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

[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.

[ppppenguin]

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.

[maxhifi]

Originally Posted by ppppenguin

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.

[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.

[maxhifi]

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.

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.

[old_tv_nut]

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.

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.

[Kevin Kuehn]

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?

[Kevin Kuehn]

Originally Posted by old_tv_nut

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?

[maxhifi]

Originally Posted by Kevin Kuehn

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/bm7e4on8q...tAU050hPa?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.

[Kevin Kuehn]

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.

[maxhifi]

Originally Posted by old_tv_nut

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

[Kevin Kuehn]

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

[maxhifi]

Originally Posted by Kevin Kuehn

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.

[maxhifi]

Originally Posted by old_tv_nut

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?

[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.