I gave the Zenith a very through cleaning using compressed air and brake-kleen (no residue, quick drying). Got rid of a lot of dust & dirt.

I also checked the HV section/cage and all of the power supply areas/tube sockets for any signs of arcing. Couldn't find anything wrong.

I powered it back up again, and it thanked me with the problem you see below. :uzi: Basically, the picture is very jittery at the bottom and only fills the top 2/3's of the screen.

I changed every tube in the vertical section (vert output tube is known good, borrowed from my roundie). Then I swapped everything in the Horz section. Finally I tried the Damper and Voltage Reg.

It strikes me that this is might not be a tube problem. Does anyone here have an idea?

Carmine:

A number of things could cause a picture like that but the next thing that I would try to do is adjust the vertical size and lin controls. These could go bad and cause that problem. I know that the 19 inch table model Zeniths of the early 70s were famous for bad vertical size controls. If I replaced one I replaced a thousand. I even remember the part number, 63-9115 (I think). Anyway give those controls a shot and see what happens.

Steve K

looks like its rolling.
the size pot is a good possibility.
changed lots of em too.
g2 pots too.
used to buy them by the hundred at vj mcgrannahan to get a price break.
bad linearity can be caused by a lytic in the output cathode too.

Really??? I hadn't even tried moving the pot since I figured it was so unlikely for the pot to go bad... Especially something as little-used as vertical size!

I'll run them back/forth tomorrow.

BTW, it's not actually rolling... I think that double image of the guy's head was a trick of the digital camera. I never see the band of video info at the top of the screen. However, the bottom of the picture is very unstable/jittery.

I just finished playing with the vert size and lin.

Vert size had litte to no effect, while the vert lin. expanded the picture a little. It does lock in for vert hold, but the adjustment is super critical.

Any thoughts? I would have expected a bad pot to make the screen occasionally "jump" to full size, but no luck. I did notice a SLIGHTLY smaller picture, with flicker at the bottom BEFORE any of the fuse blowing drama. I HOPE the cleaning solved any of this!

Also, I noticed a slight blue gas in the Vertical output tube when I tuned it on at night in the dark. I swapped it with another, and saw the gas again, in the same area! I also swapped this tube (6HE5) with a known-good tube earlier. All three tubes produced the same picture. Could something else cause the blue-gas problem???

Stumped:dunno:

carmine,

A circuit component like a cathode bypass cap on the vert output tube could be shorted causing no negative grid bias. This causes grid current to flow and can drive a grid incandescent which will make the tube gassy (blue glow inside between the elements, not on the inside of the glass) and also ruin it. Just an idea. You might be ruining every good tube you stick in that socket.

Rob

Ok, thanks Rob. I had a feeling this wasn't good. I did reinstall the known-good 6HE5 back into the roundie, and it's OK.

Now comes the part where I have to admit I'm not an electronics expert. (In case you didn't guess):D

How do I test the capacitor? I'm assuming it'll be located under the chassis, somewhere in the vertical section. I also have a schematic for this set (and most other Zeniths, thanks Ebay).

If it's bad, where do I get a new one???

The cathode is on both pins 4 and 11 on the 6HE5. You count pins clockwise from the wide indexing space between the pins looking into the bottom end of the tube. Look for an electrolytic cap from here to chassis ground. Positive end will go towards the cathode of the tube. It could be one of the sections in a combo can capacitor. I am at a complete loss as I don't have a schematic of your set. The circuit might not even be like I'm thinking. I'm only an amateur with extremely limited experience with tube color TV circuits, mostly RCA at that. My biggest claim to fame is that I got one of my vintage sets working well enough to make my avatar. :)

Let us know what kind of cap it is and then I can suggest where to get one.

