Quote:
Originally Posted by Electronic M
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.
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Attached the second page of the schematic. Makes sense that the issue could be in the detector since the DC voltages on the IF stages are spot on. I assume I'm going to need to dig up some germanium diode or equivalent, but worth trying.
Quote:
Originally Posted by Electronic M
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.
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Yeah, checked that -- the flyback winding is 5 ohms, right on the schematic value. Glad it's not that at least! Since I am seeing those 15,750KHz pulses on the plate of V8A, would that imply the AGC is at least partly working?
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:
Originally Posted by Electronic M
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.
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I've tried at least six different 6AN8 tubes that have all tested way-off-the-charts-good and all produce the same results, so not a tube. Adding the bias certainly affects the contrast but not really better. It's more like if you turn the AGC too far where it whites out the picture.
Quote:
Originally Posted by Electronic M
I'm running on ~3 hours of sleep presently so take this advice with a grain of salt...
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No problem at all! I really appreciate all of your help and thoughts!
I'll try changing out that diode and see what that does. I think I have some low forward voltage drop diodes... somewhere...
FK