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  #1  
Old 09-24-2008, 08:06 PM
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26KC20 Tuner drifting issue.

Hi all, I've been watching the black metal '63 Zenith since I finished the restoration. The last two days the tuner has been drifting all over. I checked all the RF/IF tubes and the 6EA8 in the tuner was shorted, the 6GK5 was burned up with no emissions by that point. I replaced them and the problem is still there but not as bad. But now it has drifted out altogether to a black screen, came back with herringbone lines and now can be tuned in clearly but has no color. If you tune in color it will be very fuzzy. Tuners are not at all my specialty, does anyone know where to go from here. I have pics if that helps. Thanks.
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Old 09-24-2008, 09:21 PM
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Well, it is somewhat better. I replaced the 6KT8 Color killer/Amp. and can tune in very weak color. But it still drifts around for a while until the set is at full temperature. and it is still out of tune, I think the tuner may be out of alignment, if it is tuned to a perfect picture, the color goes away. If it is out of tune, the color comes back and only gets strong when it is WAY out if tune and starting to turn to horiz. sync bars. I've successfully aligned a tuner on a B/W Zenith but have never attempted it on a color set.
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National Panasonic SA-5800
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Pioneer SX-939, ER-420, SM-B201
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Last edited by zenithfan1; 09-24-2008 at 09:54 PM. Reason: additional comment.
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Old 09-24-2008, 10:22 PM
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Well, I fiddled with the tuner, color killer and AGC and got it working. I fig'erd 'er out m'self! It looks great now. The real test will be tomorrow when I start it up cold.
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Pioneer SX-939, ER-420, SM-B201
Motorola SK77W-2Z tube console
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Old 09-24-2008, 11:24 PM
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glad you got if fixed up. If you ever need tuner repaired, there is a guy in Longview texax, "Quality Tuner Service", his name is Louis Stangle and hes and older guy at 67 and worked for sarkies tarzan and oak industries back in the 60s. He knows the old tuners really well. Hes fixed up some for me and always did a nice job. He's has a website too, just google in his name or QTS. Hes cheap too, $25 for a rebuild. I think hes the only guy left in the world who rebuilds/aligns tuners.

By the way, I find lots of intermittent signal problems due to poor connections at the IF tube sockets. For some reason those wafer sockets are not the greatest. Time after time I'll pull an IF, put it back in and the set has blank or intermittent raster or other problem. Regardless of the condition of the chassis, it seems to be common. I give those sockets a little shot of control cleaner and it takes care of the problem. Probably unrelated to what you were dealing with, but thought Id share anyway. Then again, Im sure most of you guys know about it already.
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Old 09-25-2008, 09:01 AM
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Thanks for the tip on those sockets, I've had a few intermittent IF stages on Zenith sets before and now if they do it again I'll spay 'em. This one just took patience, once I replaced those bad tubes, it made it so I had to readjust everything mentioned. It was frustrating me so I started this thread, then fixed it myself, sometimes I just need to take a break and come back later. The picture is better now than it ever was before. It now makes gold and yellow properly too. Before, those two colors could not be properly obtained and there was some "noise" in the picture that would not go away. I will do the cold start test today after work, I hope it don't go all crazy again. If it does, I'll spray those sockets. I also know what to do with bad tuners now, thanks.
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Pioneer SX-939, ER-420, SM-B201
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Last edited by zenithfan1; 09-25-2008 at 11:41 AM. Reason: additional comment
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Old 09-25-2008, 04:54 PM
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AHHHHHHHHH! This thing is making me nuts! It's back to the same ol same ol. It takes it a while to clear up but it does EVENTUALLY clear up. It starts up in B/W then has color, not right but color. Then slowly turns somewhat normal. I'm starting to think wandering resistors somewhere. Maybe in the IF and Chroma circuits?
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Kyocera R-661, Yamaha RX-V2200
National Panasonic SA-5800
Sansui 1000a, 1000, SAX-200, 5050, 9090DB, 881, SR-636, SC-3000, AT-20
Pioneer SX-939, ER-420, SM-B201
Motorola SK77W-2Z tube console
McIntosh MC2205, C26
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  #7  
Old 09-25-2008, 05:39 PM
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Do you have a tuner subber or a B+K analyst? Let the set cool down for a few hours then start it back up with the subber in place, or the B+K analyst set to IF output and hook it up to the IF input cable to the chassis. See if the set still acts quirky. If it does, the tuner is cleared and you have some kind of IF or AGC fault within the chassis.

One of the toughest yet easiest to fix problems for me was with Zenith 20CC50 chassis. Id turn on the set and it would work, then go blank intermittently. I looked around and checked the circuits and monitored voltages until it failed again. Couldnt pin it down. I adjusted the AGC and the other controls that I "knew" about. Then I looked at the front apron drawing on the schematic. There I see an "AGC Delay" control. Forgot about that one. With the flip down door down, the lable does not indicate that there is an AGC delay, they list all the other non-customer adjustables but not that, but look through the hole and there it was! So I adjust it and the picture starts flipping in and out like crazy. All the running around and confusion isolated to an darn intermittent dirty pot.
Intermittents are the true test of your ability and they can drive you nuts. I always hated having a really tough problem to fix and thinking about it at work. While you're at work you think what to check and then the light goes off and you know where the problem should be. Excited to get home to check, you find out you were wrong!!!!!!!!! Thats the worst!!! Then not being able to go to sleep because its not fixed yet and the gears are still turning. I used to be up till 3AM on a friday night until the job is done. I get so focused and into it that breaking my concentration is the worst thing to do. I am much better now. Ive learned to take a break now and its amazing how much better you feel when you come back to it. Like in my case, you find out its a god dang intermittent pot and you feel like a total idiot! Or in another case, an intermittent molex connector contact was causing weak color on my every day 12A12C52 zenith color set. That one had me running in circles too. So sometimes the easiest stuff causes the most rediculous problems. Hopefully you'll find its some control or tube or socket and make it easy. As much of a pain they can be, I love working on them.

