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  #1  
Old 11-07-2013, 09:47 PM
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Kamakiri Kamakiri is offline
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On the bench: Zenith color roundie 25MC30

Well, here we go with another set......

When I got this Zenith, it was partially recapped, and working. After making it home to Buffalo from MD, I got about 45 seconds viewing time out of it, the vertical started to shrink, and I smelled (something). Shut it down immediately, and months later, here we are at today.

Did a full recap on the set and half a dozen new tubes, and powered it up for the first time just half an hour or so ago. Zero vertical sweep (just three bright lines of color very close together, you can see the horizontal shifting around with some kind of picture), but sound and reception was right on the money.

What was odd, is that there was a prominent ticking sound coming from somewhere in the chassis, about two ticks per second, steady beat. No arcing that I could see either topside or underneath (the set has an open bottom, there was a screen of some kind there which is long departed.

I'm inclined to think vertical output transformer at this stage, but wanted to see what others thought.
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Old 11-07-2013, 09:57 PM
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check the service switch
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Old 11-07-2013, 10:22 PM
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It's in the correct position, that was my first thought. I'd be further inclined to test the switch, were it not for the very noticeable "ticking" noise.
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Old 11-07-2013, 10:40 PM
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I've heard that ticking, but on later solid state sets. Not sure what to make of it. I'd shoot some cleaner in the service switch and work it a few times real fast, just to make sure that isn't the culprit. I recall others on here having trouble with vert transformers in Zeniths.

No telling what it might be, considering that hack that worked on it before you got it
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Old 11-08-2013, 06:00 AM
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Wasn't sure if you were the one working on the set, Bryan, or if it was someone before you. Curious though, what was the set doing which led you to replace specific capacitors, when you got it? How long did you have it running afterwards, and did you notice anything that it did?

Tried spraying the service switch, and powered the set back up. When I switched it over to the service side, the ticking (which I'll call arcing, because I realized that's what it sounded like) went away.

Switched the set back to the "service" position to get underneath to listen and isolate it further, and by the time I laid on the floor, I heard what sounded like the same static that you hear when the picture tube lights up on some sets, and I lost raster. Shut set off immediately. Smelled something burned

Dammit.
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Old 11-08-2013, 08:07 AM
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I'll bet it's the yoke.
I'll just bet it's the yoke.
Vert. shorted to horizintal. Probably took a resistor with it.
Bill Cahill
I've seen that happen, even on some cheap BW tv's.
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Old 11-08-2013, 08:26 AM
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Quote:
Originally Posted by Bill Cahill View Post
I'll bet it's the yoke.
I'll just bet it's the yoke.
Vert. shorted to horizintal. Probably took a resistor with it.
Bill Cahill
I've seen that happen, even on some cheap BW tv's.
I second the yoke.
The ticking is the vert stage running, easy to hear on newer sets.
Ofter if you pull the yoke you will see a rusty spot where it
burned. Use your free test inst., your nose. The nose knows.

73 Zeno
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Old 11-08-2013, 02:44 PM
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I had a 12" b&w portable TV, bought new in 1975, that worked well for three years, then one day I turned it on and saw a curl of smoke coming up through the ventilation slots in the cabinet. (No raster, of course. ) I had no clue at the time as to what was burning, but seeing your post reminded me that it could have been the yoke, one winding shorting to the other, and possibly a resistor (as Bill Cahill suggested). Typical for a small, cheaply made, tube-powered offshore-brand portable.

Talk about BPC! To the best of my knowledge and belief, no one used that term in the 1970s, but it certainly would have applied to a lot of the small, cheap no-name offshore-made tube-type portables available at the time. The sets may have worked halfway decently when they were new, but they would develop problems after a very short time, after which many of them were junked. Today's no-name flat screens are like that too.

I guess this just goes to show that there were cheap TVs on the market as recently as the 1970s, and quite possibly even earlier than that. I don't know exactly when these junky sets first appeared on the market, but my best guess is the late 1960s. I had a Sharp 12" portable in 1970 that worked well all of three years (!), then developed a tuner problem which I was able to work around, at first, by propping up the VHF tuner shaft with a screwdriver; however, the set developed more serious problems a year or so later, after which I junked it. Too bad, because that set had a great picture, even on a monopole VHF antenna in the near-fringe TV area in which I lived at the time. Further, since it was made by Sharp (this was years and decades before offshore companies started slapping well-known TV manufacturers' names on BPC sets), I expected it would last longer than it did.
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Last edited by Jeffhs; 11-08-2013 at 03:32 PM.
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Old 11-08-2013, 06:54 PM
6GH8cowboy 6GH8cowboy is offline
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The yoke is a good guess. If the rate of the buzz/ tick is adjustable with the vertical hold control the vert trans and ckt is likely good at least on the primary side. Back in the day zenith roundie yokes did not fare well in south florida (at least) usualy a horizontal winding issue. Good luck!
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Old 11-09-2013, 07:19 AM
Gunslinger Gunslinger is offline
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And if you had a test jig, you could quickly eliminate the yoke as a problem.
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Old 11-09-2013, 08:06 AM
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Changing a yoke is easy enough, I don't mind. I'm not looking forward to doing a degauss, purity, and convergence setup on the set, but it beats the heck out of where it is now....
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Old 11-09-2013, 10:18 AM
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zenith purity and convergence should be easy.they tend to set up almost perfect.watch out for the coils though.when they become unwound,it could be a nightmare.good luck and post pics
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  #13  
Old 11-09-2013, 10:34 AM
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Oh, I've had that happen before. Nightmare is being kind
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Old 11-09-2013, 10:53 AM
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Quote:
Originally Posted by 6GH8cowboy View Post
The yoke is a good guess. If the rate of the buzz/ tick is adjustable with the vertical hold control the vert trans and ckt is likely good at least on the primary side. Back in the day zenith roundie yokes did not fare well in south florida (at least) usualy a horizontal winding issue. Good luck!
I wonder if that is the yoke with the nylon fish line strings for the horizontal and vertical centering.
I used an RCA yoke for the last replacement. The centering was close enough.
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  #15  
Old 11-09-2013, 11:11 AM
dieseljeep dieseljeep is offline
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Inexpensive B/W sets of the 70's.

When Kennedy and Cohen came to town, now defunct, they used an inexpensive B/W set as a leader. It was branded "Kenco", made by Sampo. of Taiwan. They sold them for $38.00, a lot of them!
I got several of them as freebees, as the customer thought, that it was so cheap, it wasn't worth repairing. I don't remember what went wrong with them, but was a simple cheap repair. They worked well and IIRC, I sold them for $35.00.

They also sold a Kenco branded Sylvania-built hybrid 19" color table model for $177.00. Unheard of at the time.
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