My Zenith has failed!!!

[mattdavala]

I'm sad this morning. While watching cartoons on our 66' Zenith 24NC31/z roundie the sound and high voltage completly died!!!! It was quiet, no sparking or pops. It has been our daily watcher in the bedroom for years(4 years). Since owning it I've gone through two HOT tubes and damper tube, but its been flawless in performance. Its never had anything other than a tube failure before, nothing castrophic.

It seems like a possible power supply issue, BUT the vertical section is working. I can hear it. The speaker seems to have a quiet hum, which indicates that there is power there. I have recapped the power supply and some caps in the vertical section. The rest is origional caps.

I've swaped the horizontal output, the damper, the 6U10 horizontal oscillator, even the video output tube. Nothing!!!

Two of the spare horizontal output tubes' plates glowed red. I am guessing its because with no horizontal oscillator the bias must be off. The tube I had been using in the set didn't glow red. I don't think it failed though, its only been running for a couple of months.

Two unrelated issues, sound and high voltage. This doesn't sound like a common issue, especially with Zeniths! What do I check? Anything before pull the chassis? I doubt replacing one tube will restore HV and my audio. I am not looking foward to pulling the chassis out, and if I do, how the heck do I check voltages?

I thank you for my help to get my beloved roudine working again
Matt Davala

[MRX37]

I suspect one or more of the original caps decided to go out on you...

[Old1625]

Shooting from the hip I'm going to guess you have a B+ supply issue, likely one of the larger-wattage resistors in the filter network opened up. Also if the 'lytics are original then you need to seriously consider replacing them--especially if they are showing any kind of spittle around the terminals--or are running hot to the touch....

If your horizontal oscillator isn't working then your horizontal output tube will climb up in current and kill itself. The bias it needs to keep the cathode current in check is developed by the driving signal from the preceding stage.

I would recommend you try and locate the Sams folder on the set before doing a lot in the way of troubleshooting; Zenith used point-to-point circuitry, and--while it is superior IMHO to PC construction in tube sets--it also is the very dickens to read, as component density tends to run high.

I had an early '60s Zenith roundie which I enjoyed for years. One quirk it did have however was that it ate the focus rectifier every 18 months like clockwork. Go figure....

[mattdavala]

Bingo on the power resistors.

Funny thing, I replaced these two resistors a couple of months ago, because I accidently touched one and it crumbled.

They are R190 and R191 in the power supply. R191 I had to replace with two resistors in series, its possible one burnt out. I wasn't too sure of the wattage, but it overheated. At R190 I get around 370V. R191, I tested the voltage in the middle of the two resistors and got 0 volts.

Soooo I have lost my 240 volt B+. Which powers my audio and horizontal oscillator sections. The reason my vertical worked was becuase the 340 volt source was there.

Thank you for your help! . . . . But now I need to pull that damn chassis. The resitors are on top, but on the far corner behind the HV box. Will let you know of my progress.

_
Matt Davala

[drh4683]

Glad you were able to find the problem. Sounds like you need to check that HOT cathode current. two horizontal outputs is alot for such a short period of time. See how your current is, it should be somewhere between 180-220mA. I think zenith puts a link in that chassis so you can simply disconnect it and place your meter in series with it.

[Robert Grant]

I'm not familiar with your Zenith's audio curcuitry, and don't have a schematic, but, I know some old tube sets can lose their SOUND as a result of sweep circuit failure.

This seems counter-intuitive, but many sets used a quadrature detector tube for detection of the audio signal. A Quadrature tube (also known as a gated beam discriminator) required a low B+ voltage with virtually no reactance. Getting the B+ from the usual B+ supply is out of the question (the source of the usual B+ is a huge inductor (transfomer secondary) and a huge capacitor (AC ripple filter). So, the plate of a gated beam discriminator as attached to a big resistor to the quite high voltage of the "B+ boost" in the sweep circuit.

No sweep circuit, no B+ boost, no B+ boost, no B+ on the detector, and the only sound you hear is the noise in the audio amplifier.

Of course, if your Zenith uses a ratio detector of Foster-Sealey detector, all of this is no help to you :-)

good luck with your Zenith, Rob

[mattdavala]

Hi again,
I've got to wait till Monday, tommorow, to get my resistor. Parts place is closed on the weekend.

