CT-100 B1001599 Failed Convergence Transformer

[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.

[Penthode]

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.

[Penthode]

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.

[Penthode]

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.

[Penthode]

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.