HELP with Zenith 25DC56!
 

Got TWO Zeniths this morning for $15 a pop. The color one works, but the tube is tired. Going back this evening or tomorrow to fetch them.

Now you have TV twins, sweet! You're getting into the same collecting league as Doug. Made me think of his Quasar trio.

Ha, gotta love that sign in the foreground. Looks like that stack was an accident waiting to happen before you got there.

Nice! The top set looks like a 14N22 series monochrome...It and the RCA 630 are about tied for longest lived monochrome chassis...The Zenith is in a league of it's own if you count years of production essentially unmodified from the original design.

I dig that chair in the background, I'd buy it with the sets in your shoes if the price is sane.

That is so awesome to be compared to Doug - the King of Zenith. When I got there, I only saw the color set, but when I went to pay, the ladies told me that there was yet another old console TV. It was in the back room marked "NOT INCLUDED IN SALE." I manhandled it and put it on top of the color set, and one of the ladies put sold stickers on both. We took the old beat up F150 for a spin to go pick them up. I plan on watching The Big Bang Theory tonight on the 25DC56 - it has high voltage and the 22-5001's have been replaced. What could possibly go wrong?

Famous last words...:D

According to Murphy's law the motor run cap for the VRT, the lytic under the chroma module, one or more of the 3 video out transistors, the module contacts and or the tuner contacts will ruin your program now that you've said that.:smoke:

Take this, Electronic M!

Moar.

Let's not forget the socketed transistors on the 9-57 module. :thmbsp: I've had intermittent contacts that caused reduced horizontal sweep, partial and complete loss of horizontal sync, pincushioning and no HV. One module I subbed in had a bad transistor; no HV until I changed the transistor.

That being said, did it last through the show?

It sort of made it. The only problem I'm having (besides convergence and purity) is that there is an intermittent connection in the tuner. Every once in a while, it will get a clear picture, but after a while, it just fuzzes out with snow and crap.

Yup...These days whenever I open a set I've not opened before to work on it I try to clean the tuner while I'm in there....Especially if the tuner is a pain to get at.

These CCII are pretty darn reliable aside from dirty tuners and contacts, and if the safety caps are good they should last. The other issues I've personally dealt with or seen once or twice, but are not very common.

That happened with mine as well. The small opening in the back of the VHF tuner, on the top left, is a convenient place to spray contact cleaner where it will flow to where it's needed. Better that than pulling it and opening it I figured. You mentioned Radio Shack contact cleaner which is what I use. Well, it's no longer Radio Shack here, it's now The Source. Anyway, giving it a good spray and cycling the tuner a few times worked like a charm.

I see vertical linearity needs a touch-up as well.

I'm kind of starting to prefer this set over my other 25DC56 that I got it April. I like the warm tones it has better than the darker one. Just for comparison, here is the other set.

I think this TV is trying my pi$$ me off. Remember the problem I was having with the fuzziness? I found the source of the problem. It's the connection from IF wire at the tuner, where it plugs in like a composite RCA cable. I cleaned the connection countless times, and it will get a crystal clear picture, then if you look at it too hard, the fuzzing connection messes up again. I think it is the tuner, not the cable itself. The cable is soldered together. I tried bending the prongs around the tuner connector, but nothing worked.

I tried cleaning every connection I could find on the tuner to the chassis to try to get rid of the awful fuzziness. It's incredibly frustrating when you clean and clean and you turn the set on and have an excellent picture, and then after 15 seconds it goes fuzzy and awful again. I cleaned the inside of the tuner with spray and a rag, cleaned both IF wire connections, I'm at a loss. :grumpy:

Intermittent RF amp transistor in the tuner...

Used to be a popular issue here in the day.

That's what I was afraid of. This is why I prefer tube stuff any day of the week.

I've got spare Zenith tuners of that vintage, maybe the transistors from them would work in yours...If you have the service info and do enough online cross referencing you will probably find a sub for a few cents at your local Radio Shack or 100 for 50 cents at a online parts house.

Zeniths of this era occasionally have intermittent transistors. Usually most sets have one at most...Change it and they will be fine.

It could still be the IF cable - the shield wire will open and cause problems. A simple sup of the cable might be easier than tearing into a tuner.

IF Cables were on our "save" list when junking a set - they were a savior many times. Maggies and Zeniths seemed to eat them - most likely the flexing from pulling the chassis or dropping the tuner to clean it.

Those RF amp transistors inside of the tuner's were a problem in several different model sets. Usually they opened up or shorted when someone walked across a carpet or something similar and had a static change built up on them, and then touched the rabbit ear antenna or the antenna terminals or leads. There not that hard to replace, just have to take out the tuner, remove the shield. and a good eye for soldering. Most of them have 4 leads, a ground, and of course the E, B, C.

Might have been a cable or outdoor antenna set when it got hit by lightning (or nearby discharge). That will do it. My guess is the first RF amplifier (probably an FET) is fried.

