CT-100 B1001599 Failed Convergence Transformer

[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.

[old_tv_nut]

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.

[Penthode]

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?

[jr_tech]

“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?
What was the ratio of the original transformer?

jr

[Penthode]

The original transformer was 10:1

[Username1]

.



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.....



.

[Penthode]

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.

[old_coot88]

Cool idea with the xfmr. As a young kid, i used to use two 6V doorbell xfmrs back-to-back for isolation to make B+ for projects.

[Penthode]

Hmm. I am boosting the B+ and I see the boost B+ on the CT100 is 680v. I am going to investigate using it for the FET. Would only require at most a resistor and capacitor....

[Penthode]

Here is my latest further simplified itteration. I will grab the B+ boost from the flyback transformer. The FET will be picking off about 3mA which should be relatively inconsequential. The 650VDC available as a supply to the FET drain should allow a nearly 600v p-p swing which should be adequate to provide the vertical convergence.

See the attached drawings for the modified circuit and the B+ boost source.

[Alex KL-1]

If +Boost reduces only a little with extra load, is nothing to be worried