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

[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.

[Penthode]

Originally Posted by 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.

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.

[old_tv_nut]

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.

[old_tv_nut]

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.

[Kevin Kuehn]

Originally Posted by old_tv_nut

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?

[old_tv_nut]

Was working on an explanatory diagram while you posted.



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.

[maxhifi]

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/...d.php?t=145083

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

[Penthode]

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!

[maxhifi]

Originally Posted by Penthode

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.

[maxhifi]

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.

[Kevin Kuehn]

Originally Posted by maxhifi

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.

Originally Posted by Kevin Kuehn

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.