Question about demods/drivers - Videokarma.org TV - Video

miniman82 · #1 10-18-2010, 11:48 PM

CTC-7C of course, but really applies to most of the CTC lineup since the X/Z demod scheme is roughly the same for all of them.

I've discovered that the color issue on this set is due to the G-Y amp not doing anything- I 'scoped it's plate. Blue and Red work as advertized when using the raster buttons on the sig gen (I get the waveform I'm supposed to get), but green refuses to work. The voltages on the G-Y amp are all in the normal range (within 2-3v high of the RCA manual, and only due to the 122v AC input), so what are some possible causes? It's a pretty simple amp being the 'difference' color, since it basically just takes it's cues from the other 2.

Only thing I can come up with is maybe the color matrix is still not aligned correctly (too much red/blue phase difference throwing off green?), though it seems to be pretty close. I don't have issues with rolling color bars anymore since replacing the crystal, and the tint and color controls seem to be doing their jobs. Tweaking the burst phase transformer has no effect, neither does the CW driver.

I don't think the demod is to blame, it looks to be working correctly.

It's not the amp tube either, already tried that.

cbenham · #2 10-20-2010, 07:20 PM

Have you scoped the grid and the cathode of the G-Y amp while feeding color bars to the set?

stromberg6 · #3 10-20-2010, 08:47 PM

On one of my 7s, I found wax had dripped into the G-Y socket causing the loss of signal to the CRT. Don't know if that's happening in your chassis, but thought I'd mention it. I replaced the wax caps with mylars. No more trouble.
Kevin

miniman82 · #4 10-20-2010, 10:32 PM

Quote:

Originally Posted by cbenham

Have you scoped the grid and the cathode of the G-Y amp while feeding color bars to the set?

Quote:

Originally Posted by miniman82

I've discovered that the color issue on this set is due to the G-Y amp not doing anything- I 'scoped it's plate.

I suppose it is possible that there's a bad resistor on either the red or blue amp that might cause a degraded signal at the green amp, I'll check that next.

miniman82 · #5 10-20-2010, 10:36 PM

Quote:

Originally Posted by stromberg6

On one of my 7s, I found wax had dripped into the G-Y socket causing the loss of signal to the CRT. Don't know if that's happening in your chassis, but thought I'd mention it. I replaced the wax caps with mylars. No more trouble.

Kev- I've already totally recapped all waxies on this one, and the sockets (though old) are pretty clean. Thank for the tip though, the wax was indeed all over the underside of the chassis!