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.

Two problems about adding DC restoration to a TV.

1: EHT regulation matters more with true black level. Change in mean brightness of the pcture may give unacceptable change in size. This sort of effect might explain your problem.

2: If the set has mean level AGC (common with +ve modulation, probably less so with -ve) then the DC restoration won't be so effective. Partly because contrast will vary with average picture level, partly because sync amplitude will vary with APL. Since a simple DC restorer stabilises sync tip, brightness will still vary with APL.

Two problems about adding DC restoration to a TV.

1: EHT regulation matters more with true black level. Change in mean brightness of the pcture may give unacceptable change in size. This sort of effect might explain your problem.

2: If the set has mean level AGC (common with +ve modulation, probably less so with -ve) then the DC restoration won't be so effective. Partly because contrast will vary with average picture level, partly because sync amplitude will vary with APL. Since a simple DC restorer stabilises sync tip, brightness will still vary with APL.

To clarify how it behaves: almost like losing horizontal lock a bit when the contrast or brightness are turned up high. The picture bends and pulls toward the right, and then straightens out when either the scene changes, or the contrast or brightness are manually reduced. The black level is better with this modification.

That R32 150k you crossed out, what happens if you put that 150k in series with the top leg of the diode? Without that there you're really loading down the signal when the brightness control gets to the extreme ends of it's travel.

That R32 150k you crossed out, what happens if you put that 150k in series with the top leg of the diode? Without that there you're really loading down the signal when the brightness control gets to the extreme ends of it's travel.

I used it but didn't show it, but I did this with a 100k resistor. I tried 150k, but it resulted in inadequate brightness range. With no resistor at all, it ends up being way too bright.

That R32 150k you crossed out, what happens if you put that 150k in series with the top leg of the diode? Without that there you're really loading down the signal when the brightness control gets to the extreme ends of it's travel.

You are not loading down the video when the brightness control is at the end, because the diode only conducts on sync.

However, in the original circuit, the average cathode video voltage is between 0 and 120, but with the diode, the sync peaks are between 0 and 120, and the average is lower (making a brighter picture). You may need to look for a higher voltage for the positive end of the brightness control and put a resistor between the other end of the contol and ground, to get the syncs to be cutoff and normal video to be normal.

Have you experimented with smaller cap values? Could be the RC constant with 150k isn't allowing the .1uf to fully charge. I'm just kind of guessing at what's going on there. I'd like to see the rest of the schematic. Is that in Sams?

You are not loading down the video when the brightness control is at the end, because the diode only conducts on sync.

However, in the original circuit, the average cathode video voltage is between 0 and 120, but with the diode, the sync peaks are between 0 and 120, and the average is lower (making a brighter picture). You may need to look for a higher voltage for the positive end of the brightness control and put a resistor between the other end of the contol and ground, to get the syncs to be cutoff and normal video to be normal.

Yes but the sync could be loading down, no?

Yes but the sync could be loading down, no?

It is not in Sams, but I uploaded it to dropbox just now, the link is below:

https://www.dropbox.com/sh/bm7e4on8qls4v6g/AAAoJxZzs68h_nxetAU050hPa?dl=0

I am not sure if it is loading down sync or not, but it does look like there is some effect. I agree, I am thinking maybe to try say a 0,022 capacitor and see how it looks. I also need to change the resistors to give it a bit more brightness range.

Have you observed the signal with a scope? I'm sure old tv nut gave you the correct answer. It's one of those circuits that's almost too simple to not have trade-offs.

You are not loading down the video when the brightness control is at the end, because the diode only conducts on sync.

However, in the original circuit, the average cathode video voltage is between 0 and 120, but with the diode, the sync peaks are between 0 and 120, and the average is lower (making a brighter picture). You may need to look for a higher voltage for the positive end of the brightness control and put a resistor between the other end of the contol and ground, to get the syncs to be cutoff and normal video to be normal.

I just noticed this - will review the schematic and waveform to make sure I understand

That noise immunity control in series with the sync is interesting. What effect does that have?

That noise immunity control in series with the sync is interesting. What effect does that have?

Fully counterclockwise, it has a switch. When switched off, the picture is grainy and seems weak. When turned on, the picture becomes progressively overloaded until when it is fully clockwise, it has the same effect as high brightness.

I really don't know what is is for though.

You are not loading down the video when the brightness control is at the end, because the diode only conducts on sync.

However, in the original circuit, the average cathode video voltage is between 0 and 120, but with the diode, the sync peaks are between 0 and 120, and the average is lower (making a brighter picture). You may need to look for a higher voltage for the positive end of the brightness control and put a resistor between the other end of the contol and ground, to get the syncs to be cutoff and normal video to be normal.

