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#1
04-26-2016, 08:36 AM
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Signal issue on a Motorola 7VT2?
Been working on this Motorola 7VT2 for a while, and I've hit an impasse....
http://www.earlytelevision.org/pdf/M...-Sams-83-6.pdf I've got full raster, but no signal and no audio. I've got nothing on pin 3 of V1 (6AG5), and ~180V on pin 1 of V2 (12AT7). The overload on V2 is in the neighborhood of the voltage that I should have on V1 that's not there. The only thing that connects the two pins directly is a small mica, which I robbed out of a parts chassis to no effect. I've done the obvious resistor and cap checks in the vicinity. My question is this. This set appears at one time to have had the "golden screwdriver" treatment. Everything's been fiddled with. Do the coils in that vicinity have anything to do with the voltages on those pins?
__________________
"Restoring a tube TV is like going to war. A color one is like a land war in Asia." 
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#4
04-26-2016, 10:15 AM
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Pin 3 is a filament. If rest of the set is working, that means the other tube's string filaments are lighting. Unplug the set and ohm out the filaments in this area. Pin three should have a choke and not a cap! James
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#6
04-26-2016, 11:32 AM
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What voltage do you measure on pin 4 of V1.... does it light up? Do you observe any tubes in the set that do not light up?
What do you mean by "The overload on V2". I'm scratching my head on that one.  jr Last edited by jr_tech; 04-26-2016 at 11:48 AM. Reason: add another question
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#7
04-26-2016, 12:30 PM
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Everything's lighting, and I have a full raster. No audio, no picture. It appears that someone at some point had their hands in this section extensively, a few changed resistors, and the screws on the pots are obviously not in their original positions. As well, there's pencil labeling showing what each does.
I made a mistake when I said the "overload on V2". To clarify: On pin 3 of V1, I have nothing. Should be 52 VAC. On pin 1 of V2, I have 180 VDC. Should be 107 VDC.
__________________
"Restoring a tube TV is like going to war. A color one is like a land war in Asia."
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#8
04-26-2016, 12:31 PM
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__________________
"Restoring a tube TV is like going to war. A color one is like a land war in Asia."
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#11
04-26-2016, 12:47 PM
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Quote:
__________________
"Restoring a tube TV is like going to war. A color one is like a land war in Asia."
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#12
04-26-2016, 12:52 PM
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jr how about a picture of the area around V1... perhaps the collective can figure out what mods were made.
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#13
04-26-2016, 01:00 PM
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I hadn't thought of that. Hm.
Since one pin on one tube had no voltage, and an adjacent tube had one pin with high voltage.....and the two of them were connected by componentry, I automatically assumed that's where the problem was. I'll take a couple shots tonight if I have chance to get back into it.
__________________
"Restoring a tube TV is like going to war. A color one is like a land war in Asia."
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#14
04-26-2016, 01:35 PM
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If it is similar to a TS-4 chassis I'd check the chokes around the video detector diode. Mine cooked it's diode and one of the chokes.
__________________
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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#15
04-26-2016, 01:41 PM
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180 V on V2 plate means its not turned on so either
dead osc or mixer stage. Clean the crap out of the switches first. Then tubes. Look for bad socket or broken pin internally. Rocking the tube will show it sometimes. If V1 stage dead but everything after OK will display snow & a strong signal make it through anyways but will be snowy. GL Zeno 
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