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

[old_tv_nut]

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.

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?

[Penthode]

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.

[maxhifi]

Originally Posted by Kevin Kuehn

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

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.

[Penthode]

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.

[maxhifi]

Originally Posted by Penthode

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

[Penthode]

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.

[old_tv_nut]

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.

[Kevin Kuehn]

Originally Posted by maxhifi

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.

[maxhifi]

^^That's a very nice looking TV

[Kevin Kuehn]

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?

[Kevin Kuehn]

Originally Posted by maxhifi

^^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.

[maxhifi]

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

[Kevin Kuehn]

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.

[maxhifi]

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.

[Kevin Kuehn]

Originally Posted by maxhifi

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.