Rob

Hey Carmine,

This isnt to bad of a problem to solve. Ive delt with it before. What youve got is a shorted electroytic thats for sure. Take a look at your schematic around the 6HE5. Youve got a 100uf @50V lytic from cathode to ground (C62 on schematic). If youve got a volt meter, take a voltage check at the cathode, pin 4. You should have 26volts right there if the cathode cap is ok. Also, while you're at it, take a voltage test of pin 10, you should have 280 volts. Comming to pin 10 are two electrolytics, C54 (10uf@475V) and C65 (30uf@475), this C65 is a combo cap inside a can, its the "squared" designation. Just follow the wires untill you find the lytics. But check your voltages first.
Heres how to test your capacitors. clip one of the cap leads out of the circuit, but be shure to clip near the middle incase you need to solder the wires back together should the cap be ok. Then, set your vom to RX1 test it with your meter, the needle should whip up and slowly fall back down, This indicates a proper working cap. Reverse the meter leads, the same thing should happen again. Should the meter fully deflect and stay that way, the capacitor is shorted and thats your problem. As for that C65, just test it by putting the probe on the capacitor lug and the other to chassis. If the meter fully deflects on rx1, its shorted. those caps are really easy to find on the chassis, they are the biggest capacitors and are held to the chassis by a metal band. Those caps are either black, and brown paper types, or they are orange and blue. Whatever the case may be, they will stick out at you when you open up the bottom cover. They will be in the middle of the chassis twards the row of user controls.

Hope this helps, It should get that set working again! As for finding new capacitors, go to "mouser.com" they have all the hard to find super high voltage electrolytics like those 475 volt things you've got to deal with. You'll probably spend about $5 a capacitor or less. I buy from them all the time. Its kinda tricky to learn there system but whatever cap you find is wrong, just let me know and I'll give you there part number for the right one as Ive got there catalog too.

By the way, my little photo caption is a 20Y1C48 copied out of a 1968 zenith brouchre!
Good luck-Let us know how it works out!, Doug

This should really help make things go easyer. What you see here is a 20Y1C48, exactly like yours. Ive circled the 4 potential troublemakers. Chances are high that "2" or "3" is your trouble maker. Ive never really had a problem with the paper lytics in zeniths. In "3" look for the "square" shape underneath the can, it should be the middle lug and a lead from "4" should be soldered onto it. If you're blowing fuses, "3" is a prime suspect as its in the B+ cicuit supplying 280 volts to the grid of the 6HE5.

Heres a zoom up of "3" Notice the "square" shape. Just put your meter probe from chassis to this point on RX1 and see what happens.

First of all, many thanks to Rob and drh4683. You obviously went through a great deal of troble to remove the bottom of your set, and photograph it! That helped a great deal. I really do enjoy this list.

First of all I've only got a digital VOM, so there will be no needles whipping to the end of a scale! There is no RX1 either. Can I still test with this meter? Do I test the capacitor voltages acroos the component (AC) or to ground, as in DC?

I didn't bother testing anything, because I might have found something visually. The cap circled in yellow is loaded with melted wax. This is bad, I assume. Also note the cap circled in green, which does not show up in your chassis...???

So where do I get this part, and which is the correct way to test the others? I've got a "real" electronics store just down the street, might I find it there???

Look at that!, that big green cap in there is a replacement to the one in the can that I was talking about. Make absolutly sure that there is NOTHING connected to the lug on the original "square" cap in the can, if there is, its still shorting out and thats your trouble. Change that waxy looking thing, thats junk. Wax coated electroylitics cause lots of problems. If youve got a real electronics store (something I dont:( Give them a try, they could have it. Keep in mind, barely anything today uses capacitors rated at 475 volts so dont be surprised if they havnt got anything like that in stock. As for your digital VOM, its kinda tricky for testing caps. Make sure your meter setting is at its lowest multiplier. Touch the two leads on the meter an watch for a jump in the digits then they will probably fall back down to "0" or close to it. Then, touch the leads to the capacitors while they are in the circuit. If you observe nothing, no changes in the meter, the cap is probably oK, however, should the digits start to move around, its shorted. As for voltages, set your meter to read out DC. Basically, an analog meter is alot nicer for cap testing kind of work. You might want to pick up a cheapy analog at the hardware store. They are generally those pocket sized ones and usually $10 or under. But anyway, the digital should get you through these tests.
Good luck!-Doug

Doug, thanks again for your help. I also sent you a private message last night, thinking it might pop-up, since you were in the forum. I guess it doesn't.

