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?

Did you mean pin 5 on V1?

Gregb

Nope, pin 3 is where I've got squat. Should be 52 VAC.

Forget your "golden screwdriver treatment" for the moment, you have a circuit wired wrong.

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

Possible 1n34

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. :scratch2:
jr

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.

First thing I thought of, but that's not it. The 1N34 tested good. Replaced it anyway. No change. Tested every resistor in that little cage too.

Voltage on pin 4 of V1?

jr

Spot on. 48 VAC.

Sure looked like it could've been a mica. In any event, I replaced it with the unit from a parts chassis that was in that exact spot, figuring that the likelihood of both being bad was negligible.

If the tube is lit up normally then pin 3 has to be about 6 volts higher, perhaps the pin is corroded where you are probing?

jr

how about a picture of the area around V1... perhaps the collective can figure out what mods were made.

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.

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.

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:smoke:

If you have zero volts on filament pin 3 of V1 then it ain't lighting!

You should have 6.3 V AC between pins 3 and 4 of V1.

If V1 filament is not lit, then it will draw no B+ current and your voltage on pin 1 plate of V2a mixer will be too high, as you mentioned above, because both are after the same B+ load resistor.

James

The wife was a bed hog last night, so I woke up at about 4 AM and decided that I was up. Here's where I'm at.

I cleaned the snot out of the pins on V1 and now have correct voltage on pin 3. As to whether it was lighting before, I was fairly certain, but who knows....it was early this morning....so I just went ahead and cleaned the pins and the voltage was there.

The voltage on pin 1 of V2 is still unchanged.

Going to toss this set in the van and have Tom (Electronic M) take a peek, as long as we'll be seeing each other at ETF. I might bring the other tough dog 7" for my replica pre-war too....spread the fun around a little, and maybe I'll learn something in the process :)