Ok so after looking at it closely, here is my understanding of what is going on. I could well be wrong in some detail:

Looking at the service manual, the AC signal at the plate of the video output tube is shown as being 16V peak to peak, with the big peaks being on the sync pulse. This makes sense from the perspective that during sync, the picture should be blanked. So assuming that there is no DC restore circuit, it looks like at maximum brightness, the signal at the cathode of the CRT ranges between the 0V and +120V, plus the 16V of the video output tube. So at maximum brightness, the AC value of the signal at the CRT cathode is between 0V and 16V, and at minimum value of brightness, the signal ranges from 120V to 136V.

The DC restore function appears to work like this: During the sync pulse, the highest voltage is present, which charges the capacitor. As soon as the sync pulse ends, the voltage at the capacitor is higher than the voltage on the cathode of the CRT, so the diode switches off.

Now, why would elevating the positive end of the brightness control help things? It is usually operated closer to the middle position, or higher. Even if this end were elevated more, how would it affect the DC voltage at the junction of the capacitor and the volume control wiper?

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.

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!

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.

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/bm7e4on8qls4v6g/AAAoJxZzs68h_nxetAU050hPa?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.

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.

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

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. :scratch2:

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.

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.)

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?

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

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?

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?

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.

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.

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.

https://antiqueradios.com/forums/viewtopic.php?f=3&t=194179

http://www.videokarma.org/showthread.php?t=248278&page=2

Max,

I did some more poking around and ran across this old thread that you started and possibly you'd forgotten about it. Andy pretty much identified the problem with this post I've quoted below. Andy confirms the sync stability remark with another post at the very end. This Marconi set you're working with definitely fits the category of having a cheap sync design where the sync is taken from after the video amp. Another way of doing this would be grid-leak DC restoration which takes place in the grid circuit of the video amp. That may give you the needed isolation. I'll see what I can dig up on that method.

http://mail.videokarma.org/showthread.php?t=250452

Most sets drive the cathode with the video, and it's coupled to the video output tube via a cap. The brightness control is coupled to the cathode with a resistor and controls the DC voltage on the cathode. All you really need to do is replace this resistor with a diode (cathode toward the brightness control). Adding a .1uF cap between the brightness control and ground will improve the DC restoration a little, but might cause sync, or blanking problems on some sets (some old/cheap sets take the sync off the video output tube, and don't have any blanking circuit). You will also need to add a resistor in series with the grounded side of the brightness control to keep the maximum brightness level reasonable.

As a quick test, you can simply clip the diode in parallel with the brightness control to cathode resistor, and turn down the brightness a little. If you connect the diode backwards, it won't do anything.

Also, if you haven't already, try experimenting with making your .1uf cap value smaller or eliminating it. Possibly you can find a workable compromise.

Kevin,

I do remember that now! I was trying to improve my little 8" RCA, it is incredible how fast time passes, I can't believe it was already 8 years ago.

Thank you for looking into this for me, I will pull the chassis today and try some smaller capacitors. It is looking like I am going to.have a lot of time over the next few months. The Marconi was sitting half finished under the desk I'm using to work at home, so when I was preparing to work at home I pulled it out. I'm using it to play the news, and concert DVDs while I'm passing the time. It has a better picture, and a substantially better speaker then my Predicta does, and this one little improvement would be nice.

I also.got a bunch of tubes for it over the weekend from VK member Gregb,

I noticed the resurrection of this thread and would like to add my own comments. The RCA 630TS employed 1/2 a 6AL5 diode as DC restorer in the circuit depicted below). The RCA sets from chassis KCS28 thru and up to I think KCS47 utilized DC coupling and no DC restorer diode. From about 1953 RCA abandoned DC restoration.

In every case, RCA did not employ 100% DC clamping and compromised with only partial clamping. You can examine the degree of how well black level clamping is achieved by watching active video cutting between dark to bright scenes while examining the vertical blanking bar on the screen (by misadjusting the vertical hold control so the bar is central on the screen). The bar should remain the same shade of grey or black when cutting between scenes.

The later sets without any DC clamping the bar changes with the scene cuts. The earlier RCA sets the bar shade remaiens "reasonably" constant in shade. This doesn't means that there is no noticeable shift in the bar's brightness.

I think the RCA engineers compromised a little and did allow some AC component. Perhaps this was because of normal temperature drift of components which would result in having to constantly adjust the brightness control. Nevertheless, look at the 630 TS and compare it with your original modification. It looks as if R148 was added to isolate the diode capacitance from the video path. Also note R149 the 1Meg ohm resistor across the clamping diode which is necessary as a DC return.

Also note that in the earlier sets, the clamping diode is across the video path to the CRT grid input. Later sets applied the signal to the cathode. The grid circuit does not draw current whereas the beam current passes through the brightness control and thru and around the clamping circuit. A fundamental error in your earlier circuit is that there is no DC path to allow beam current to flow!! So you are going to have to place a lower resistance across the diode which will compromise the clamping. In other words, not to sound to pessimistic, I do not think it will be a simple task getting the cathode driven circuit to work.

