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#16
11-09-2021, 05:39 PM
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#17
11-09-2021, 05:46 PM
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The other clue was the resistance reading. 1.1 Meg normal at V6 pin 9 with (680k +22k)
(R505 +R506) shorted across it = 428k. You read 436k, close enough.
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#18
11-10-2021, 04:02 AM
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Well, unfortunately the fix was short-lived. The pic became quite weak again and the agc needs to be biased again to produce an acceptable picture and sound. This time pin 9 is in range of the specified voltage, around 1/2 a volt, but V6, pin 3 is now double what it should be, around 74v. So, normal voltage on the plate of V6B but too high voltage on the agc keyer grid. Too high of voltages coming of the plates and screens of the IF and video stages too. A feedback loop seems to be removing gain from the video stages without proper bias, as something is now loading the screen of V6B and making the keyer think the video signal is too high.
Although voltages on AGC pot measure where I think they should the control has absolutely no effect. I have the board and chassis out of the set and am going over it with a fine tooth comb. There must be some kind of intermittent condition. Really scratching my head on this one. Last edited by Jon1967us; 11-10-2021 at 09:47 PM.
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#19
11-10-2021, 06:29 PM
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Open ground in that section?
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#20
11-10-2021, 10:36 PM
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I don't see any grounding issues in the area of the AGC. Both R196, 193 test ok to ground. AGC control still does nothing, however.
Sync and limiter diodes check out. Vid det diode checks. Although I've tested V6, and it tests good for emissions and shorts, would like to swap out with another, as there may be leakage. I just don't have another 11BT11 handy at the moment.
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#21
11-20-2021, 02:42 AM
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Pulled the board - touched up a couple of iffy solder joints. Trimmed up the excessively long factory thru-hole leads - at least a couple appeared shorting/making contact. Because a bracket broke off which engaged fine tuning, I needed to open the tuner to actually better tune my RF, via the little gear and a mini screwdriver, which wasn't far off, but this helped a great deal (obviously). Tightened several loosey goosey tube socket pins, and my AGC returned. Looked like the AGC loop might have been disrupted somewhere between the 3rd IF and the video amp.
 https://www.youtube.com/watch?v=2WY6...l=Silvertone58
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Linear Mode
