Adding DC restoration to a Sony TV-411

[ChrisW6ATV]

Hello-

I decided to try adding DC restoration to a Sony TV-411 portable B&W 4-inch TV/radio combo I bought a while back, since I had the Photofact manual. After reading numerous discussions of DC restoration and circuit modifications, especially VK member Andy's comments, it seemed pretty simple to try, but I have had no luck. The Sony's circuit is posted below, along with some of the modified versions. I did the following modifications and changes:

-Replaced R409 with a 200V, 1A fast-recovery diode (NTE 587), cathode toward RV402 (brightness control).

-Added a 0.1uF/100V capacitor from point 89 (diode/brightness junction) to ground.

Those changes made the brightness control unable to darken the screen enough, so I jumpered across R407 to fix that. I still had no change in DC ability, so I tried these changes:

-Reinstalled R409, putting it in parallel with the diode. (Note: R409 is actually 470k. Good old "reliable" Photofacts!)

-Put the diode in series with R409.

-Changed R409 to 47k as in the diagram, and tried it in parallel/series with the diode.

-Changed the 0.1uF capacitor to a 4.7uF electrolytic.

-Changed the diode to a 1N4007 or similar.

NONE of any of this accomplished any change at all in DC restoration. All I ever got was changes in the position needed for the brightness control. Note that the filament of the CRT is powered by a small separate transformer powered from a winding on the flyback. The Photofact schematic shows the CRT cathode not connected, but I would expect that that is a mistake and it is connected also at pin 7; the wiring on the board does match the drawing and CRT pin connections as shown.

I would appreciate any comments or suggestions. Thanks!