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#1
01-20-2008, 10:15 AM
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'47 magnavox - gutless high voltage (help!)....
Ok, so I'm going over my '47 maggie, and it's working sort of, but the high voltage to the CRT is absolutely gutless. We're talking maybe 5kv and no balls to back it up.
I've so far scoped the horizontal, and I'm getting a 60 or so peak to peak drive waveform to the 6BG6 grid. Which doesn't sound even close to what I need. And the horizontal's not eager to start. Anyone got an idea on what I should be seeing on a 6BG6's grid? I'd imagine it'd be a heck of a lot more, and that my problem is that I'm missing out on the drive somewhere along the line. The HV cap, might be popped, too, but that shouldn't adversely affect things should it? (The tube does have an external coating that's grounded...) Anyway, here's the schematic: http://farm3.static.flickr.com/2249/...115dedb7_o.jpg 
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#2
01-24-2008, 12:31 AM
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60 volts p-p on the 6BG6 grid is probably correct. The grid should be -7 to -10 volts dc measured from the cathode. I would take the 500 pf doorknob out and see if your HV comes up to 9KV or more. I thought the one in my old Philco was OK because it measured not shorted with a digital ohm meter, but it was shorting out when the voltage built up to -- 5KV! Also check the 0.05 from the screen to the cathode. It could be partially shorting and knocking the screen voltage way down. If the H osc. is slow to start try another 6SN7.
Best Luck Cliff
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#3
01-24-2008, 10:30 AM
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More information would be helpful.
Is there any light on the screen at all? If so, is the picture full width? Is the picture trapazoid or a rectangle? Is there any high voltage or damper cathode voltage at all if you remove the damper tube? Have you replaced tubes including the 6BG6, damper tube and 1B3? What is the 6BG6 screen voltage? What is the 6BG6 control grid voltage? Does the AC arc on the top of the 1B3 increase with the tube out? Some answers will help us be more especific. Good luck,
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#4
01-24-2008, 11:20 AM
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Quote:
I did find two out of tolerance resistors the other day that helped the slow start issue, and slightly boosted the P-P up (maybe 2 or 3 volts) Is there any light on the screen at all? Yes. If so, is the picture full width? Yes/no. If the brightness is a bit low, it'll go full wide. Is the picture trapazoid or a rectangle? Honestly, I haven't looked. The CRT's aimed at the wall now. I can spin the chasis around to look. Is there any high voltage or damper cathode voltage at all if you remove the damper tube? Didn't even think to check that  Have you replaced tubes including the 6BG6, damper tube and 1B3? Changed all but the 6BG6, which I'm going to order a few next week (I need 2 or 3 anyway). What is the 6BG6 screen voltage? I'll measure and report this. What is the 6BG6 control grid voltage? Ditto. Does the AC arc on the top of the 1B3 increase with the tube out? AC arc? I'm not getting any arc or hiss at all I'm pretty sure the 500pf HV doorknob is bad, since the set holds NO HV charge at all, so that's gonna be replaced, too. Vishay still makes 'em, Mouser sells 'em, but a bit pricey ($50-75 for one, depending on voltage) 60 volts p-p sounds a bit low, but I've never measured on a working set. I was always under the impression that the drive pulse beat the snot out of the output tube... What's odd is the flyback seems to ohm out right. I don't want to point my finger there until I rulle out the easy stuff, though
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#5
01-25-2008, 10:46 AM
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1. The 6BG6 is a real possibility.
2. Test the "doorknob" by removing the CRT socket, run the TV for a couple of minutes, shut off and see if it retains a charge at the anode. You should test at the 1B3 bottom since disconnecting the anode will eliminate the capacitor formed by the CRT aquadag coating. 3. Check the resistors under the 1B3 if/when you replace the doorknob. There are usually two. One is a low value (2-5 ohms)filament dropping and the other is a high value (470K-2.2 meg) filter. They don't need to be exact; +/- 20-30% is OK for now. 5. Remove the doorknob and see if you get any more high voltage. If everything else is OK, the TV will work without it. (There may be some other minor symptoms.) 6. Do you have a chassis or model number?
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#6
01-26-2008, 04:53 PM
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1. The 6BG6 is a real possibility.
Gonna be on order monday... 2. Test the "doorknob" by removing the CRT socket, run the TV for a couple of minutes, shut off and see if it retains a charge at the anode. You should test at the 1B3 bottom since disconnecting the anode will eliminate the capacitor formed by the CRT aquadag coating. Oh, it's, well, dead as a doorknob. Holds no charge at all. Not anything like the Bendix I have where I swear the thing's haunted - ground it and it pops back from the dead a few minutes later. 3. Check the resistors under the 1B3 if/when you replace the doorknob. There are usually two. One is a low value (2-5 ohms)filament dropping and the other is a high value (470K-2.2 meg) filter. They don't need to be exact; +/- 20-30% is OK for now. Hmmm, That's another place I'll have to check... 5. Remove the doorknob and see if you get any more high voltage. If everything else is OK, the TV will work without it. (There may be some other minor symptoms.) The HV's pretty much the same, maybe an extra kv, if that much. 6. Do you have a chassis or model number? Ahh yes, it's a Magnavox chassis CT238.