Also note that C143 0.25ufd is charged thru R150 which effectively further dilutes the clamping effect.

The bottom line is that it is not a perfect world and engineering is all about compromises. Especially when dealing with analog circuits! I would suggest you carefully think thru this as there are a few gotchas that need to be addressed.

I noticed the resurrection of this thread and would like to add my own comments. The RCA 630TS employed 1/2 a 6AL5 diode as DC restorer in the circuit depicted below). The RCA sets from chassis KCS28 thru and up to I think KCS47 utilized DC coupling and no DC restorer diode. From about 1953 RCA abandoned DC restoration.

In every case, RCA did not employ 100% DC clamping and compromised with only partial clamping. You can examine the degree of how well black level clamping is achieved by watching active video cutting between dark to bright scenes while examining the vertical blanking bar on the screen (by misadjusting the vertical hold control so the bar is central on the screen). The bar should remain the same shade of grey or black when cutting between scenes.

The later sets without any DC clamping the bar changes with the scene cuts. The earlier RCA sets the bar shade remaiens "reasonably" constant in shade. This doesn't means that there is no noticeable shift in the bar's brightness.

I think the RCA engineers compromised a little and did allow some AC component. Perhaps this was because of normal temperature drift of components which would result in having to constantly adjust the brightness control. Nevertheless, look at the 630 TS and compare it with your original modification. It looks as if R148 was added to isolate the diode capacitance from the video path. Also note R149 the 1Meg ohm resistor across the clamping diode which is necessary as a DC return.

Also note that in the earlier sets, the clamping diode is across the video path to the CRT grid input. Later sets applied the signal to the cathode. The grid circuit does not draw current whereas the beam current passes through the brightness control and thru and around the clamping circuit. A fundamental error in your earlier circuit is that there is no DC path to allow beam current to flow!! So you are going to have to place a lower resistance across the diode which will compromise the clamping. In other words, not to sound to pessimistic, I do not think it will be a simple task getting the cathode driven circuit to work.

Also note that C143 0.25ufd is charged thru R150 which effectively further dilutes the clamping effect.

The bottom line is that it is not a perfect world and engineering is all about compromises. Especially when dealing with analog circuits! I would suggest you carefully think thru this as there are a few gotchas that need to be addressed.

Thanks for posting, this helps a lot! I have been using the TV all week, and what I've noticed, is that high contrast scenes tend to lose cause the TV to lose lock horizontally, and there are also rare scenes which affect vertical lock.

This TV actually wouldn't lose horizontal lock through the entire range of the horizontal hold control previous to being modified.

Maybe it is best to just put it back as it was? I suppose if it was good enough in 1958, it's probably still good enough!

I also wondered about where will the beam current flow, and realized that it flows to ground through the diode when the diode is "on", and when the diode is "off" it flows through the video output tube. There is really no other path. This must be what is going on, because the circuit does work, it just doesn't work well.

Did you get a chance to try any smaller cap values? And if so was there any difference on the hold interaction? I looked at grid leak bias restoration at the video output grid, but that requires DC coupling from the video output to the CRT cathode. Could be done, but it's a lot of re-engineering.

Did you get a chance to try any smaller cap values? And if so was there any difference on the hold interaction? I looked at grid leak bias restoration at the video output grid, but that requires DC coupling from the video output to the CRT cathode. Could be done, but it's a lot of re-engineering.

Not yet, I haven't had a chance to pull the TV chassis out of the case again yet.

Let me ask a dumb question. Is the DC signal we are attempting to restore present at the output of the video detector?

...Is the DC signal we are attempting to restore present at the output of the video detector?

Yes. The DC component is broadcast. One of the consequences is that the RF power at sync tips or blanking or black level does not change due to variations in the scene content. Therefore (assuming the TV's AGC circuit is working on sync tips) the output of the video detector is similarly stable.

However, carrying this all the way to the CRT by using DC coupling all the way is not practical in tube sets (too much drift over time/temperature, plus changes every time you replace a tube). It is also not practical in solid state sets even if the transistors do not vary, due to component tolerances. Every stage would need the bias circuits replaced with adjustable potentiometers.

The only reasonable way to stabilize the video DC level is to do it close to the CRT, (or measure it at the CRT and use feedback to adjust it at an earlier point).

The ultimate solid state CRT TVs used feedback to control the CRT bias individually for red, green, and blue, thus achieving not only DC restoration but automatic CRT tracking. The video IC restored DC in the low-level luminance, then added a low brightness pulse at the top of the raster just after retrace (where you can't see it) to measure the CRT beam current near black.

Why not measure zero current, exactly at black, to set cutoff? Because a CRT has a power law current vs. voltage characteristic, so at zero current, the incremental current gain is also zero, which means there is no feedback signal.

