Zenith color set, no HV. Any ideas?

[RetroHacker]

Recently I accquired a Zenith color television. It's a 19", looks to be from the early 70's, has a plastic simulated woodgrain cabinet, and that wonderful Zenith "spinner" UHF tuner. A hybrid tube/solid state set, this one has only four tubes, but it has that quality Zenith metal chassis. I'll post pictures and get the model number when I get a chance, but I figured I'd post a quick, general thing here since I have a moment this morning...

OK, so the set looks good, and when I power it on I get sound instantly (solid state sound). No HV or deflection whine though. Tuning appears to work, I can get the sound for various channels. Inspecting the inside doesn't tell me much, it's dusty, but these Zenith sets typically work fine, and hold up very well. The electrolytic can doesn't get hot or even warm. The filaments on the tubes all light. The horizontal output tube, however, looks odd. It's still got a silver getter, but there are black marks on the inside of the glass, looks to be streaked down between the two mica spacers inside. It almost looks like burnt something, and the pattern is unlike any I've seen on a tube before. Normally you get the brown marks on the glass, but this is different - it's hard to explain, I'll post a picture as soon as I get a chance though. The top cap on the tube is a bit loose, but it still appears to be making connection properly, and the getter looks fine, don't think the tube is gassy. It's a 20LF6, and I don't have another one (that I can find, anyway) to swap in. Normally I never suspect a tube just because it looks odd, but this one looks really odd. The flyback looks fine, no burning or cracking, but once again, that doesn't mean much. I just thought I'd post this and see if anyone knew of any common failures on these sets, and had some suggestions of where to start. I just don't have time to go to the library, track down the schematic, measure voltages, etc, etc.

While I don't know where I put my digital camera at the moment, it looks a _lot_ like this set I found in the gallery. Might even be the same model.... http://www.audiokarma.org/gallery/sh...&cat=all&page=

Just hoping someone could give me a hint as to where to start. I'm really hoping I have some actual time soon to work on stuff... Also more space would also be nice...

Thanks!
-Ian

[Chad Hauris]

1. remove the plate cap lead from the horizontal output tube. Connect an alligator clip lead to the lead and to a voltmeter (carefully). Don't put the cap back on the tube--turn on the set and check for DC voltage...if there is around 300-400 volts or so then the damper tube, flyback primary winding, and B+ power supply should be OK...if no voltage, troubleshoot in these areas till you get DC at the plate lead.

2. If this test passes, attach a neon bulb to a nonconductive stick and reconnect the plate lead to the HO tube. Carefully hold the neon bulb near the insulation of the HO plate lead (no direct connection) and the lead going from the flyback to the tripler. If it lights, there is HV AC being generated in the flyback, and there may be a problem in the tripler.

3. If bulb does not light...
Check grid voltage on horizontal output tube...should be -30 volts or more.
If not, there is either weak horizontal oscillator output or the tube is open.

I have had some sets with no HV and the problem was a bad HO tube that was destroyed because of no horizontal oscillator output. Check the voltages on the oscillator tube and if you have a scope, scope the grid input to the HO tube and make sure you have a signal. You may just need a new output tube, but do check the oscillator to make sure the new one doesn't burn up.

[OvenMaster]

I found out that loads of the Zenith color sets from that era had been recalled or had huge warranty claims because of bad HV sections... like the one in our house that started smoking and had no picture when I was a teenager. The one in that AK gallery pic looks realllll close to the one we had, too!

Tom

[jstout66]

The Zenith's that got recalled were the solid-state Chromacolor 2 chassis. On a 4 tube hybrid set the most likely problem is a bad horizontal output AND Osc tube. IIRC when the osc tube got weak, it would cause the output tube to fail. Those 20LF6 tubes are PRICEY. Check out these 20LF6's on eBay and see if the marks on your tube are like this. http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WDVW

[drh4683]

That set on the gallery has NO HV either. Its waiting on a new flyback. The fly is shorted and the damper tube plate glows red hot. Make sure thats not your situation.

Do you have a HV probe? Very important when servicing TV's.

As for the 20LF6, I know exactly what you are describing. The tube is just fine as long as it tests OK. Even NOS 20LF6's have that streaked brown appearance.

If your set has a 20LF6, then its a 72 model 19CC19. Of all the different 4 tube hybrid chassis (theres about 6 or 7 of them), this is the only chassis that used the 20LF6.

That being said, if you have NO voltage at the plate cap of the horiz. output tube, then check pin 5 to ground of the 20LF6. You must have -80 DC volts! If not, or very low, suspect oscillator circuit issues.
Ive enjoyed working on these sets. When you get it working and if the CRT is ok the picture will blow you away.

[peverett]

Do you have flyback part numbers? I have a few Zenith flybacks.

[mbates14]

"The picture will blow you away"

sad to say, not anymore with todays zeniths. they suck ass today.