Anyways, I've had some time to work on the set. Had lots of time during the blackout, but you know why that didn't happen :(

I replaced the waxy-melted cap (you labeled #4) without doing any testing. No effect.

Today I tested some voltages. I had 18 volts at pin #4 (should be 26) and 316 volts at pin #10 (should be 280). Any thoughts?

Also on the previously replaced cap, what do you mean by NOTHING connected to the square cap? We are talking about those big vertical metal tubes on the top of the chassis right? Cause underneath, there are teminals marked with triangles, circles and SQUARES... you mean nothing to the square TERMINAL correct? Not the entire cap? Either way, there are things still connected... But I assume this thing worked properly at some point (actually when I first tried it out) so how can the wiring be wrong?

Finally, I'm still a bit confused about tesing caps with the digital meter. Do I need to clip them out of the circuit? Am I testing between the two leads, or from each lead to ground? Power is recently applied, but off I assume? This may sound embarassingly obvious, but you do mean RESISTANCE (OHMS) when you say RX1, right?

Carmine, All,

The idea of using an ohm meter to check capacitors for leakage is simple. The ohm meter has a battery inside that sends voltage out to the test leads through the meter circuit. When you place a dead short (touch the leads together) this is zero ohms and the maximum current can flow in the ohm meter which is callibrated to read zero ohms under this condition. The higher the external resistance placed across the leads the lower the current flow. The meter is calibrated to read higher ohms as this circuit current goes down. There is a low DC voltage across the leads of the ohm meter which comes from the battery inside the meter case. When you place a capacitor across the ohm meter, at first the cap will read low ohms because it looks like a short circuit because it needs current to charge itself. As the internal voltage of the capacitor under test grows (cap is getting charged up by the meter) the ohms reading will increase because the cap is looking like an increasingly larger value resistor while the charging current drops. Eventually the cap, if not leaky, will, charge to the full potential supplied by the ohm meter and the meter will go to infinity ohms or OL if a digital ohm meter. For the most accurate testing of small coupling capacitors in circuits they should be tested with one lead disconnected from the circuit and the highest ohm range available on the meter, 2 meg ohm, 10 meg ohm or 20 meg ohm should be used. For electrolytics it is best to also disconnect one side, usually the positive connection in the case of a chassis mounted can type cap where the negative terminal is almost always the negative terminal. When testing electrolytics the lower ohms range may be needed because the lower ohms range can supply more charging current to the capacitor and large electrolytics will take more current to charge, and even a good electrolytic cap has some leakage meaning it will never read infinity on the ohm meter. With an electrolytic you should have the ohm meter red test lead connected to the positive terminal of the capacitor. You can shorten the test time on the big electrolytics by starting at low ohms range to help charge the cap quickly and then keep switching to higher and higher resistance ranges as the ohms reading continues to climb. If you can reach over range or infinity reading on the meg ohm scale of your meter with an electrrolytic that shows either a very good low leakage electrolytic (if it took time to get to thet reading) or if it reads high right away it is dried out and 'open' inside and should be thrown away.

When working on testing capacitors in a chassis it is important to unplug the chassis from the mains and also to discharge any of the capacitors in the circuit with a shorting wire placed across their terminals for several seconds before unsoldering them or attaching the ohm meter. Touching an ohm meter to a charged capacitor can blow out the ohm meter.

Rob

did you try testing and/or replacing "1" and "2"? Go ahead and replace those and see what happens. Should you still have trouble, youve got some other problem. I'll check out my schematic!
LOL!!! Today my 20Y1C48 lost the focus! Ironically Ive had more trouble with my zeniths than the rcas!!!!!!!! Darn focus diode! The rcas colors are holding up really well for me. Cant say that for zenith. Im having too many hv and focus troubles and Im getting irritated!

Another quick test is to measure the capacitance, if your digital meter will do so. Very low capacitance compared to what it's supposed to be indicates a bad cap. I found an electrolytic cap in a Kodak projector that was supposed to be 1500 mfd. and it only measured 30! (it was the source of the problem). A good cap will often be slightly above the marked value.