Also in your case, check that 6JU8 color AFC tube. Those are troublemakers too, almost always responsible for color problems in zeniths.
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Old 09-25-2008, 06:36 PM
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I do have a B&K 1077 B but, I just found it a couple weeks ago and haven't checked it other than to see if it powers up. I also don't have the book, what do I check to make sure it's aligned and working properly? It seems to be in good shape and has the probe too but someone hacked it, it has tape wrapped around it and both wires are red and look the same, that don't seem right to me. I don't have a tuner subber either. Man, this is really starting to suck.
The set has once again warmed up and the picture looks OK but is not right. It was B/W and smeared on startup, tuning drifted back and forth and once again, I had to turn the color killer to the right a little. I think is in the IF or AGC somewhere, but would that make the color off? It drifts around and has herringbone lines and those slowly dissapear. I'm still getting the feeling it's resistors in these areas. I kind of hope it's the tuner.
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My TV page and YouTube channel
Kyocera R-661, Yamaha RX-V2200
National Panasonic SA-5800
Sansui 1000a, 1000, SAX-200, 5050, 9090DB, 881, SR-636, SC-3000, AT-20
Pioneer SX-939, ER-420, SM-B201
Motorola SK77W-2Z tube console
McIntosh MC2205, C26
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Old 09-25-2008, 07:40 PM
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only thing to say about the 1077B is that it will probably work, but the vertical linearity will be way off. No big deal as you are looking for a good signal. Turn the channel dial to IF, then connect the RF output into the IF input of the TV. Turn up the RF gain control and tune the IF dial to around 4.5 mhz. A picture should show up, adjust the RF gain as needed so you dont overload it. I assume you have some kind of immage or test pattern in the CRT slot inside the unit.

Problem isn't in the chroma circuits. Like when your fine tuning is off on a normal working set, you lose color because the color subcarrier is not tuned to the proper IF freq and you lose gain to the point where the color is gone. Really sounds like tuner trouble to me based on how you describe it.
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Old 09-26-2008, 11:19 PM
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Well, I fixed the damn thing. You were right about those sockets, I did find 2 intermittent ones after all. It was a 6EJ7 3rd IF and the 6BN6 Sound detector. That loose connection on the 6EJ7 was causing the drift, and to think I already checked it once and totally missed it. When I wiggled that tube the picture went haywire. The 6BN6 was causing the whistle and pop in the audio. It is still hard to properly obtain yellow, it likes to be a bit green. If you turn the tint to fix it, it messes up the flesh-tones and red areas. It makes them "pinkish". This is why I think there is some issue with Chroma, I'll flip 'er on it's side sometime this week and check the plate voltages on the 6JU8s and go from there. But other than that the picture is very nice and stable from cold to warm, greyscale looks great, bright white and true black. Checked CRT again to be sure it can be ruled out, excellent and equal emissions. Thanks for your advise Doug.
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Sansui 1000a, 1000, SAX-200, 5050, 9090DB, 881, SR-636, SC-3000, AT-20
Pioneer SX-939, ER-420, SM-B201
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Old 09-26-2008, 11:56 PM
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Cool. Glad that fixed it. So common for IF tubes to make intermittent contact in the color sets.

I know what your talking about. That greenish/lime looking yellow, yet the flesh tones look correct and you can go from green to purple on the tint range. I had this exact same problem with a 24MC32 roundie. I found out the injection transformer was out of alignment. Its a dual slug coil and went to adjust it with my meter hooked up like the manual points out. As I was adjusting it, I peaked the VTVM out in both directions and then the slug just siezed out of no where with practically no force. I had to litterally dig out and crumble the slug to remove it, but the ID of the tube was enlarged to the point where a new slug just slid around. Out of pure luck, I had a good one from a parts chassis that brought it back. I aligned it and it works fine now.

That transformer gets really hot and it bakes that tube that the coil is wound on. When you adjust it, the coil will be very squeaky and sound rough when turning it. I have the factory data for aligning it, its really easy. All you need to do is hook up your VTVM to a couple spots and peak out the meter., max neg and max pos for R-Y and B-Y injection.

In my case, I think it was most likely out of alignment due to the fool who previously owned the set. I bought it off ebay and the guy tells me he "aligned" it. The set was so far out of alignment that there was barely no sound and an annoying sync buzz. I got my favorite excuse on that when I powered it up in front of him "Oh, the antenna isnt good enough" or "it would probably work if its hooked up to cable".

Hope the rest goes well fixing it back up. Atleast you got the intermittent solved!
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