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On V7B: Tube biasing 101 says the less negative the grid the more plate current is drawn, the more plate current the less plate voltage (at least in a common cathode circuit like V7B)....If your grid bias is off it will effect the plate.
I'd like to see more of the schematic to the right of V7B. The grid circuit wanders off the edge of your attached scans...Seeing where it goes could give some insight into why it is not negative enough. There is a slight chance the video detector diode is going bad (I've seen it in some TVs from time to time) and reducing video amplitude. The AGC wiper appears to basically be the start of the AGC bus that biases the tuner and IF grids. If the control is dead till a voltage is injected it probably means the AGC circuit is not working. If all the tube voltages look right I'd be looking that fly winding between the plate of the AGC tube and the AGC control. Unless you can get proper/much better contrast by injecting -9V at the AGC control (and adjusting the control) I think the AGC is not the main issue....Even if you can I'm still a bit more concerned about V7B. With what I see I'm not inclined to think it is an IF alignment issue....And even if it is part of the problem, you need functional AGC and to work out the issue with V7B first before any alignment can be attempted. If you try to align circuits with defects the results often are not an improvement over the defective circuit as it was. I'm running on ~3 hours of sleep presently so take this advice with a grain of salt...
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
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I've replaced every passive component on the AGC circuit between the grid of the 1st IF and the flyback. The control works perfectly too... no jumps and smooth resistance all the way across. I suppose I could try changing out some of the small mica and disc caps in the adjacent circuits. Quote:
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I'll try changing out that diode and see what that does. I think I have some low forward voltage drop diodes... somewhere... FK |
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What I do see is that there is a slow brightness increase over 5 mins or so when starting from cold. As in, having adjusted the bright/contrast the night before, it will start very dim - almost not visible. Could this just be the age of the CRT and it's poor life test? I can set all of the controls so that all of the voltages on the CRT are exactly on spec, and this is how the picture appears https://youtu.be/jzV57jRqrcI. I can adjust for better picture, but it changes the cathode and g1 voltage by 50-60V away from the schematic spec. Does it make logical sense that I would want to keep the controls set to produce the schematic voltages on the CRT because that will always be it's optimum operating point? As in, this should be the best picture, and there's something else causing it to be whited out like this that still needs to be found. FK Last edited by FrankieKat; 09-13-2017 at 09:40 AM. |
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