Admiral 20B1

[wa2ise]

Originally Posted by bandersen

I did some experimenting and found 0.0022 and 1M did a pretty good job suppressing the retrace but I got some twisting at the top of the picture and hum in the audio.

Does the video signal feed into the cathode of the CRT? And if so, often in these sets the same video signal also feeds the sync circuits and the sound IF. This can confuse the vertical sync circuit, as it starts to lock onto its own output rather than the TV channel's. Many later sets would inject the video into the CRT cathode, and the vertical blanking pulse on the CRT grid 1. This avoids the above interaction. Only thing is that the blanking pulse needs to be inverted or not inverted correctly. Get it backwards and all you'll see are retrace lines, and no or very dark picture. Oh, and be careful of the vertical output tube's plate circuit, the retrace pulse can be a kilovolt or 2 tall. Usually most later TVs grabbed this pulse off the yoke tap. There it's about a hundred volts tall. If the blanking pulse turns out to be backwards to what you need, you could try moving the B+ or ground connection going to both the vertical output transformer and yoke from one end of the yoke winding to the other end, but I'd only recommend that to people who are experts. This may effect the height and maybe centering. And then get the blanking pulse from the end of that winding not connected to B+ or ground. Thru a cap. I have an Admiral that could use it, but I haven't tried this yet. YMMV.