By the way, if the DC component was not broadcast, it would of course not be present at the detector, and the whole exercise would be somewhat futile (although it could have some effect by clamping the darkest thing in the scene as black).

Iconoscope camera tubes had inherent AC coupling. The DC level had to be made up by clamping the darkest parts of the scene to not go below black, plus there was a "shader" (a live person) adjusting it along with the spurious shading across the image that the iconoscope was prone to. Someone who does camera electrical alignment is still called a shader today, although they are not doing it all real-time during use.

The reason you are losing sync is because when the brightness is turned up high or the contrast advanced, you are clipping the sync with the diode! I suspect the sync take off is at the plate of the video amplifier.

Compare your circuit with the RCA 630ts and note R148 which will alleviate the sync clip.

It is a mystery how it works all unless the diode was installed opposite to how you depict it. The CRT electron current flow in the original design is from is ground through the brightness control up thru R32 the 150K resistor to the cathode of the CRT. The diode depicted in your drawing would obstruct the beam current and there is no other path unless C19 the 0.05ufd coupling capacitor is very leaky!

The reason you are losing sync is because when the brightness is turned up high or the contrast advanced, you are clipping the sync with the diode! I suspect the sync take off is at the plate of the video amplifier.

Compare your circuit with the RCA 630ts and note R148 which will alleviate the sync clip.

It is a mystery how it works all unless the diode was installed opposite to how you depict it. The CRT electron current flow in the original design is from is ground through the brightness control up thru R32 the 150K resistor to the cathode of the CRT. The diode depicted in your drawing would obstruct the beam current and there is no other path unless C19 the 0.05ufd coupling capacitor is very leaky!

Agree with the possibility of clipping the sync. Adding a series resistor as in the tube circuit would fix this.

Disagree with the "can't work" statement. The added diode is in the right direction for restoring DC at the CRT cathode. The sync tips are positive here, and C19 is charged by diode conduction during sync, then discharges through the CRT during active video. In the case of grid drive, since there is no grid current during active video (or at any time) to discharge the coupling capacitor, a resistor is added across the diode.

Investigation of the charge/discharge time constants with a scope by looking for tilt on the cathode waveform and sawtooth on the added 0.1 cap would help optimize the capacitor values.

I don't think I've ever seen a textbook or manufacture example of a diode in conjunction with the video going to the cathode. The diode orientation is confusing because early on there was a double standard depending on function of the circuit. I always assumed that came about because of conventional vs electron current flow.

I don't think I've ever seen a textbook or manufacture example of a diode in conjunction with the video going to the cathode. The diode orientation is confusing because early on there was a double standard depending on function of the circuit. I always assumed that came about because of conventional vs electron current flow.

I agree. Why I suggest it can't work or at least the concept is faulty is because there is no path for the CRT beam current. The diode is in fact reversed to the flow of beam current. This may explain why the clamping diode was only used for grid driven CRTs.

The path for the beam current is the coupling capacitor. Electrons are drawn into the coupling capacitor throught the diode during sync pulses, then flow from the capacitor into the cathode and the CRT beam during active video.

On average over time, the charge drawn through the diode equals the discharge through the beam current.

Note: I have built DC restorers that work with a diode at the base of a transistor amplifier. They work just fine, even though there is base current that must be supplied. Current flows into the input coupling cap during sync and out of it into the transistor during active video.

I understand it could work. But how do you set up the quiescent point? If there is no video, how do you control brightness? The capacitor and diode block the beam current so with no video the screen is black.

That is why, perhaps facetiously, I suggested perhaps the coupling capacitor has not yet been changed, was leaky, conveniently providing the quiescent current.

Even if the video is totally black, there will be sync pulses, so I'm not sure I understand the question about "if there is no video..."

If the picture is black, and the brightness control has been set so that the CRT draws no current at black, then the diode will not supply any current during the sync pulses, and the cathode voltage will remain at the same voltage as usual.

But, you have pointed out that the CRT is a non-linear load (very non-linear when the current goes to zero), so this results in a variable time-constant for decay of the capacitor voltage, hypothetically infinite if there is no leakage whatsoever. So if this results in odd transient behavior (for example, when the picture suddenly cuts from bright to black or vice-versa), it can be reduced by putting a high value bleed resistor across the diode, just as is done in the grid-drive case.

You have made me realize that in the case I mentioned of a transistor amplifier, it is biased such that the current never goes to zero, so such problems cannot occur.

Reading your post again, it is not the capacitor and diode that block the beam current, it is the CRT. If/when the CRT conducts beam current, the diode and capacitor will supply it.

Reading your post again, it is not the capacitor and diode that block the beam current, it is the CRT. If/when the CRT conducts beam current, the diode and capacitor will supply it.

