CT-100 B1001599 Failed Convergence Transformer

[Username1]

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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.....



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[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

[Penthode]

I am currently searching for a MOSFET with Vds of minimum 1000v. In these days of switchmode power supplies, devices of 1000 volts plus abound.

The convergence waveform amplitude of max 350 p-p was so close I believe the additional B+ boost should be just enough to achieve good vertical convergence.

The circuit replacement will be more reliable and not subject to the fragile transformer leakage and failure.