'51 Zenith Porthole

[Charlie]

So far, the cracked gasket does not appear to be giving any problems. I've run the set several times. Last night, I put new caps/resistors in the power supply/audio output chassis and then ran the set at least 1.5 hours.

As far as the shock hazzard goes, I think I'm safe by simply having the front bezel grounded to the main chassis. However, if this gasket gets worse, I'm sure there's a good chance of HV leakage.

At this point, I'm kinda leaning towards using grey silicon, but I'll give it more thought once I get the rest of the caps put in the main chassis.... which will likely be done today.

[John Marinello]

Hey Charlie, Those insulators pop up now & then, so I wouldn't try to sub it with anything else just yet. I've seen many many porthole Zeniths, and only 2 have had brittle insulators. The sets have been in barns, attics, basements, you name it. So, I don't know what causes it. Maybe the supplier goofed and had a bad lot? I do have a cracked insulator that you can have, but wouldn't be worth shipping it. But, I could bust some pieces off of it and maybe you could cut some puzzle pieces to make it look better for now?

[Charlie]

John, perhaps I will hold off on trying to doctor my insulator up and just keep an eye open for one.

Right now, I'm trying to figure out a puzzle. I was changing some stuff around the vert osc and came across a tired looking 100k resistor off the plate of the 6SN7 that was reading 1.4 meg instead of 100k. Put in a fresh resistor and ended up with two pictures... top and bottom of the screen. I double checked the color bands on the old one I pulled out... brown-black-yellow... and then put the meter on it again and it was still over 1 meg. Looked at the Sams, and compared to a factory Zenith schematic... both diagrams show 100k... and that's what was in there... just way out of spec.

I went back and tacked in a 1.2 meg resistor, and the picture went back to normal. While I obviously had a 100k resistor that went way up in value, changing it got the oscillator out of whack.

I'll have to go thru the rest of the vert circuits and see if there's something else that's causing this issue.

[Charlie]

First, keep in mind the set worked pretty good as is when I first got it.

After changing a 100k resistor that went up in value to about 1.4 meg on pin 5 of the vert osc, I got a double picture. I then tacked a 1.2 meg resistor in series with the 100k and the picture went back to normal.

Checking schematics, they do indeed call for a 100k resistor on pin 5. I then looked for other issues around the oscillator, but all seems well.

The vert hold control checks good within specs.

I checked voltages. If I leave the 1.2 meg resistor in there (that's not supposed to be there), voltages are off. Pin 5 should read about 65 volts... I get 12.5 volts. Pin 3 & 6 (cathodes) should read 2 volts... I get .4 volts. Even though these numbers are wrong, the picture is fine.

When I remove the 1.2 meg resistor and only use the 100k as I should, all voltages become exact to what the paper work says... but yet I get the double picture.

So... question is... should I leave the wrong resistor in there since it produces a perfect picture? If no, then what else should I look for?

While I realize schematics can contain errors, remember that this started because I replaced a 100k resistor that was reading 1.4 megs. In addition, you'd have to be legally blind to mix-up brown-black-green and brown-black-yellow.

Thoughts?

[tvdude1]

Sometimes the more you change ,the worse they get.

[Charlie]

I feel like I've been running around in circles chasing my tail. I can't really find anything else at this time causing the vert osc to lock in a 30 cps when I put the right resistor in there. So, instead of trying to make the circuit "right", I'll just make it "work" by leaving a 1 meg resistor in there.

If I wouldn't have noticed the original resistor being so far out of spec, I would have never known something was fishy.

The Zenith service notes had lots to say about the verticle and sync section of this chassis. It seems they had quite a few issues. The verticle integrator is one of those couplet units. I thought it was simply a disc capacitor at first... then noticed it had three legs instead of two. There's no diagram to build your own.

[bandersen]

I know how you feel. I'm having the same headaches with the Sentinel I'm restoring. It was working OK, but I found resistors that were way off from the schematic. I replaced them figuring things would get better only to find they're worse now

Hang in there - I figure the hobby wouldn't be much fun if everything worked perfectly the first time around

[Phil Nelson]

More than once I have put an original component back in, when a replacement made things worse.

I don't know about this particular case, but there are certainly instances where a design change never made it into documentation.

When the component is clearly original, my theory is that the TV must have worked when it left the factory. So if the chassis and the schematic don't agree, my first vote goes for the chassis.

Phil Nelson

[Charlie]

It does not help matters much when you start to realize how many differences there are between schematics. As I was going thru my work, it looked as if I mixed up two lytics... put a 40 mfd where an 80 goes, and the 80 in the 40 place. Looking at Sams, it said I did it wrong, and looking at Zenith, it says I got it right.

There were also many voltages between the two schematics that did not match. I lean towards the Zenith schematic as being the right one, but then, they could make a mistake just as easily I suppose.

Currently, I'm looking for the source of ringing in the video. It's not severe, but it's there enough to notice it.