CT100 Convergence Transformer Solid State Replacement Update

[Penthode]

In my final design I used a 1200v MOSFET. I used the IGBT as well and they both perform similarly. The bias might have to be changed slightly. The best way is to just build it and turn the parabola amplitude up and you should reach close to 800v p-p without clipping.

The edge convergence is the problem. Best to start with a symmetrical parabola and only vary the amplitude at first. All the convergence controls interact and I had to patiently circle over and over all the convergence adjustments to get best convergence. But edge convergence pretty good now.

Had a set back tonight as a second metal film replacement cap appears to have failed. Earlier the 0.001ufd cap across the vertical output primary failed. It was a 1600v unit. I replaced it with a 2000v cap and after working for a couple of days I have a compressed distorted scan. Will have to figure that one out tomorrow.

[Penthode]

I found the problem with the video sweep. I will mention it here as it may affect others.

While adjusting the convergence, I found the Vertical Conv. Amplitude control would not adjust linearly. So i removed the control to more closely examine. Testing with a VTVM, I saw no problem. I replaced the control and found when powering the set the vertical sweep was non linear and compressed.

Disconnecting the 0.22 uF capacitor connected to the vertical amplitude shape control I measured only 6kohm to ground. Looking at the circuit diagram, it should have measure minimum about 7 megohm. The 0.22 was shunting the output of the vertical oscillator to the vertical output grid. Moving the potentiomenter lead, the resistance jumped to anbout 7 megohm. Replaced the lead powered the chassis and the vertical output signal still compressed. Measured the vertical amp. shape pot lug to ground, the resistance was again 6 kohm!

I carefully washed and brushed with an artist's paintbrush the area around the pot connections. Problem gone.

It looks like 70 years of grime and the the high circuit impedances (around two 5 megohm pots!) caused the intermittencies. The pots looked quite clean and I was not as scrupulous as I should have been around the pot.

It was following the horizontal hold control exhibiting the same leakage problem suddenly last week. The control runs at about 200 vdc above ground and a minor leakage made the the oscillator control section of the 6SN7 to pull the horizontal oscillator off frequency. Odd as both these pot problems happened suddenly without warning.

[Penthode]

Convergence Update.

I have run the CT100 about 24 hours and the convergence remains very good. It is actually better than the transformer that was in the set before it failed.

The transformer failing first sign was the drifting focus and convergence on warm up. With the convergence pot fixed, the convergence is shows minimal drift from warm up from cold. Also the focus drift is very much reduced.

I used three 10kV .01 uF capacitors in parallel in my final circuit although I earlier used 0.02uF (two in parallel) worked fine.

I also noticed an error in my earlier as built diagram: the 0.018 uF coupling between 12AU7 and IGBT should have read 0.1uF.

With regards to the device I used I was mistaken. It is a 1200 volt IGBT type GF3NC120HD. They are available at Mouser for $1.62 each. (I would be inclined next time to buy 10 for $1.02 each).

https://www.mouser.com/ProductDetail...8Tvd683-o9yPh_


The 0.01uF 10kv capacitors I bought on eBay:

https://www.ebay.com/itm/32496656125...F+10kv&itmmeta

Lastly is attached my most recent as built diagram.

[Penthode]

Doodling with the CT100 schematic, you can see the connection to the main power supply and the circuit inserted within the circuit itself in the attachments below.

[ChrisW6ATV]

I am highly impressed with your work on this tough issue, and your good results and documenting and sharing everything. Thanks!

[Penthode]

Thanks Chris.

After earlier toying with a transformer rebuild and pondering the focus and convergence high potential had to traverse over 2 /1/2 miles of AWG #44 wire to reach the CRT with the strict insulation requirements, it seemed appropriate to examine an alternative approach requiring capacitive coupling. My alternative approach is much simpler and cheaper and in my opinion works better.

I have been running tests. I have gotten the convergence best I can on my tube. It matches what I was able to get prior to the transformer failure. It has the benefit of less drift: from switch on to warm up there is very minor drift which is due to the 12AU7 coming up to temperature. The convergence becomes optimized at about 4 minutes and holds well after that. I have been running the set for two to three hours at at a time and the convergence box remains cool.

Here is a snapshot of the crosshatch with the convergence about the best I can get. I could never get the lower corners as good as I would like but as it is a rebuilt tube and I am extremely fortunate the tube is still good and appears to have a lot of life left, I am extremely grateful with what I have.