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#76
10-20-2010, 06:56 PM
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Good eye, Bruce! I had Dick Van Dyke playing on the DVD player. I have this set I bought at Wal-Mart with 65 hours of old tv shows. I think this episode was about an italian guy painting their living room.
Yeah, I'm not sure about the contrast. I think I've cancelled out the chance of any bad tubes, unless 1B3 tubes get weak and cause poor contrast somehow? I don't think that would happen, but it was mentioned in one of those repair books I've got. I have a picture of my contrast/video section. I had a hard time matching some of these weird components to the schematic:  Wow, my labeling didn't work out so good. Anyways, there's some capacitors listed that I couldn't find, as I recall. Those little cylinder/disk things around the sound take-off coil looked like tiny resistors or something, and I'm not sure about that green thing that's marked with paint dots like an old resistor. There's a similar one that's red like this and where I'm looking on the schematic, it mentions a capacitor. I wasn't sure if these were were caps, or resistors. The peaking coil, that someone had suggested might be open, appears to be that "blob." Looks like a resistor with a coil around it, coated in wax. Am I right? It said 10 ohm on the schematic and this measured 8 ohm on my multimeter. I've been meaning to apply that video signal to see if the problem is before or after the video amp, but I don't seen to have any spare .1 capacitors to protect my DVD player. Last edited by RitchieMars; 10-20-2010 at 07:03 PM. 
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#77
10-20-2010, 07:20 PM
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ID peaking coil Correct!
Green cap with the yellow/purple/black dots = 47 uuf shown across sound takeoff xfmr ... 22k resistor just below the green cap. Little disk circled is likely the 3.3 uuf cap on the sound takeoff xfmr. jr
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#78
10-20-2010, 07:29 PM
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Wow, so those are capacitors... weird. Are these considered ceramic or wax, or what? I've never seen much information about these. Reliable? I know they're small values, but they are right there around the contrast control, namely that C32. It's hard to believe that such small caps could cause my contrast to work the way it is, but who knows. Also, my set doesn't have a screen register control on the front, so I'm guessing that R25 would be a fixed resistor in this case rather than a pot. I'm not sure if the screen register has much bearing on contrast, though.
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#80
10-20-2010, 09:48 PM
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I'm having a little trouble reading the schematic because of the resolution and I don't remember everything you've tried.
Have you checked the contrast control? It has four terminals instead of three so it's not as straightforward to check as a normal pot. For it to affect the brightness the way it does it must be shifting the DC level at the cathode of the CRT. One side of the pot goes towards the plate of the video output tube which is at about 98V and the other towards the screen of the same tube which is at about 155V, if I've read it correctly. It looks like the cathode of the CRT is at about ~130V ? So, the DC level will shift some when you adjust it, but it should be over a limited range. around 130V. If the 4uF electrolytic tied to the screen, pin 8 of the video output tube, were leaky it could cause that level to be off which could cause a larger shift in the DC level at the cathode when you adjust the contrast. Probably not much help, but just a thought. John
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#81
10-20-2010, 10:49 PM
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Uh-oh...
 I was just running my set for about 10 minutes and while I was adjusting the bulls-eye tuning a bit, I started to notice this little "hiss" coming from inside my set. I hadn't ever heard that before. Awhile awhile, it started to get louder. I turn my brightest up and the sound get's a bit quieter, but if I turn it lower the sound get's louder. I'm almost certain it's coming from the high voltage cage. Smells like... static electricity. As much as I hate to ask, can a failing flyback cause contrast problems? Last edited by RitchieMars; 10-20-2010 at 11:44 PM.
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#82
10-20-2010, 11:00 PM
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Quote:
__________________
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#83
10-21-2010, 07:22 AM
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Well, that sound coming from the HV cage could be causing your problems. Maybe not a failing flyback, but rotting wires, etc could be discharging some of the HV before it gets to the picture tube. My CTC-9 had lousy HV until I listened in and heard a hiss like yours. I switched the lights off, and I could see the tube cap was glowing purple, like with a heavy static discharge. Looking closely at that cap, I saw it was cracked, and bleeding HV.
Of course, most of your HV stuff is UNDER the chassis, so you'll probably have to pull the whole mess out, and run the tv on its side on your workbench to get a visual on that hissing sound's source....
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#84
10-21-2010, 12:26 PM
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Yes, powering it up in total darkness may let you locate the Evil Spot quickly. That has worked more than once for me.
I would not run it for long periods in this condition. High voltage is escaping to a place where it's not supposed to be -- 'nuf said. Phil Nelson
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#85
10-21-2010, 06:24 PM
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Alright, well as it gets dark tonight I'll be able to turn out the lights and monitor the chassis on it's side. The idea of a HV leak sounds pretty scary, but needless to say I won't be touching anything. I can tell you that a piece of wax had fallen off the underside of my flyback awhile back. Most of the wires were in okay shape from what I could see before, but I'll just have to see.
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#86
10-21-2010, 07:27 PM
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Found it!
Well, that was creepy. But here's a photo I illustrated to show where it is. I could barely see it, even with the lights out:
 This is the bottom side of the flyback where that wax fell off and it seems to be running towards that bracket with the green wire you see there. That wire goes directly to the HV rectifier.  The white wire comes from the HV rectifier and goes to the anode of the CRT. The blue wire on the side of the flyback there goes to the top cap of the horizontal output tube. Anyways, I couldn't tell if the voltage was leaking into the actual wire or just into the bracket.
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#87
10-22-2010, 11:58 AM
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Corona dope? No, I am not calling names
 ...General cement made/or makes a thick gooey black paint (in a small bottle with an applicator brush) that is painted over the problem areas. This may still be the "product of choice" for these problems... don't know... people might be using RTV or something else these days. jr Update: Looks like the stuff is still available... even listed on Amazon (with awful shipping charges): http://www.amazon.com/GC-Electronics.../dp/B000B62YO8 Likely it can be found cheaper elsewhere. Last edited by jr_tech; 10-22-2010 at 12:07 PM. Reason: Add link to Corona Dope at 'zon
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#88
10-22-2010, 12:43 PM
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I believe Corona Dope is classified as a rather nasty substance so the shipping tends to be high.
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#89
10-22-2010, 12:52 PM
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I have used GC corona dope with great results, but on smaller areas than this flyback.
People talk about recoating entire flybacks with RTV. If you search the forums, those discussions should turn up. I assume the arc is jumping to the bracket because that's the closest path to chassis ground. You want to seal up the fly where the spark originates, of course (i.e., not paint over the bracket). Again, I would not power up the set at all, or only for a very brief period, before restoring the insulation. I would inspect that flyback very carefully before covering it up. Let's hope the arcing has not damaged the flyback. Phil
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#90
10-22-2010, 03:07 PM
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Alrighty, well I think I can probably get some RTV around here somewhere. My only concern would be how to get to the flyback so I can work on it. It has a few wires connected to it that aren't easily removed from the chassis, such as the little plastic cap on top of the horizontal output tube. Should I actually desolder these from the flyback? I guess I'll see how much I can reach by just taking the screws out first.
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