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

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

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.... :yikes:

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.... :dunno:

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

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.

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

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

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.

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

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!

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.

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

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

Damnation - that's puts a jumbotron to shame!

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

$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!

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!

About 20 years ago, I had a ham radio transceiver which used 6JS6 final amp tubes; my first amateur transmitter had a 6146 final, and the Kenwood rig that preceded my current transceiver used two 6146s in parallel. I replaced the 6146 in my first transmitter in the early '80s and paid $17 for the replacement. I'm glad as anything my present setup is all solid state, no tubes, as I don't think I could afford to replace the finals in those old rigs anymore.

Fifty-six dollars does seem steep for one 6JS6 (and AES' price of $76.10 for one 20LF6 seems high as heck as well), but vacuum tubes are all but obsolete these days so those tubes you can still get your hands on will cost you quite a bit; supply and demand, of course. The demand for vacuum tubes is nowhere nearly as great today as it was, say, 30-35 years ago.

The day may come when new (or NOS, new old stock) radio and TV tubes may cost a king's ransom. However, you can still get good used tubes at bargain prices from online sources such as John Kendall's Vintage Electronics (www.vintage-electronics.com) in Fallston, Maryland, outside Baltimore; I purchased a 12AU6 from him recently, to replace a broken one in one of my vintage radios. But beware: The tubes themselves may be inexpensive (the 12AU6 I purchased was only $2.25 or so), but the shipping charges ... sheesh. This tube was sent to me via UPS, in a huge box; the shipping charges were something on the order of $8, so a tube that should have cost $2.25 wound up costing me just over $10. I think John Kendall could have used a smaller box (the box he sent the tube in was the size of a large cube), which would have knocked a bit off the shipping charges. I was surprised, to say the least, when UPS arrived at my door with this cube-shaped box that contained one small seven-pin tube--and nothing else. A padded mailing envelope would have served the purpose just as well, and saved even more on shipping charges to boot. Also, he could have sent the tube via USPS mail in such a mailer; the tube would have reached me sooner and there would have been no shipping charges.

I've dealt with two other online tube sources in the past and have had excellent results.

http://www.hamtubes.com and http://www.vacuumtubes.com

No connection with them other than a satisfied customer.

Both of these guys provide excellent service and quick shipping. In one case, a tube ordered one afternoon was on my doorstep by the next. Looks like the prices on 6JS6 are 1/2 to 1/3 of the AES price.

-Jim

AES is very expensive . I was unhappy with my tube purchases i made from them in the past . I got a RCA box with a Quasar tube in it and i think it might have been used . Lucky for me, over the past several years i managed to pick up a lot of new old stock tubes fairly reasonable from retiried tv guys . Usually at radio shows i tend to find tv tubes cheap becuase everyone is looking for radio and amp tubes .

Judging from the way UPS (united parcel smashers) treats their freight, I'd say packaging the tube in the manner described above would be the only way to ensure that it arrived in one piece with vacuum intact.

Sticking the tube in a padded envelope is a recipe for disaster as you can bet with reasonable certainty they'd place (ok, drop) a 40lb box on top of the envelope.

I make sure I buy quantity enough to displace the shipping costs, since the larger box can hold many more tubes safely.

There are many horror stories about UPS on the forums and 'net.

Yeah, a padded envelope wouldn't adequately protect even a 7 pin tube if it were run through any kind of automated sorting equipment. You'd receive a padded mailer full of glass dust and tiny metal bits.

At my job, I frequently order printer parts from a supplier, and have been suprised when something like a couple of small rollers arrives in a medium sized box. Sometimes when shipping(especially a large distributor), it's easier to simply use no smaller than an X sized box - no need to stock little mailing supplies, and far less likely that the parcel becomes lost in shipment.

Also, at my job, we ship using UPS only. We have the UPS WordShip software, and daily pickups. It's not worth the time and effort to seek out a small priority mail mailer and send something out that way. Also, UPS WorldShip goes only by weight, and one pound is pretty much the lowest limit (I think). So, a tiny VHS tape sized box and a 12" cube box could cost the same to ship, and the larger box would not be as easily folded, spindled or mutilated.

-Ian

I only buy tubes through ebay now. The 6JS6C is still pretty expensive, but you can get a better deal than AES. Just type in the tube number and you see about 20 different NOS tubes. Just pick the brand you want. I dont like that most tube sellers on the internet wont let you pick the brand name. For tubes that are rarely requested (like TV tubes) those guys should care less and give you what you want.

I was wondering, If anyone could make a list of the TV tubes to be on the look out for. I go to several Radio shows/Ham fests a year, and have seen many TV tubes for sale. Unfortunately I don't know which ones are desirable and hard to get. maybe this could be a new topic and make it stick to the top of the TV forum.

