CT100 Convergence Transformer Solid State Replacement Update

[ohohyodafarted]

Originally Posted by Penthode

Yeah I hear you. That happened to me and led to a sudden brakedown. Fortunately I caught it before burning up the focus pot.

I am running the set right now and it is looking very good. I am finding the focus is more consistent and I think the focus drift was due to the failing transformer.

Cost of parts is about $20 do this.

Only real concern I have at the moment are the 450v electrolytic. When I switch on the set about a half hour ago, the voltage across them starts at 475v abd settles back to 450v where it is now. Tried looking for 500v electrolytics at Digikey and Newark and they are not available. Digikey had a 26 week lead time and you had to buy 1000 units! I have decided to put the capacitors in series as I want to put bleeder resistors in anyway

I am happy with result and will proceed with the proper construction of it to fit in the original can. It will in effect be a direct drop in replacement and feel ultimately will be more reliable that the the original transformer.

Just Radios in Canada stocks 500v electrolytics,

[Penthode]

Originally Posted by ohohyodafarted

Just Radios in Canada stocks 500v electrolytics,

Thanks! I will look into that.

I ended up going to DigiKey. Shipped from Florida and with some other items. Shipping was only C$8 via FedEx. Ordered Friday afternoon and items arrived Monday morning to my door. Not bad!

[Penthode]

Been busy on CT100 this past week. Successfully installed my circuit permanently in the CT100. Convergence is very good if not better than the original transformer.

(Was delayed unfortunately by not one but three totally unrelated issues that required a bit of sleuthing! All part of the fun and more about that and related issues later.)

For the construction and installation of my solid state replacement unit, I shaped three perforated boards to fit in the transformer can. They are stacked and held in place with stanchions that would neatly fill the box. The picture describes my construction.

The tapped secondary of the original transformer connects to three points: to the convergence electrode, the focus electrode and the focus control 3000v source. I replaced the tapped secondary with a resistive divider. The focus control may be considered as signal zero or return so the capacitive coupled output of my circuit connects to the opposite side of what was originally the transformer secondary with a divided down parabola output to modulate the focus. For increased reliability, I saw no need to bring these three leads carrying 3000v into the box and so mounted the resistive divider consisting of 2w 560kohm and 330kohm resistors across the three points previously connected to the secondary.

The leads from the box are yellow the capacitively coupled parabola output, red to +400v, blue to the 12AU7 vertical convergence amplifier, black to ground and white to the diode junction of the main rectifier doubler. The white wire is to feed the internal doubler circuit to effectively create a voltage quadrupler to provide the +850vdc for the mosfet parabola amplifier. The white wire I discretely ran with the set wiring along the rear chassis underside and up to and after the main 400 volt rectifier fuse.

After installing I was concerned about the heat dissipation of the load resistor and the resistive bleeder to discharge the 10ufd capacitors. The resistors get only slightly warm but after an hour the box was reaching about 40 degrees Celsius. There was ample room to mount the resistors on the rear panel externally and had the added benefit to allow me to scope the 750vp-p parabola and measure the 850volt supply.

Below are some photos of the final install. The diagram I will add to the next post.

[Penthode]

Here is my final as built diagram. I left out the 620kohm bleeder resistor which is connected between the +850v supply and ground.