stepped up drain supply is attached.