I apologize for the awkward/ miscommunication here. But the CRT DC quiescent beam current used to flow thru the resistor from the brightness control center tap to the crt cathode. With this resistor removed and replaced by the diode, note that the current flow is now blocked by the diode because it is reversed biased. The coupling capacitor cannot pass DC. The circuit is therefore indeterminate and depends on component leakage currents to sustain the CRT current.

The CRT beam current is generally quite small at about 1ma maximum. But removal and replacement of the resistor not only makes the CRT bias determined by undesired component leakage but also will lead to clipping of the video at high amplitude. As there is minimal or no proper DC return path it will behave like a grid leak detector and effectively clip the video sync.

Look at the grid-drive version. Without the resistor in parallel with the diode, it is similarly indeterminate. But, that resistor is not supplying current, it is draining it, just as the CRT does in the cathode drive circuit. The cathode drive circuit is not indeterminate if there is some beam current, only when the picture is completely black. Fixable the same way as the grid crive circuit with a resistor across the diode to drain a little current when the CRT does not.

The diode does not block the average current, it supplies it as a pulse of current during sync, putting a charge in the coupling capacitor that then supplies the beam current during active video.

Think about the flyback high voltage supply: same thing. The HV rectifier only conducts during retrace, but charges a doorknob capacitor or the CRT dag, which supplies the beam current during active video. The rectifier does not need to conduct during the active scan while beam current is supplied. In the case of the HV supply, the other end of the capacitor is connected to ground, so its voltage slump between pulses is in reference to a DC level. In the case of the DC restorer, the other side of the coupling cap is not connected to ground, but is connected to the video output. So, it holds a fixed (slightly slumping) DC difference between the video output and the driven CRT element. That DC difference is reset to the desired value every time the diode conducts.

Summary: a DC restorer is like a power supply where the bottom of the filter cap is connected to video (which is the source of the charging pulse) instead of ground.

A DC restorer is the same as a power supply circuit, with the ground moved from the usual place. The pulse still appears across the diode, the cap still holds a DC voltage across it. It's just the reference point for voltage readings to chassis that has moved.

Pulsed current comes in, but continuous current can go out.

Summary: a DC restorer is like a power supply where the bottom of the filter cap is connected to video (which is the source of the charging pulse) instead of ground.

A DC restorer is the same as a power supply circuit, with the ground moved from the usual place. The pulse still appears across the diode, the cap still holds a DC voltage across it. It's just the reference point for voltage readings to chassis that has moved.

Pulsed current comes in, but continuous current can go out.

Would you consider drawing us the circuit as you see fit for the set Max had asked about?

Was working on an explanatory diagram while you posted.

https://live.staticflickr.com/65535/49728465717_13abd14610_o.jpg

Of course, you want the sync tip voltage to be adjustable, not tied permanently to ground (zero volts) as shown in this simple comparison diagram, so in the actual circuit you tie the diode to the adjustable brightness control.

If drawing zero current through the CRT during black scenes is a problem, you can add a high resistance load to draw a little constant current.

Wow, there is a lot of interesting discussion to catch up on here! I really appreciate all the effort being put forth.

In the meanwhile, I found this thread on another forum which is quite interesting

https://www.vintage-radio.net/forum/showthread.php?t=145083

The circuit in post 10 looks interesting, because the cathode follower would entirely isolate the diode from affecting sync.

Here is how I would revise your circuit. The the upper resistor should eliminate the sync clipping and the 680K diode bypass resistor will limit the clamping and can be varied to the degree of clamping required.

It occurred to me afterward that because the CRT is conducting, the diode remains forward biased even with no sync or video. Nevertheless the bypass resistor should make the brightness level less likely drift over time.

I guess all of us are suffering cabin fever!

Here is how I would revise your circuit. The the upper resistor should eliminate the sync clipping and the 680K diode bypass resistor will limit the clamping and can be varied to the degree of clamping required.

It occurred to me afterward that because the CRT is conducting, the diode remains forward biased even with no sync or video. Nevertheless the bypass resistor should make the brightness level less likely drift over time.

I guess all of us are suffering cabin fever!

You almost made it look like part of the original schematic! Thanks for taking the trouble! I will give it a shot, if it works well this is sure an easy modification to add.

I had a few minutes free so I installed Penthode's modification. I'm going to call it a success, the black level seems consistent from bright to dark scenes, and the sync interaction is absolutely gone. I'd be curious what others think of it, if anyone wants to try with their TV. Modern program material like DVD video appears to have quite a high contrast range, which really exaggerates the issue caused by lack of DC restoration, I think this was a very worthwhile thing to do.

I had a few minutes free so I installed Penthode's modification. I'm going to call it a success, the black level seems consistent from bright to dark scenes, and the sync interaction is absolutely gone. I'd be curious what others think of it, if anyone wants to try with their TV. Modern program material like DVD video appears to have quite a high contrast range, which really exaggerates the issue caused by lack of DC restoration, I think this was a very worthwhile thing to do.

