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

[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.