On the bench: Zenith color roundie 25MC30

[Kamakiri]

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

[dieseljeep]

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.

[DavGoodlin]

Originally Posted by dieseljeep

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.

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+

[bgadow]

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.

[Electronic M]

The nylon strings are centering controls.

[Kamakiri]

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

[DaveWM]

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.

[DavGoodlin]

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

[Zenith26kc20]

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.

[old_coot88]

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.

[DaveWM]

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.

[Kamakiri]

Originally Posted by old_coot88

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.

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.

[Kamakiri]

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

[old_coot88]

Re. the plate load resistor of the vert. oscillator, Kamakiri wrote,

...And drift high it did. Supposed to be 473K, tested at 567K.

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.

[zeno]

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