Also check the 100 ohm (brown black brown) resistor Called out as R19 by Zenith, may be a different callout in the Sams) on the tuner. Zenith mentions it as causing snowy pix on all channels.

Keep in mind that the problem is intermittent. One minute the picture is perfect, and the next thing you know, it's snowy and distorted.

I've had intermitent video out put transistors in Zeniths of that era. Screen would change tint towards a primary. Damn thing bugged me for years cause it would not stay bad long enough to meter for a source...Eventually it did and I confirmed my suspicion by switching the three color video out transistors around...When the primary color of the intermittent changed I knew I'd found the culprit. Was mechanically/electrically sensitive too a smack would sometimes fix it, sometimes adjusting the tuner would fix it.

Indeed, almost any component can become intermittent, including solder joints as well as active components. Sometimes heating, cooling or mechanically stressing the suspect part will provide an answer, sometimes "blanket replacement" in the suspected area of the circuit is necessary. There is also a possibility that the problem is further downstream, like in the IF amp or AGC circuits. Can you substitute another tuner or do you have a pattern generator that can provide an IF output, and eliminate the tuner from the test?

jr

First off can you get the pix back by hitting, moving
tuner etc ?? OR once snowy do you have to shut off
& cool down to get it back ? Also is it clean snow like
an OTA fringe station ? Last time this came up here
it was just the external matching transformer !!

The set almost has to have a Zenith SGVG tuner that
is good cause its easy to service. BUT I would NOT change
the RF amp without proof. Too many things can go wrong.
Much more info & trouble shooting are needed first.

73 Zeno:smoke:
LFOD !

Danny replaced the transistor in this thing's tuner yesterday, and the picture is very much improved. However, it still does its thing where it distorts and gets snowy every so often. More often than not, it is a perfect picture though.

May want to try another channel. I have a table CCII that randomly flakes out on 13, but will go all day on 3.

IIRC there was also a 'lytic in the AGC that caused snow on these.
Not on a board but on the chassis.

73 Zeno
LFOD !

I think it's a mechanical problem this time. If I have it on channel 4, and it flakes out, all I have to do is wiggle the knob.

Have you tried giving the tuner a good shot of tuner cleaner? If your set has a turret strip tuner as do most Zeniths of that era, that may cure the problem. The channel strip itself could have dirty contacts, but a good cleaning will get the others as well. The only thing I'd watch is the price and/or availability of tuner cleaner these days. The last can of tuner cleaner I bought from Radio Shack in 2002 or so cost about $8; now, almost 15 years later, it could be a lot higher, if you can find it at all anymore.

BTW: How on earth do you find the room for all those TVs you have? :scratch2: I would think you would be just about out of room for consoles by now, especially since you have your '63 Admiral three-way roundie and now a second Zenith. The Admiral looks by far to be the largest of your consoles.

Good way to clean turret tuners is with a big eraser. Pop
cover off & turn channels, moving along as you go.
If one is still flaky bend up that strips contacts a VERY
little bit.
Can also tune an adjacent such as ch 3 on ch 2 or 4
most the time.

73 Zeno:smoke:
LFOD !

Ha, yeah, I once fine-tuned my own 25DC56 to pick up a local FM station. Lots of static, but not too bad. I had a CTC117 with an electronic tuner that was probably a little better at it though.

Yes, I should have mentioned that. When I got the set, I cleaned all the contacts inside the tuner very thoroughly with Radioshack cleaner and lubricant, and I wiped the contacts clean with a smooth rag. I take cleaning very seriously. It worked perfectly last night for two hours. Got up this morning, and I saw this mess...

Did you use the RS stuff that comes out as a sort of semi-frozen oil...IMO that stuff is nearly useless on it's own. Your better off with the eraser (white drafting works better than pink), or better yet combine a good cleaner with the eraser. There is a product called De-Ox-ID that you can get online for <$7 for a decent sized can...It is as good as Deoxit, but less than half the price.

Great news. I adjusted the AGC Delay pot like Danny said to do, and it hasn't had the problem since. That's not to say it won't crap out again soon though. Only problem now is that sometimes the horizontal won't lock. The horizontal hold is all the way to one side.

For hoz check the 330 ohm on the 9-57 board.
Use a 1W film. This is the same as on the 9-90
in upright chassis 25EC45 etc, very common.
For control pull out the white sleeve over the shaft
center control then center the sleeve. Almost all
Zeniths have this in one form or another.

73 Zeno:smoke:
LFOD !

Been watching the 25DC56 for about ten days now, and surprise, it's got a new problem. Intermittent red. It usually will work good after half an hour, but the before that, it comes and goes, and is very streaky. I'm thinking bad transistor, or at least a bad connection somewhere.

There should be a trio of color video output transistors....Swap the red and blue OR the red and and green....If the color you swap with becomes intermittent the transistor is bad....I had such a transistor failure in a 71 zenith hybrid, and diagnosed it that way.