I think they use NTE resistors and they don't have a 15 watt resistor. I need a 450 ohm, 15 watt resistor. They have 10 watt and 25 watt.

Is it better to go with the higher wattage? I want to use a single resistor, becuse I used two previously, adding up to 15 watts. One burnt out giving me the failure described in this post.

Thanks!
Matt Davala

[jpdylon]

stick with the 25 watt. Never go down in heat dissipation.

Glad to hear you found the difficulty. Sounds like your well on your way to getting the set up and running again. I definitely agree to check the HO current. There is no way you should be going through tubes every 2 years.

[mattdavala]

Thanks for your help guys. Its nice to have people to talk to about these old TV's. I tell all my co-workers what I collect and its always " waaaa what?"

My Sams doesn't give a procedure for checking HO current. I'm surprised too, because it tells you how to adjust the high voltage to adjust current through the regulator.

Its chassis 24NC31/Z. Where do I hook up my meter? Once I know where to hook it I know I need to adjust the efficinency coil. I'm usually good at reading schematics, but where the heck is it connected to in the circuit? Its not listed as efficiency coil in the parts list and I don't see Horizontal Linerarity. I assume its off the damper tube?


Thanks again,
Matt Davala

[jpdylon]

A common way to check HO current to disconnect the cathode lead on the HO tube and put your ammeter in series with the cathode of the HO tube and the lead you just disconnected.

On some zenith chassis the efficiency coil is located in front of the HV cage. You'll need to pull the chassis to get at it safely. This is where having a test jig would absolutely rock!

[Jeffhs]

If your set's horizontal output tube plate is glowing red, the tube isn't getting a grid drive signal from the oscillator, as was mentioned in a previous post. However, no grid bias on the HO tube can also ruin the power transformer due to the heavy current drain (if the high-voltage fuse or the TV's own line fuse do not blow for any reason). With no grid bias, the HO tube's plate current will rise until either the tube destroys itself or the transformer burns out. Series-string TVs cannot be damaged this way by horizontal-drive failure due to the nature of the filament circuit; if the horizontal oscillator tube burns out, it will open the string and save the output tube. Note as well that if the horizontal oscillator is far off frequency the power transformer as well as the HO tube can be destroyed for the same reasons (no drive on the grid of the output tube, causing the tube to draw more and more current until the fuses blow or the power transformer/horizontal output tube fail). It's much cheaper and easier to replace a defective output tube (after finding and correcting the problem that is depriving the HO tube of its drive signal, of course) than it is to replace a burned-out power transformer.

[mattdavala]

Wooo Hoooo!


Well got my resistor today from the parts place. A single resistor this time, 470 ohm at 25 watts. I lucked out that I didn't have to pull the whole chassis out, I just had to angle it to get behind the HV cage.

Replaced it and I've got my sound and HV.

BUT!!!!!!!! When R191 failed it took out my HOT!!! The lack of Bias on the tube cooked it in a minute! The 6JS6C! Thats a 56 dollar tube from AES.

If you read the first post I said that I went through two 6JS6c. Well I replaced one and was using the second. The first I replaced because it gave "horizontal ringing" in the picture. It wasn't worn. It gave me faint vertical bars in the middle of the screen, just an annoyance. Since the replacement tube just gave up the ghost I've got that origional 6JS6 back in the set. It works fine except for the "ringing" in the picture than I guess its ok. These tubes are too darn expensive to swap out for a minor issue. I'll leave it until it really does wear out.

Thank you everyone for all your advise. Always good to know I can come here for help.

Thank you Thank you Thank you!!
Matthew Davala

[drknstrmyknight]

Originally Posted by mattdavala

I'm sad this morning. While watching cartoons on our 66' Zenith 24NC31/z roundie

Damnation - that's puts a jumbotron to shame!

[eberts]

$56 for a 6JS6C ?
I used to buy those brand new for $2 !

[radiotvnut]

Originally Posted by eberts

$56 for a 6JS6C ?
I used to buy those brand new for $2 !

Yes, and you can blame ham radio and CB operators for that. Those HO tubes are used in linear amplifiers. Remember the 20LF6 that was used in some of the Zenith 4-tube hybrid color sets of the early '70's? Antique Electronic Supply wants $76.10 for that animal!