Videokarma.org TV - Video - Vintage Television & Radio Forums

- Early B&W and Projection TV (http://www.videokarma.org/forumdisplay.php?f=19)

- - picture question (http://www.videokarma.org/showthread.php?t=150247)

Quote:

Originally Posted by electroking (Post 1669763)

Hello Old1625,

To answer your last question, I guess the following:

1 - Opening the cathode with the switch would mean routing the video
signal through long wires going to the power switch, not a good way
to deal with a wideband signal like this.

2 - A relay is costly.

How right you are, Jeeves! And that's why all the sets in the day had that spot after shutoff!

But when doing an antique resto what the heck is money....? :D

The solution would be as someone suggested earlier that happened in IIRC Electrohome, which would be to have the second gang switch on the on-off control introduce a stored + DC voltage to the cathode circuit, which would supply enough reverse bias to make the CRT cutoff until the HT bled down and the heaters cooled sufficiently.

(I may need to rescind my resonse to that post and apologize, as what was said about involving B+ seems to be making more sense than it was at the time--talk about clumsy reverse-engineering here.... :o)
.....

Quote:

Originally Posted by Old1625 (Post 1668762)

Some later model TVs had a "spot killer" circuit added. Perhaps a relay in the focus circuit, or cathode circuit of the CRT could provide such feature in an older set... :scratch2:

Some sets had the G1 grid bypassed with a cap to B+, and when the set gets turned off, the B+ quickly goes to zero, and the voltage charge on the cap gets pulled to a negative voltage, thus cutting off the electron beam that would otherwise go to the screen. The bypass cap would take longer to discharge than the collapse of B+, and this time constant was selected to last longer than the residual heat on the cathode. Thus no spot.

Further comment on the Electrohome spot-killer

Hello everyone,

Reading all these contributions, and looking back at the schematic I posted
a few days ago, I remembered something. This TV was purchased by my
parents in 1961, when I was less than a year old. As a child I would
sometimes play with the controls, and I found that if I set the brightness
to the max, the spot would not be killed when the TV was turned off, which
may explain the presence of the small burn. If you take a look
at the schematic, you see that when brightness is set high, the wiper
of the control is grounded, so that the positive bias applied by S401 has
no chance of reaching the CRT cathode. So this circuit design was far
from perfect...

Despite what I mentioned above, I was not too hard on this set, nor was
the rest of my family, as it has survived to this day. I used to run it a
few tens of hours every year but the last time I tried (last year), it was
dead except for the pilot light, pointing the way to a busted LV rectifier
(5V3 tube). I should start a serious restoration effort this year.