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

[Penthode]

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.

[maxhifi]

Originally Posted by Penthode

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.

[Kevin Kuehn]

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.

[maxhifi]

Originally Posted by Kevin Kuehn

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?

[old_tv_nut]

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.

[Penthode]

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!

[old_tv_nut]

Originally Posted by Penthode

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.

[Kevin Kuehn]

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.

[Penthode]

Originally Posted by Kevin Kuehn

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.

[old_tv_nut]

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.

[old_tv_nut]

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.

[Penthode]

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.

[old_tv_nut]

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..."

[old_tv_nut]

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.

[old_tv_nut]

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.