CT-100 -> TV tuner
 

Since my 15GP22 is dead, I've finally decided to break down and do
what I've been threatening since it happened.

I am going to turn my CT-100 into a color TV tuner that will feed any
standard CRT or LCD TV monitor. This is so I can watch TV with
CT-100 quality decoding. As a glorious stunt.

The video will of course be easy. I'll just DC couple the CRT drives
into some op-amps and adjust the level and gain.

Sync will need a bit of work but will be equally straightforward.

The biggie from my point of view is the HV. Without a CRT there is no
need for it. If I do nothing the poor regulator tube will sit there
overloaded and generating Xrays. So I will unplug the HV and focus
rectifiers and the regulator. Without these the pulse amplitude
from the flyback will go far too high. One can't remove the horizontal
output tube or the AGC and horizontal sync won't work at all.

So I want to get the pulse level and therefore boost voltage
well below spec, like half or 2/3, so the pulses will work the AGC
and sync OK.

Turning down (or up) the horizontal drive
is not really useful as it does not change DC bias.
Two others method would be to increase the output tube cathode
resistor, currently 12 ohms, or its screen resistor, currently 2.2K. Would it
matter which I did? Or would using whichever was easier to do be OK?

Or should I just plug in a 6BG6, which is pin and rating compatible but
has about 1/2 the current at a given grid-plate voltage?

You advice on this is sought.

Horizontal output/sweep is a switching circuit, not a linear amplifier, and the current is not set by the tube, but by the B+ supply voltage and the yoke/flyback inductance (which is how you get a sawtooth sweep current: constant voltage across an inductor). If the current was regulated by HO tube conduction, the tube would be vastly overheated and burn out. If you must have the AGC and burst gating pulses from the flyback, I think your best bet is to leave the circuit as it is and let the regulator do its job.

That's true. However, if I could get the tube to conduct less time,
the inductance would charge up to less current. Then, when the tube shut off, the flyback would go up to a lower voltage. The average tube current would also go down.

I think I should simulate the circuit and also try playing
with a much cheaper tube TV!

I think you are right. I believe I should simply put a resistor and filter cap
in series with the HV fuse (in its B+ line) and adjust it until the pulse voltage
is say a couple of kV below the standard voltage, for transformer safety,
and the tube average current is also a bit below normal.
I have a high voltage scope probe that works for this measurement.

Another option might be to add a second parallel HV regulator tube to reduce the load on the one tube.

ElectronicM, your post reminds me: the other method I mentioned should
also work.
Just consider "half" instead of your 2. That is, cut the present horizontal
output tube in half, or remove half the cathode. . For the same grid input, the
output current would be cut in half (at saturation that is). The other tube
I suggested isn't quite that, but its saturation current is half.

The other thing to think about is .. what happens when the
main HV rectifier's filament dies? the load on the output tube and the
transformer goes way down, and I suspect that the pulse
voltage goes way up. This of course actually does happen.
What I'm worried about is really only the output transformer.
The convergence transformer is immaterial, it could just be disconnected.

The alternate tube I propose might generate a bit too little drive for the
AGC and sync windings to funstion properly.

I would like to add my two cents. Many latter day NTSC decoders besides wider chroma and luma bandwidth also employed comb filtering to better separate the chroma from the luma. Not to fully dissuade you, I think it is better to leave the set as is until a replacement can be found.

I have mused to myself what would it take to build a display tube replacement employing a flat panel display. Resistive loads could be included to properly load the power supplies. The RGB applied the tube could drive the flat panel display. The biggest problem would finding a flat panel to fit the cabinet aperture without modifying the set or the cabinet in any way. It would just be a slip in replacement. Certainly the volume normally occupied by the original CRT could easily accommodate the modern interface electronics required.

There is a fair bit of work involved (mechanical and electrical) but it at least would provide a means to display an image in absence of a 15GP22.

Please be as cynical as you like in your response. :scratch2:

FWIW, I would wait for a replacement CRT.

I'm not proposing to modify the TV at all, except possibly to add
a power resistor or two. The video adapter would connect to the wires going
to the CRT grids and cathodes. Sync can be picked up from lots of places,
even by capacitance coupling from the output tube plates. The display would
simply sit on top of the TV set.

I tried the alternate tube yesterday. It worked in that nothing overheated and the
voltage generated was down nicely. Unfortunately it was down to 1/3 or thereabouts,
which surely generates sync/AGC/color killer pulses that are too small.

Another method, after looking at the tube curves, is to cut the screen voltage.
The waveforms I saw on both tubes were clearly "switch" action, i.e. Class C
operation. Thus, cutting the screen voltage is just like screen modulation
of a Class C AM radio transmitter and should work nicely. Only a modest cut is needed. The chassis has to be removed anyway to properly install a new fuseholder in the power supply area, so I'll add a power pot on a temporary bracket.

Could you wind a custom pulse transformer with no HV windings and a more favorable ratio for the pulses that you need? :scratch2:

jr