Revisiting an old topic - Adding DC Restoration to a 1950s TV

[maxhifi]

Originally Posted by old_tv_nut

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.

Here's the image from the manual. It struck me as odd too, it seems quite low, especially compared with range of brightness control as you say. Also, I would expect a pentode to have more gain. I will have to measure it to be sure. So you think 16V is a mistake in the manual?

Note to self: stop putting the chassis back in the cabinet until all is settled!

[old_tv_nut]

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.

[maxhifi]

Originally Posted by old_tv_nut

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.

0.5) Thanks for paying attention to this thread, I appreciate the advice and interest. If you are interested, I did upload the circuit to dropbox, here https://www.dropbox.com/sh/bm7e4on8q...tAU050hPa?dl=0

1) Not really all that touchy, but only the top of the range shows a good picture. The picture tube is not super strong though, so this is not a big surprise.

2) With the brightness control full counterclockwise, there is no image visible on the screen at all.

[old_tv_nut]

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.

[maxhifi]

That does make a whole lot more sense than 16V. I did check the 12BY7A just for fun and it checks good.

[Kevin Kuehn]

Originally Posted by old_tv_nut

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.

Will the b+ in this series string set even allow those kind of voltage swings? I looked at a similar US made set and they show 30vpp from the video out.

[old_tv_nut]

The 33 ohm cathode resistor reduces the gain such that the max possible plate swing is 136 volts. Knock off a bit for the plate resistance, and it's maybe 128 volts.

BUT all this was quick small-signal analysis based on spec sheet data at a plate voltage of 250 volts. You are right - The B+ of 130 v in this set means that the analysis should be done graphically and the gain will be considerably less. I'll attempt this later for fun, but it may be that the answer is 30 or even 16!

That's what I get for going back-of-the-envelope as I'm falling asleep.

[old_tv_nut]

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.)

[maxhifi]

Originally Posted by old_tv_nut

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.)

Thanks for doing all this. I suppose it is not a big surprise that the tube is not used to its full potential in a transformerless TV with such low B+. Out of curiosity, during what time period did you work at Zenith?

[old_tv_nut]

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

[maxhifi]

Originally Posted by old_tv_nut

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

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?

[old_coot88]

Originally Posted by old_tv_nut

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

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

[old_tv_nut]

Originally Posted by maxhifi

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?

Actually, I can't answer off the top of my head. I'm sure someone else here could be more exact. I was in Advanced Development, so stuff I worked on was 2 years or so ahead of production, and sometimes didn't pan out in the end. To paraphrase Edison, the time wasn't wasted, we just found out what not to do.

[old_tv_nut]

Originally Posted by old_coot88

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

Actually, not a lot. I was fascinated by color TV technology from the first time I saw it at age ten, and I lucked out in getting to work on related stuff, so I felt I was being paid to play.

[Kevin Kuehn]

Originally Posted by maxhifi

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.

So getting back to your sync disturbance issue - your sync is being pulled off the bottom of L7 with very little isolation from your DC restoration diode. So it's no surprise that there is some push-back loading effect on the sync when the diode conducts heavily. I'd be curious to read the thread that this method your using originated from.