CT-100 B1001599 Failed Convergence Transformer

[kvflyer]

Originally Posted by Penthode

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

[Penthode]

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.

[Doug]

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

[Penthode]

Originally Posted by Doug

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.

[Doug]

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

Your second images doesn't work

Doug

[old_tv_nut]

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

[Penthode]

Originally Posted by old_tv_nut

@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.

[old_tv_nut]

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!

[Penthode]

Originally Posted by old_tv_nut

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.

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!

[old_tv_nut]

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.

[trinescope]

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.

[Penthode]

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.

[old_tv_nut]

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.

[Penthode]

Originally Posted by old_tv_nut

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.

[Penthode]

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.