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Originally Posted by Username1
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Hi So if that scope pic you posted is from the Base of Q402 then it loos like to me, and
my Squirrel companion, that there is a good bit of overshoot on the rising edge. I don't
know if'n yer schematic picture is of better clarity than what is posted, but I don't see
the same overshoot there..... But before we go there..... since we are looking at a
square wave thingie, does your scope have the test calibration post, where you
put your probe on it and then adjust the variable cap on the probe to adjust
it's LC balance so you don't have probe induced overshoot on your scope picture?
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Yes, the probe is fine. It tests fine on the calibration source on the scope.
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So if all that is sorted out, then you may have found your source of noise that
you are seeing on the screen..... See that decaying oscillation at the rising
edge is much higher in frequency than the horiz frequency, so that may
be making your "Jail Bars" looking stuff.....
I think, and the Squirrel Concurs, that you should poke around in that area and
see if there is a bad soder joint or something else, since you said you replaced
all that stuff over there.....
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I went down that route a couple years ago (I've been working on this for awhile) and all components in that area test good and all the solder is good. I've been in there and had everything in and out multiple times, undoing and redoing the solder joints. Most components have been replaced, including the main video IC, and absolutely nothing has changed. All the scope readings are the same as they were when I started.
The only parts in that whole section I haven't replaced are T401 and the two inductors L418 and L412. I have spare parts in a donor TV set that is identical to the one I'm working on which I intended to try out, but it's in storage and I probably won't get to it for awhile.
In any case, the transformers and inductors are not likely to go bad as they are very simple components. I also tried using freeze spray on all the components in the section while scoping waveforms and I saw absolutely no movement or change from anything. The inductors were tested with a good LCR meter and read correct values, which is the best I think I can expect from an inductor.
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Good work, on that stuff, and finding a possible cause....
You may be able to add a very small .pf value ceramic cap
to the base of that transistor to see if you can dampen
it out a bit, and if it does, dampen it on the scope,
then see if it does it on the TV screen as well....
PS - While you are in that area, you should
check all wave forms, and check voltages
too & Please post any differences in
detail. - Thanks & Good Evening.
May the Squirrel be with you!
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Well, thanks for your input, and the Squirrel as well.
I hope I can figure it out, because if what you are saying is correct, I don't know how to make any difference. I can try a capacitor, but there is already a ceramic cap C425, 0.0022uf in that spot, and C405, 680pf, upstream from it on output of the horizontal signal from the video IC. I have already removed them and tested them and they are spot on.