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

[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.

[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.