Hello..my first TV

[Donj]

No, the photo shows part of the vert sweep circuitry, only a few of 5 ceramic bodied caps under the hood. I already put safety caps across the line.
Do you think I should be safe leaving the ceramic bodied caps there? They are not original, as the parts list specifies paper/wax where they are.

[JohnCT]

Originally Posted by Donj

No, the photo shows part of the vert sweep circuitry, only a few of 5 ceramic bodied caps under the hood. I already put safety caps across the line.
Do you think I should be safe leaving the ceramic bodied caps there? They are not original, as the parts list specifies paper/wax where they are.

Unless someone else has info that I don't know about, I'd leave the ceramic body caps in there assuming they're the same value (or really close) and equal to or higher than the original voltage ratings.

John

[Electronic M]

Originally Posted by Donj

No, the photo shows part of the vert sweep circuitry, only a few of 5 ceramic bodied caps under the hood. I already put safety caps across the line.
Do you think I should be safe leaving the ceramic bodied caps there? They are not original, as the parts list specifies paper/wax where they are.

Flat ceramic disc caps are usually best to leave unless tested defective, BUT tubular ceramic bodied caps (as the last picture on the last page shows) should always be changed.
Disc ceramics use ceramic dielectric and rarely fail. Tubular ceramic caps usually are paper dielectric and are VERY high failure. Paper dielectric capacitors are not just paper/cardboard tubes they were sold in plastic shells (I see some in your pictures too), ceramic shells and metal shells....No matter the shell material paper dielectric capacitors are BAD and should be changed.

The plastic shell caps and ceramic shell paper caps are known to explode when they overheat.

[Donj]

I can't express how much I appreciate the guidance you folks have been sending my way. Makes this task a learning experience..
So the recap continues along with checking resistor values. Have only found 1 resistor that hasn't measured almost right on as have all the others. Most have been within 3% of marked values. The one bad one was marked 10k and measured 251k.

Next question regards a selenium rectifier within the filament string. I have yet to test it and am just wondering if I should just replace it since I'm in there.
Would a couple of 1N4007s do the trick? Part sheet just describes "selenium rectifier" nothing further. What purpose does this serve?

[old_coot88]

V2, whatever tube that is, gets filtered positive DC on its heater, supplied by that circiut. The little selenuim is a full-wave rectifier. If replacing with Si diodes, you may need to bump the value of that 1 ohm resistor up a bit to avoid overvolting V2's heater.

[jr_tech]

Scratching my head here... why is V2 heater also connected to the upper leg of the AC heater supply?

What is V2 and what is its function?

jr

[Donj]

Coot,
V2 is the oscillator tube on the tuner... pretty sensitive area to be messing with if not necessary I expect. Maybe I best leave it be for now.

[Donj]

jr,
Tube is a 12at7. Split filament maybe?

[jr_tech]

Originally Posted by Donj

jr,
Tube is a 12at7. Split filament maybe?

Yes, so indeed, the heater for 1/2 of the tube runs on AC and the heater for the other 1/2 runs on DC.... I don’t believe I have seen that before. Curious!

jr

[old_coot88]

Yes, the whole 12AT7, 'AU7, 'AX7 family have split heaters with pin 9 as common. So they can be run in series on 12V, or in parallel on 6V. But V2 in your set evidently has 6V DC on one and 6V AC on the other.

(Edit) jr_tech nailed it first. I never seen that arrangement before either. Just a SWAG, but since V2a is the tuner's oscillator, the DC heater scheme is to minimize 25 cycle "FM-ing" of the oscillator.

[Donj]

Just a quick pic of the schematic showing the tube's full use.

[jr_tech]

Thanks! oc’s theory is likely correct.
Interesting!

jr

[JohnCT]

Originally Posted by Electronic M

Tubular ceramic caps usually are paper dielectric and are VERY high failure. Paper dielectric capacitors are not just paper/cardboard tubes they were sold in plastic shells (I see some in your pictures too), ceramic shells and metal shells....No matter the shell material paper dielectric capacitors are BAD and should be changed.

This made me wonder as I had never seen a ceramic tubular fail, but then, my experience has been limited from 70s TVs until present.

I took this cap from my old TV stock:



...and cracked it open. What I found was that this capacitor is a foil and plastic film, not paper. This cap would have been factory installed from the 60s to the 80s in a transistor TV, so perhaps paper ceramic caps were gone by then.



But what the heck, caps are cheap enough and cheap insurance. Change all tubulars seems to be good advice on tube TVs. I suspect any ceramics in later transistor TVs are fine.

John

[Electronic M]

They made those ceramics as far back as the 1950s. Most caps before 1960 were paper. I believe 58-64 were the transition years from paper to film. I have seen those ceramic tubulars bad in mid to late 60s TVs recently. I have also found some defective orange drops in 60s sets. And some 30s-50s sets are having substantial mica cap failure. Anything over 60 years old that isn't in a vacuum is potentially subject to degradation so suspecting everything is valid. I tend to consider ceramic tubulars as being much much worse than the other caps mentioned.

It is possible those tubular ceramics are good in 70s sets but they were the among the safety caps recalled in early 70s SS Zenith sets, and (8 years ago when I was buying parts local) I had a NOS 3KV tubular ceramic I was told was film that had a higher voltage rating than the 2K the circuit specified dead short after a few months... Because of all that I just change them all so I don't have to worry.

[JohnCT]

Originally Posted by Electronic M

It is possible those tubular ceramics are good in 70s sets but they were the among the safety caps recalled in early 70s SS Zenith sets..

Those Zenith caps weren't ceramics. They were wrapped plastic made by American Radionics IIRC. I think I may have one. I'll look for one tomorrow.

I changed a hundred or more of those for that recall campaign (along with some yokes, vert output modules, and CRTs) and the replacements were made (I believe) by Sprague. They looked like huge four lead Orange Drops.

But I did see some of those ceramics fail in GE SS TVs now that I think about it. Those were retrace capacitors on the horiz output, and if one opened, they would cause excessive HV and over voltage of the collector of the output, like the Zeniths did, but never blew a yoke or CRT. We learned early on that if a GE had a blown horiz output, just change the caps under the heatsink.


John