With the exception of the 680k bleeder across the diode it seems we're right back where we started from. ;)

That R32 150k you crossed out, what happens if you put that 150k in series with the top leg of the diode? Without that there you're really loading down the signal when the brightness control gets to the extreme ends of it's travel.

http://videokarma.org/attachment.php?attachmentid=200498&d=1584623306

http://videokarma.org/attachment.php?attachmentid=200579&d=1585867061

I had a few minutes free so I installed Penthode's modification. I'm going to call it a success, the black level seems consistent from bright to dark scenes, and the sync interaction is absolutely gone. I'd be curious what others think of it, if anyone wants to try with their TV. Modern program material like DVD video appears to have quite a high contrast range, which really exaggerates the issue caused by lack of DC restoration, I think this was a very worthwhile thing to do.

Great!

I'm curious to know how much the voltage on the wiper of the brightness control (and on the 0.1 uf cap) changes between a bright scene and a dark/black one. To be clear, I mean with the brightness control set for normal operation and not moved. Is it still accessible for a quick check?

Yes I am curious about the wiper position as well. My guess is the clamp will raise the sync tips to the average level pushing the brightness control towards the +250v.

With the exception of the 680k bleeder across the diode it seems we're right back where we started from. ;)



http://videokarma.org/attachment.php?attachmentid=200498&d=1584623306

http://videokarma.org/attachment.php?attachmentid=200579&d=1585867061


I think the 150k resistor is what is preventing the diode from messing with sync. The picture now does not tear sideways when there is a picture with certain characteristics.

I cannot easily access the control.with the set together to measure voltage, this TV has a fairly awkward chassis to remove from the cabinet. I actually had it on all morning, and it plays quite nicely now.

But can you say if the brightness control is near one end or the other? That is assuming before the modification the control was roughly central.

But can you say if the brightness control is near one end or the other? That is assuming before the modification the control was roughly central.

It is turned up almost all the way, but that was also the case before the modification. I think it has slightly less range on the high end now though

That makes sense. The CRT is closer to cut off and the wiper is at the ground end. Placing a little plus DC bias on the CRT control grid will centralize the control. The diagram is cut off but I suspect the grid goes to ground through a resistor with a coupling capacitor from the vertical output transformer feeding it?

Placing a single additional high value resistor between the +250v rail and the CRT control grid will form a resistive divider which should apply a positive bias of roughly +20 volts on the grid. This will bring the brightness control closer to the center of its range.

I wouldn't bother changing anything if you have sufficient range now. The reason I asked about the voltage change with scene variation is that it represents a decrease in the accuracy of the DC restoration if the voltage varies. There should be less variation if the wiper is nearer one end than the center.

I'd be curious what others think of it, if anyone wants to try with their TV. Modern program material like DVD video appears to have quite a high contrast range, which really exaggerates the issue caused by lack of DC restoration, I think this was a very worthwhile thing to do.

I have this Setchell Carlson P65 I restored several years ago that suffers form this same issue. Same basic circuit and CRT type as your TV, although it is a transformer powered set. Sams shows 50vpp video going to the CRT. If I recall correctly I can easily operate this one outside the cabinet. Hopefully I'll be able to take some voltage measurements to see how closely this regulates sync pulse level.

https://live.staticflickr.com/65535/49732477497_f0f4d06f59_z.jpg

https://live.staticflickr.com/65535/49731607383_fd686efa76_z.jpg

^^That's a very nice looking TV

Got the chassis out and warmed up. The video source is a Pioneer VCR/DVD combo with internal rf modulator playing a DVD. The 2nd and 3rd pictures are with the screen brightness set in the middle, contrast about two thirds up. The last picture is what this set has always done between scenes, goes to medium grey with retrace lines. I suppose that happens because of the lack of DC picture level? Have to take a break but hopefully I'll get back to this later this evening. Max, do you recall what type of diode you used?

https://live.staticflickr.com/65535/49732800912_9f819787a4_b.jpg

https://live.staticflickr.com/65535/49732479146_dc3bca818b_z.jpg

https://live.staticflickr.com/65535/49732800882_9b63577131_z.jpg

^^That's a very nice looking TV

Thanks. They are quite common around here as I live about 100 miles from where they were manufactured.

I used just a regular 1N4007, because I have a bulk pack of them. Just make sure whatever you use has an adequate voltage rating

OK, as a baseline I get about 1.5 volts of average DC variation at the output of the detector diode, from bright to dark scenes. When I say average, I mean that's what my scope is telling me when I look at the composite signal at the detector output. Then at the cathode of the CRT, without the restoration circuit, I see about 10V DC in variation, but that appears to be faster transients. With the restoration circuit in place I only see about 5V DC change. My first thought was that that small variation can't be very useful restoration, but then I don't know what would be considered acceptable. This was while watching my first season Gilligan's Island DVD. Right now I'm having a hard time convincing myself it's beneficial. I need to spend some more time with this as it's much harder to analyze with the scope than I anticipated. It's possible I'm one of those individuals that's not sensitive to background level. :scratch2:

Try watching something more recent, which was filmed at night. Old TV shows were made for old low contrast TV sets, so they probably will show the least benefit.