That is a good idea,the expensive ones are mostly output tubes {Horizontal Vertical ect}. Yep, we should make a list.

Yeah, a padded envelope wouldn't adequately protect even a 7 pin tube if it were run through any kind of automated sorting equipment. You'd receive a padded mailer full of glass dust and tiny metal bits.
-Ian

I didn't consider the automated systems the post offices use these days, but you're right. Anything containing or constructed of glass, be it a vacuum tube or anything else, will be smashed going through those automated sorters. However, I have medications delivered to me from a pharmacy through the mail and they almost always arrive in my mailbox unscathed. The only time anything ever went awry with this system for me so far was once when the pharmacy goofed on my ZIP code. Other than that, it's been very reliable. Of course, the pills are in plastic bottles, not glass, so they can withstand going through the post office sorting gear without so much as a scratch.


There is one advantage to using an oversized box to ship a very small item: I can and do reuse the boxes for storage, once I throw out the bubble wrap and packing peanuts.

UPS can be very rough on packages, as has been mentioned here (for example, last night I related an incident with a rather large radio that was sent to me via UPS from Arizona and arrived at my apartment with two loose potentiometers under the chassis). United Parcel Smashers, though? I remember AK member Nolan Woodbury telling the story in the Antique Radio forum some time ago of how a radio he had won in an ebay auction arrived at his home irreparably smashed, so these things do happen. However, if smashed parcels were a real problem (read a higher percentage of ruined/mutilated packages than those safely delivered), I'm sure UPS would be out of business by now.

That's one thing I always wondered about - why is it that some tubes are so *&^# expensive? Especially the color TV tubes. I mean, there aren't that many of us that actually fix these old sets - why are the tubes so costly? I was looking for a spare 6BK4 for my CTC-15, and I was suprised at the cost - nearly $40 at AES! And I better hope the horizontal output never burns out, because I'm not going to be able to afford to replace it.

Is there some secret society of people (besides us, that is) that collect old color TV's? Or are there that many of them out there still in use? Or do television tubes have some other application in industrial situations? Someone once told me that certain tubes are used in ham radio sets - but to be that popular?

And some tubes, really common ones, seem to be pretty expensive for what they are. Like the 6SN7. It's a simple dual triode, used as vertical oscillators and the like. The prices on that one seem to be all over the place. The GTB is signifigantly cheaper than the GT version, and the only difference is the heater warmup time! And then there are BRAND NEW ones - as in currently made, brand new, not sitting on some dusty shelf for years - and those are even cheaper!

Gah! It boggles the mind!

-Ian

---

Would it be possible to modify the circuit for a currently produced horizontal deflection tube like the EL509/6KG6?

Most TV tubes ARE cheap but there are those few that are used in something else besides just old TV sets. I remember in the 80s when I first started getting the AES catalog, they were cheap. Back then my only other ways to get a tube were via Radio Shack (which had decent prices but didn't stock anything, so you had to wait a week and make a second trip back to the store) or visit your friendly local TV shop. I recall buying a horiz output tube for a very early Zenith rect. color set, probably in '90, and it cost me $36 at a local shop. And that didn't even fix the problem.

Some of the tubes that were "junk" just a short time ago have been "discovered" recently. A guy I swap with sells audio stuff on ebay and has recently been looking for a few common types that all of a sudden have a demand. I can see the day when this could be a problem, not so much with all tubes, but with horiz. outputs. I would like to know the same thing fsjonsey asks.

Most TV tubes ARE cheap but there are those few that are used in something else besides just old TV sets. I remember in the 80s when I first started getting the AES catalog, they were cheap. Back then my only other ways to get a tube were via Radio Shack (which had decent prices but didn't stock anything, so you had to wait a week and make a second trip back to the store) or visit your friendly local TV shop. I recall buying a horiz output tube for a very early Zenith rect. color set, probably in '90, and it cost me $36 at a local shop. And that didn't even fix the problem.

Some of the tubes that were "junk" just a short time ago have been "discovered" recently. A guy I swap with sells audio stuff on ebay and has recently been looking for a few common types that all of a sudden have a demand. I can see the day when this could be a problem, not so much with all tubes, but with horiz. outputs. I would like to know the same thing fsjonsey asks.
People have been converting the old Futterman OTL amplifiers, which originally used 6lf6's to the EL509/6KG6. I was also wondering if the same thing could be done with TV sets.

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!


I'm glad that I have been saving tubes for over 40 years! I think I have about 35,000 at this point. Over the years I have been buying out tube collections at hamfests, etc. I knew that someday my ship would come in!