View Full Version : Adding DC restoration to a Sony TV-411


ChrisW6ATV
02-16-2013, 03:05 PM
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!

ChrisW6ATV
02-23-2013, 03:10 AM
I may try the same modification on a Panasonic TR555 next.

old_tv_nut
02-23-2013, 02:53 PM
I would think that the second diagram (diode across R409, cap from point 89 to ground, R407 bypassed) should work. Added cap should be should be 4.7 uF, not 0.1, I think. Try reducing value of R409. If DC restoration is working, but you go too small on R409, you will see some shading from left to right in the picture. Current value of 470k makes discharge time constant with C403 of about 1/20 of a second, does not have to be that long. 47k would be more reasonable and should give about 1% tilt.

Last illustration (resistor in series with diode) definitely will not work, it slows down the charging of C403 on sync tips. The goal is to charge very quickly, then discharge relatively slowly until the next sync pulse.

old_tv_nut
02-23-2013, 03:14 PM
It just dawned on me to look at the video output. It appears the load is 15K ohms. This makes the charging time constant with C403 1.5 ms, but it has to be much smaller to charge on the sync tips (about 300 times smaller!). Therefore, leave R409 as 470k, but reduce the size of C403. It may not be practical to reduce it enough to fix the charging time constant without affecting the video response due to the high value of R406 - you will need to experiment.

ChrisW6ATV
02-24-2013, 01:03 AM
Thank you for the comments and suggestions. I will try it with R409 at 470k (the original value), the diode in parallel with R409, the capacitor from the diode cathode to ground at 4.7 uF, and then try different values for C403.