Try watching something more recent, which was filmed at night. Old TV shows were made for old low contrast TV sets, so they probably will show the least benefit.

Good idea. For now I came up with an alternative that I think is valid. I figured my B&K 1077 would produce a representative light and dark raster that should allow me to take steady DC voltage measurements at the brightness pot wiper. B&K rf output connected through the tuner of the TV. The DC V measurements were taken at the brightness pot wiper which is also the cathode of the diode and top of the .1uf filter cap going to ground.

1. The B&K target measured about 90V DC.

2. Bright raster with no slide measured about 100V DC.

3. Black plastic in place of the slide produced 82V DC.

So a total DC swing of 18V at the wiper.

It clearly shows the diode is in fact rectifying the sync tips. How useful that amount of restoration is I have no idea. To me, visually it's a subtle effect, but like Max mentions I need to find some much better night video to put this to the test watching program material.

Disclaimer:

It's about impossible to take a picture of a dark raster, so the last image is fudged a little. But there were no controls adjusted on the TV while measuring the 3 DC voltages.

https://live.staticflickr.com/65535/49732708783_d0944b8018_z.jpg

https://live.staticflickr.com/65535/49732708808_3a16713609_z.jpg

https://live.staticflickr.com/65535/49733575117_fdb23d5a7f_z.jpg

This is how my TV was modified for the above measurements. And I just realized I didn't have the bleeder resistor across the diode when I did this. Not sure what if any effect that will have on the measurements. I had one in there early and didn't see much difference and then forgot to put it back. :scratch2:

https://live.staticflickr.com/65535/49733334676_f444b2989b_z.jpg

Good idea. For now I came up with an alternative that I think is valid. I figured my B&K 1077 would produce a representative light and dark raster that should allow me to take steady DC voltage measurements at the brightness pot wiper. B&K rf output connected through the tuner of the TV. The DC V measurements were taken at the brightness pot wiper which is also the cathode of the diode and top of the .1uf filter cap going to ground.

1. The B&K target measured about 90V DC.

2. Bright raster with no slide measured about 100V DC.

3. Black plastic in place of the slide produced 82V DC.

So a total DC swing of 18V at the wiper.

It clearly shows the diode is in fact rectifying the sync tips. How useful that amount of restoration is I have no idea. To me, visually it's a subtle effect, but like Max mentions I need to find some much better night video to put this to the test watching program material.


I think you misunderstood. In an ideal case, the brightness voltage and voltage on the 0.1 uf cap would be completely steady, so the sync tips would be clamped to a constant voltage no matter what the video content is.

The indication of the effectiveness of the DC restoration is the DC voltage measurement at the cathode. With no DC restoration, the change from bright scene to black scene will be minimal - it will just be set by the brightness pot. But with the DC restorer, the DC voltage will be high on the black scene and lower on the bright scene. Looking with the scope, set to DC coupling, the dark parts of a bright picture will be the same voltage as the black of a black screen (or almost the same, depending on how complete the DC restoration is).

Looking at your screen pics, you can see plainly that with the test pattern, the blacks are properly black, and with the black image they are still properly black, not gray, so the DC restoration is doing its thing. If you get an old movie that has spooky night scenes where nearly everything is dark, the picture will actually be dark like it should be, instead of foggy medium gray. This will not be visually subtle!

Your measurements at the wiper would indicate that the black level probably shifts about 8 volts when going from bright test pattern to full black. This is small compared to the full 50 v p-p video signal, so is much better than the non-DC- restored original.

If you look with a DC-coupled scope at the cathode, you will be able to see how little the black level shifts compared to the huge changes without DC restoration.

I think you misunderstood. In an ideal case, the brightness voltage and voltage on the 0.1 uf cap would be completely steady, so the sync tips would be clamped to a constant voltage no matter what the video content is.

The indication of the effectiveness of the DC restoration is the DC voltage measurement at the cathode. With no DC restoration, the change from bright scene to black scene will be minimal - it will just be set by the brightness pot. But with the DC restorer, the DC voltage will be high on the black scene and lower on the bright scene. Looking with the scope, set to DC coupling, the dark parts of a bright picture will be the same voltage as the black of a black screen (or almost the same, depending on how complete the DC restoration is).

Looking at your screen pics, you can see plainly that with the test pattern, the blacks are properly black, and with the black image they are still properly black, not gray, so the DC restoration is doing its thing. If you get an old movie that has spooky night scenes where nearly everything is dark, the picture will actually be dark like it should be, instead of foggy medium gray. This will not be visually subtle!

