Grainy color, what causes it?

[miniman82]

This is definitely not something with the video (luma), it's something with the chroma section of my CTC-4. The set works fine with low levels of color but when I advance the color control to where I think it's pleasing, it gets 'grainy' for lack of a better word. It looks like noise, travelling horizontally through the frame. At first I though maybe the chroma section was being overdriven, so I suspected auto chroma gain issues. I did find a drifted resistor (a 3.9k which read 4.3k) going to the 6AZ8 bandpass amp tube's grid (pin 6), but it didn't improve things any. What else can cause this?

[old_coot88]

Originally Posted by miniman82

This is definitely not something with the video (luma), it's something with the chroma section of my CTC-4. The set works fine with low levels of color but when I advance the color control to where I think it's pleasing, it gets 'grainy' for lack of a better word. It looks like noise, travelling horizontally through the frame. At first I though maybe the chroma section was being overdriven, so I suspected auto chroma gain issues. I did find a drifted resistor (a 3.9k which read 4.3k) going to the 6AZ8 bandpass amp tube's grid (pin 6), but it didn't improve things any. What else can cause this?

Possibly 9.20 khz beat? That's the difference "note" between the 4.5 sound and 3.58 color intercarriers, and will show up as colored 'rice grains' when the 920 khz trap is off the null point. Can you locate the trap? It's an adjustable coil that should be just after the vid. detector and before the chroma takeoff. Maybe post a pic of that area of the schematic(?).

If it's a 920 beat problem, the 'rice grains' usually display a squirminess (for lack of a better word) that varies in step with the sound.
Bill(oc)

[miniman82]

Here's the schematic: http://www.mcmlv.org/Archive/Radio/CTC4.pdf

Only trap I see is 4.5mc.

[old_coot88]

In this design it looks like they're trapping the sound intercarrier out with the "4.5 trap" located in T108, the chroma take-off xfmr.