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.

check the service switch

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.

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 :)

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 :sigh:

Dammit.

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:smoke:

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. :no:) 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. :no:

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.

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!

And if you had a test jig, you could quickly eliminate the yoke as a problem.

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....

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

Oh, I've had that happen before. Nightmare is being kind :eek:

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. :thmbsp:

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. :D

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

Tim, that TV had probably been sitting since the 70s, I got it from the estate of a deceased repairman-his nephew told me the man never owned a color tv (?) I sorta think he never did anything with it. I think the main thing wrong with it when I got it was a bad power switch. I seem to recall that I eventually replaced the filter caps because I noticed them running warm. I don't recall any vertical issues on that one.

Zenith color 25MC30
 

I have this Zenith TV now and I'm trying to get it to work. I got a nos yoke for it but I don't know what the nylon strings are suppose to do. I also can't find the vertical height adjustment pot on the chassis.

The nylon strings are centering controls.

IIRC, the vertical height is on the front auxiliary control panel. It should be identified on the label. You have to remove the knob to adjust it.

It could likely be the vertical output transformer. My set went through two of them. The convergence winding goes bad and it can take out the two resistors that provide B+ to the vertical stages. It is not a difficult part to replace but may be a little hard to find. If you already have a new yoke, I hope that fixes it. These sets really perform well when correctly set up.

I'll take another look. Thanks for the help.

I do have a changer in a mid 60's rca home entertainment center, I'll look and see what model it it.

Well, this set is now back in my possession from a trade deal :)

So here's where I'm at.....

Now that it's got a new yoke (installed by Preston), it's got a picture, but it's rolled up at the bottom. Picture shows it with the vertical size absolutely maxed, and it still won't go to the bottom of the screen.

Now, before I go any further, he gave me the old yoke back in the box, along with this paperwork which was included in it. I'm thinking that this yoke has to be modified somehow to work in this set, but I'm not wholly sure anything was, or how to do it if it does need to be done.

Take a gander, I've scanned the paperwork in. Suggestions are appreciated :)

I recall Kennedy and Cohen being in the Delaware Valley (media speak for Philly-Jersey-Delaware). I also remember the cheapest set was a 1975 Sanyo 12" BW for $59.66 when all the others were $80+

Tim, from what I can see in the instructions, you need to look at the original yoke (if you have it) and/or the schematic for the Zenith set, and remove (or add?) parts to make the new yoke match what the old one has. The schematic of the new one is in the one page you posted. Other may have a better way to describe this. I have not worked on any Zenith color sets, though.

I doubt its the yoke, I would take a look at the PP drive signal, and/or voltages in the vert circuit. This is another place the TV analyst comes into play, a good source for a Known vert drive.

Yep, just got a text from Preston. He made the appropriate changes to the yoke before installing. Now, to get a Sams....

if you don't have the scope or tv analyst, the 1st place to go is a sub of the vert out tube and if used a vertical osc, IIRC Zeniths used two tubes for vert sweep (unlike the multivibrator dual triodes used in RCA).

I would test by subbing tubes.

after that you would want to check the voltage from the height pot. Assuming it uses boost voltage in the osc circuit, there is often a high meg resistor in series with a height pot. check to see if the pot and or resistor has drifted high.

The other likely suspect would be any eletrolytic that is a cathode bypass of the vert out. If that cap opens it kills the gain and output of the tube.

If you have a scope the 1st thing to look at would be the PP at the grid of the vert out, this would quickly narrow down to either the drive signal (vert osc tube) or the output (vert out tube). from there you can focus more closely on where to look.

Tim,
I checked my Zenith service manual and it confirms DaveWM on a resistor that feeds 870 volt boost to the V height pot.

Also noted that the .1 mf/600 V cap from boost to one leg of the horiz eff coil will cause the HV to be intermittent.
It notes this should be replaced with a 1000 volt cap.

BITD, I worked on maybe one or two of these sets, compared to RCA, Zenith roundies were not seen much.
Good luck

For the problem pictured, I would go for the vertical integrator. Looks like a three legged brown ceramic disc but it has three wires coming out of it.

The plate load resistor of the V.osc is a very common failure. It drifts high due to constant hammering by high level pulse.
Common problem not just in Zeniths but all designs using B Boost to feed V.osc.
B&W as well as color sets.

may have been mentioned but look for any caps in the boost filter supply to the vert osc plate load cap. think its a .1 leakage here could drop the supply voltage. IIRC there are two one right off the boost the other is upstream a bit, closer to the vert circuit. Again voltage measurement would prob point this out. I use a vtvm for checking.

And drift high it did. Supposed to be 473K, tested at 567K. Picture is much improved, but still has foldover on the bottom. After about 5 minutes of folling around with the adjustments, the picture went somewhat out of focus, and I stopped there.

One interesting thing to note....every time the screen changed scenes drastically (dark to light), I'd lose vertical lock.

After letting it cool down for about 15 minutes, the focus came back. Here's where we're at.....

Have you checked to make sure that the vertical output transformer is working properly? I had a foldover issue with my Zenith 24NC31Z.

I did the resistor check and solder job upside down with the chassis still in the set :)

Think from here I'm going to pull the chassis and start checking all the resistors in the vertical section and ohm out the VOT. I had some time to spare last night, but I didn't want to go yanking the chassis until I get my 730TV2 buttoned up.

Re. the plate load resistor of the vert. oscillator, Kamakiri wrote,
Tolerance-wise, that's really not a big drift. It will often increase by a factor of 10 or more, playing havoc with the height and lin.

If you havnt change the integrators. Transformers went but IIRC
you got a very small pix & a resistor burned. 'Lytic in cathode
of vert out also a high fail on any TV, B&W or color. After all
the tests start with caps in feedback loop.

73 Zeno:smoke:

The Vertical integrators here I remember having to replace on my grandmother's Zenith of this chassis. One of them twice. Vertical output transformer shorted out too. That and the caps in the vert section and it ran 9 years, the CRT kicked the bucket. It became my bedroom set. A donor CRT from another Zenith roundie saved it's life. Lasted me until 1990 when I moved. Didn't have room for it.

I was thinking about that too, as I've replaced integrators before. But only with integrators from parts from other sets.

This site shows how they map out, but not wattage of resistor or working voltages....

http://www.boxcarcabin.com/integrator.html