Your measurements at the wiper would indicate that the black level probably shifts about 8 volts when going from bright test pattern to full black. This is small compared to the full 50 v p-p video signal, so is much better than the non-DC- restored original.

If you look with a DC-coupled scope at the cathode, you will be able to see how little the black level shifts compared to the huge changes without DC restoration.

Thanks for explaining in greater detail. I installed a 1 meg bleeder across the diode which resulted in less change at the brightness wiper, and the voltage change at the cathode being greater. Seems like what was lost at the brightness wiper was added to the cathode. I find it very difficult to monitor the black level of a playing video on the scope. Using the B&K test pattern was the only way I could see it change, or not.

I believe a portion of this lack of black level comes from some sets having lower B+ headroom. The B+ on this TV I'm experimenting with has close to 250v vs Max's set at 150v. My contrast control has a range that allows high black level's even without the dc restoration circuit present. I find that most video I watched growing up was fairly consistent within a given program. I suspect there was some type of video compression used during the production process back in the 60's though 70's. I can basically set it and forget it. I'm not sure if modern day video content would be worse in that respect. Honestly I don't have much desire to watch new programing on my vintage sets. I do find the audio processing on today's movies quite poor in comparison to older material. Or possibly there's a trend back towards less processing? With my ageing ears I struggle to hear the quieter passages, and soon after being blown away. IMO too much dynamics. Similarly I don't see the range of video dynamics in nature that we expect to see from from our television sets. When ambient lighting is low the human eye tends to see less contrast.

I think the human eye sees a far greater range of contrast than vintage TV sets can display. In old TV shows, it seemed common to light up night scenes quite brightly by today's standards in order to get any image on the film, but the more modern trend seems to be a huge range between the brightest and darkest scenes. I would say it is almost analogous to the difference between the noise floor and the loudest possible recording on a 78RPM record versus a digital recording.

This large contrast range really asks a lot of old TVs, and that's why I wanted to try and do something to improve the Marconi, if only slightly. Better yet it seems to me would be some kind of signal processing to dynamically compress the contrast range, like say using a variable mu tube as the video amplifier, to have the effect of turning the contrast down in dark scenes, but I'm afraid I'm not smart enough to design that.

Well of course the contrast on old sets was/is limited to whatever the CRT face reflects in any given ambient lighting. That's the main reason modern flat screens can produce such apparent contrast. But in IMO it's pushed over the edge, to the point of now being an effect more so than anything you'd see inside or outdoors in natural lighting. Keeping in mind that everyone's eyes aren't calibrated the same, not anymore that our ears are. I've met a few folks that don't see color, or so they say. :)

Well of course the contrast on old sets was/is limited to whatever the CRT face reflects in any given ambient lighting. That's the main reason modern flat screens can produce such apparent contrast. But in IMO it's pushed over the edge, to the point of now being an effect more so than anything you'd see inside or outdoors in natural lighting. Keeping in mind that everyone's eyes aren't calibrated the same, not anymore that our ears are. I've met a few folks that don't see color, or so they say. :)

There have been a few shows which give my CRT based Runco HDTV projector fits, because the night scenes are so dark, that if I adjust contrast to suit them, then the picture looks horrible on bright scenes. What tripped up the Marconi was concert videos where they alternate between dark very bright. The DC restoration circuit seems to help these.

I agree though, it is kind of pushing things too far, but unfortunately I have no influence over what sort of signal the TV gets. I actually use my vintage sets for 100% of TV watching, simply because I like them. I can't convince the family to live in black and white though, so the main living room set is a CTC38.

Don't get me wrong, I'm not suggesting the DC restorer isn't helping out on your Marconi. I was simply pointing out that there's a multitude of things than can compound the situation. I've watched modern over the air TV from the S video luminance output, through my Blonder Tongue agile modulator, which even then the contrast can seem very washed out, and this on a set with DC coupling from detector to CRT. It's not solely a mater of DC component. [edit] well it might be a matter of DC component gone missing from some point in the signal chain, but not necessarily within the vintage TV.

Don't get me wrong, I'm not suggesting the DC restorer isn't helping out on your Marconi. I was simply pointing out that there's a multitude of things than can compound the situation. I've watched modern over the air TV from the S video luminance output, through my Blonder Tongue agile modulator, which even then the contrast can seem very washed out, and this on a set with DC coupling from detector to CRT. It's not solely a mater of DC component.

I'm not thinking it is a miracle solution, but rather a situation where every little bit helps. The Marconi is never going to perform like something newer, but if three little $0.10 parts can make it 10% better then why not? The elephant in the room with the Marconi is that the CRT is on the weak side, which already diminishes the contrast range. Which reminds me I ought to get my blonder tongue modulator up and running!