Originally Posted by old_tv_nut (Post 3221580)

I seriously doubt that the black to white swing of the video signal on the cathode is only 16 volts peak-to-peak on a normal picture. Could you post the waveform from the manual? I suspect it's for an odd signal that's mostly black. Another clue is that the range of the brightness control is 120 volts. If the usual video was only 16 v p-p, this would mean the brightness control has terrible excess range and would be very touchy to adjust.

Suppose that the video amplitude is a more reasonable 60 V p-p (just a wild stab) and the picture is average (not all white or all black). Then without the DC restorer, the cathode voltage swing at min brightness is from 150 to 90 (120 +/- 30). At max brightness it swings +30 to -30.

Add the DC restorer, and the swing at min brightness is from 120 to 60; at max brightness it's zero to -60.

The offset of the effective bias will be on average half the peak-to-peak (depending on scene content), so to darken the picture back to normal, you need to add about half the normal peak-to-peak voltage to the range of the brightness control.

Edit - for my guessed case, this means the brightness pot needs a range of 150 v to 30 v.

Originally Posted by old_tv_nut (Post 3221585)

I think I've reached the limit of my knowledge, especially without seeing the whole circuit.

Meanwhile,
1) How touchy is the brightness control? Does it have alot of excess range? This will give you an idea of how much the video signal swings compared to the brightness control voltage.
2) Can you cut off the blacks with the brightness control? If so, no adjustment to its voltage range is required, and you may just have to back off contrast (video drive p-p) to make sure bright pictures don't overload things.

Originally Posted by old_tv_nut (Post 3221600)

12BY7A data sheet says transconductance is 11000 micromhos, or 11 ma per volt. Load is 5.6 kohms, so 5.6 volts per ma. Service data says input is 3v p-p.

3x11x5.6 = 185 v p-p. This could be different in this TV because minimum cathode resistance is 33 ohms (I may bother with actually calculating everything tomorrow). Anyway, 160 v p-p at max contrast does not seem unreasonable. 16 v p-p seems indeed to be a misprint.

Originally Posted by old_tv_nut (Post 3221605)

Ok, here are the plate characteristics with the load line. You can see that most of the tube's capability at higher plate voltage is wasted.

Transconductance varies around 4000 to 7000 micromhos; call it 5000. Then the output for 3 volt input can vary from 30 to 72 volts p-p depending on the contrast setting.

Linear approximation:
Cathode current swings 5 ma per volt Vg-k.

For 1 v Vg-k swing, 33 ohm cathode resistor swings 5x.033 = .165 V; for 3v input, V g-k swing is 3x1/1.165 = 2.575; 2.575x5x5.6=72.

For 1 v Vg-k swing, 363 ohm cathode resistor swings 5x.363 = 1.815 v; for 3v input, Vg-k swings 3x1/2.815 = 1.07; 1.07x5x5.6 = 29.8

Could it be as low as 16 vpp at minimum? Probably.

Edit: fixed a couple of typos above.

Note: In circuits lab I PO'd the TA when I asked how much the tube characteristics could vary and he obviously had no more idea than I did. (We later became good friends and colleagues when I went to work at Zenith.)

Originally Posted by old_tv_nut (Post 3221612)

Motorola '66-'75, Zenith '75-2015.

Originally Posted by old_tv_nut (Post 3221612)

Motorola '66-'75, Zenith '75-2015.

Originally Posted by maxhifi (Post 3221623)

This sounds like a great career - almost 50 years in the US TV industry! I can't imagine how much change occurred during that time frame. What was the last US designed series of television which Zenith produced?

Originally Posted by old_coot88 (Post 3221625)

No doubt you gained some electronics skills prior to '66. Did you do a lot of homebrewing, ham stuff and the like?

Originally Posted by maxhifi (Post 3221554)

Per very old posts, I added the attached circuit to my CMC TV, a rather standard 17" B&W table top set from 1958 or 59.

It evens out the blacks, but when brightness is high, there is now sometimes a bend in the middle of the picture. It appears more with dynamic images than with static ones. I wonder if this would be improved by reducing the value of the 0.1uF capacitor, or if maybe some sets are unsuitable for this mod.