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#7
02-07-2008, 12:37 PM
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Ok, I put in a new doorknob, but that didn't do it. i also replaced a few bad resistors, no effect.
The P-P waveform is still 60ish volts. The raster is square, as brightness goes up, it blooms, loses focus, then goes away totally. There's no picture at max brightness. Brightness settings affect the drive waveform slightly. Also, the waveform's a pure sawtooth, no little squared portion. I'm suspecting either the waveform's wrong, or the flyback's bad
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#8
02-07-2008, 02:12 PM
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Quote:
If this is the case, if a flyback went bad, wouldn't there be no HV at all? I'm assuming that if one of the windings breaks, the circuit is broken and the flyback wouldn't produce anything... Also, wouldn't a flyback arc or something if it failed?
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#9
02-07-2008, 02:36 PM
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In the case of blooming, i changed out my 1B3 even though my tester said it was okay and my problem was solved...
The HV rectifier was good enough for the tester, but not the tv i guess..  Steve
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#10
02-07-2008, 02:36 PM
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How do the DC voltages on the horiz oscillator plate, grid and screen, and the horiz output tube screen voltage compare to the schematic? How are all of the voltages from the power supply section?
Have all the capacitors been replaced in the set? It sounds like a power supply/DC voltage issue. Could even perhaps be a boost voltage issue...the horiz output tube starts up on just B+ but then runs on boost after the set is fully warmed up. If I read the schematic correctly, the caps which develop the boost voltage are drawn in right under the flyback. Also what else connects to the damper fuse? There appears to be something else which apparently uses boost voltage and may be dragging it down if shorted or leaky. Last edited by Chad Hauris; 02-07-2008 at 02:42 PM.
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#11
02-07-2008, 03:49 PM
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I've checked the volts to the horz section AND the boost, both are good according to the chart, which says about 200 to the B+ and 300 from the boost. (the Boost feeds the vertical and the 1st anode).
I've replaced most of the papers, except a few minor ones that aren't in that area.
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#12
02-07-2008, 05:32 PM
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I'm just going to throw out some thoughts here on this matter.....FWIW....
As far as tubes are concerned never trust a tube checker of any kind for tubes in power and rectifier applications. A checker can only reliably tell you if a tube is bad. A flyback coil has a large number of windings between the HOT anode tap on the winding to the top end where the HT diode anode connects. Any of a number of things can be going on--several shorted out turns, etc, that can drag down on the efficiency of the generation of HT without necessarily greatly affecting horizontal sweep and such. Also never assume that a newly-bought tube or component for which you may have paid serious dough is actually of good merit, and not defective. I've witnessed techs wasting many hours because they refused to believe that a new part they installed could be the culprit. 
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#13
02-07-2008, 05:50 PM
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I'll agree about the tube tester. I've seen horizontal output tubes in color TV's check good on a tube tester but wouldn't do anything in the TV. In many cases, the best test for a tube is another tube. A friend of mine who fixed TV's during the tube days told me that all a tube tester was good for was selling tubes.
I've also run into new parts that were bad. Usually, these were cheap generic transistors, IC's, and flybacks that I ordered trying to save a dollar. Like they say, you get what you pay for.
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#14
02-07-2008, 08:06 PM
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Quote:
But sometimes you don't. Sorry, but I've had some bitter experiences, and know well that spending the extra dough doesn't guarantee anything. I think it was David Bogen that had affixed tags on the power cords of each of his manufactured gear that advised the new owner to read the owner's manual first before sticking that plug into the one-ten. It said "NEVER ASSUME ANYTHING!" Sage words that have portable meaning in my book.... You're on the same wavelength as I am. Sorry for the grumpy-old-man response. I've just seen much too much more of the last two decades for an old electronics enthusiast to have to see. 
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#15
02-13-2008, 09:35 PM
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*sigh*
Well, i changed to Horz discharge cap, two bad resistors, and the HV cap. it helped a bit, but not much - the HV still drops like a rock at full brightness, the raster still changes size, and does the bloom and byebye thing as the brightness goes up (past a respectable 'peak' in brightness, but not enough width). I'm aiming the finger at the flyback or yoke. One thing I noticed - there's left side wave from too much drive, AND, the raster 'wiggles' a left and right - maybe 1/8th inch. The wiggle speed's a function of the vert section. Yoke? The horizontal waveform shrinks ever so slightly at full brightness. At full darkness  I get about 60 volts p-p drive....I'm gonna try swapping 6BG6s as a last resort - the one in there should be good, though (it worked in the Dumont...)
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