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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sat 06, 2018 1:25 pm 
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Thanks for the feedback. In my testing, I did not have the CRT connected to try to isolate that as part of/not part of the problem. All other connections were made appropriately.

So far, the two things that I'll check are the 12AX7 and the unintended burnt solder/flux conducting issue. I can totally see that this might be since I'm made so many checks and changes to the caps and resistors trying to identify the issue.

I'm in Germany for another week, but will get to it next weekend and report back!

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sat 06, 2018 6:48 pm 
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irob2345 wrote:
Sorry! Didn't read the whole story! To me, a 12AX7 is a valve, not a tube! A CRT is a tube!


I'm guessing you're not from the USA...This forum is USA based so to most folks here THIS is what we picture when a valve is mentioned. :P

Image

And this is what we think of as a tube/vacuum tube.
Image

Also, there is no such thing as line and frame oscillators; Only horizontal and vertical (otherwise what would the Outerlimits introduction control? :mrgreen: ).

By reading this you have now passed the first course of American TV english 101.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Feb Sun 04, 2018 2:44 am 
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Ok. Have made some progress and feel that I am at the make or break point.

After a number of sessions looking over the circuits and validating my work, I now have the negative voltage problem resolved (don't ask) and have all the correct voltages going through the cable to the picture tube. See below:

Image

My voltages are as follows:

Pin - Target V - Measured V
-------------------------------
12 --- 6.3 VAC --- 6.5 VAC
11 ---- 50 ---- 30 to 130 (variable resistor directly impacts)
10 ---- 340 ----- 310
1 ------ 0 -------- 0
2 ------ 25 ------- 26

The problem that I have is that there is no light appearing in the picture tube. No fuzz, no nothing.

Reiterating what happened was I bought the tv with no picture signal, recapped a few of the electrolytics, got a picture. Wanted to then recap the entire thing, but after 10 minutes of trying to dial in the picture, the tube quickly started getting dark so I turned the tv off. The picture shrunk to a single point on the screen and slowly went away.

I have spent the last year recapping and checking resistors and now finally have the correct voltages going to the tube, but no light.

I know that the picture tube *did* work. What can I test or look for to identify if the picture tube is dead or not?

Thank you all for your help so far!

craig


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Feb Sun 04, 2018 8:25 pm 
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Location: Dallas, TX
Maybe the high voltage died. Check to see if the horizontal output is working. Lately my favorite way of doing
a quick check is to hold a compact fluorescent bulb near the horizontal output tube, the flyback or even the yoke
wires. If it lights your good. Another way is to see if you can draw an arc (about 1/4 inch) with a grounded screwdriver
from the high voltage tube top cap. Careful you don't get zapped with that one.

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It's not the Destination, It's the Journey.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Feb Sun 04, 2018 10:25 pm 
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Joined: May Sun 07, 2017 11:35 am
Posts: 346
Yes check the HV. Your voltages look OK, but G2 a bit low points to low B+boost which in turn could mean a problem with the flyback. A shorted turn in the overwind would shut down your HV.

But before we go there:

Is the CRT heater lit?

Is the CRT good?

Check it with this method:

viewtopic.php?f=3&t=331576


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Feb Mon 05, 2018 12:36 am 
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Location: Dallas, TX
The CRT was working.
The fluorescent bulb doesn't even have to be a working one. Keep one in your tool box.
Attachment:
FluourHorzE01.jpg
FluourHorzE01.jpg [ 86.3 KiB | Viewed 869 times ]

If it lights then the horizontal output is running. The HV rectifier could be bad in that case or the frequency could
be way off or something else.

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It's not the Destination, It's the Journey.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Feb Tue 06, 2018 9:32 am 
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buy the hv probe for your vtvm or alternatively a stand alone hv probe and check the hv. clip the ground lead to the chassis and slip the tip of the positive probe lead under the rubber cap for the second annode. that will tell you what your hv is. if it matches what the schematic calls out for the hv for your set then you have a bad crt or a bad connection between the crt second annode and the hv lead. if the hv is low or nonexistent then more troubleshooting is required in the hv circuits. do not use the normal probe for your vtvm.

i would also check the circuit that is supposed to be 50 volts but reads 30 volts. you mentioned a variable resistor in that circuit that changes the voltage. when adjusting it does the voltage ever get near 50 volts?

remember all voltages are + - 20 percent unless otherwise noted in the service notes.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Feb Tue 06, 2018 5:13 pm 
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Posts: 2513
With any vintage CRT set including your 1955 Zenith black and white console, I wouldn't have spent a dime towards the repair unless the CRT was checked out good using a CRT tester.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Sun 25, 2018 11:17 pm 
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Location: Chicago
Thanks for all the feedback. I've been busy checking tubes and trying different things, but finally went through the effort to create the Quick CRT test as mentioned earlier.

Based upon Pin 2 of the CRT being equal to G1 and Pin 1 being next to G1, I connected everything according to the suggested graphic:

Image

I used 4 D batteries to achieve a 6.4V DC source. Pin 12 was connected to the negative voltage h and Pin 11 to the k with the 1/2 watt resistor. Unfortunately no result on the voltmeter.

This is my schematic for the CRT:

Image

Feeling disappointed that this may be the end since if this test shows no life, seems to be no other option. Note that the CRT did previously work for me so strange that I would have seen it fail.

craig.

UPDATE: Looks like I found the mapping, as expected:

Image


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Mon 26, 2018 2:08 am 
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Location: Dallas, TX
Are you sure the HV is OK?

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It's not the Destination, It's the Journey.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Mon 26, 2018 2:34 am 
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Well, I did use a CFL to test the HV and the CFL lit up. However, when I did this specific CRT test, I did not have the power on because it seems that this specific test is designed to test a CRT without any other power source. Just wanted to be clear about what I did when in case my answer could be construed wrong.

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Mon 26, 2018 3:20 am 
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Location: Dallas, TX
cvsolfari wrote:
Well, I did use a CFL to test the HV and the CFL lit up. However, when I did this specific CRT test, I did not have the power on because it seems that this specific test is designed to test a CRT without any other power source. Just wanted to be clear about what I did when in case my answer could be construed wrong.

craig.

OK. That CFL checks that the Horizontal output is running. The frequency might be off but it is running. Next logical test, which you might have done but could be repeated, is can you draw an arch from the HV rectifier cap to a well grounded screwdriver (or similar). Someone already described this test. The voltage here is high frequency, high voltage AC.
Lastly, can you get a spark to jump from the CRT anode connection, still connected to the CRT, to the grounded screwdriver?
The voltage here would be DC, you are actually discharging the CRT capacitance so you would get a snap.
Be careful that you don't ground either point (touch it) with the screwdriver while the set is on. Make sure the screwdriver stays grounded and you don't touch anything metal when you make these tests or you will get bit!
If the Ion Trap magnet is mis-adjusted it could cause the screen to be dark. I don't think you changed it.
I suppose the CRT filament could have burnt out while you have been working on the set but that would be bad timing.
Have you checked the CRT filament continuity (pin 1 to 12) lately?

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It's not the Destination, It's the Journey.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Mon 26, 2018 5:20 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 346
I'd be checking the connections again for the quick CRT tester.

A reading as low as 0.1 volts will still give you some light on the screen. I tested one yesterday in that category, a 60 year old CRT that had been in hibernation for 40 years.

You should be able to temporarily short G1 and K and get 6 volts. That will tell you if the rest of your tester is wired correctly.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Mon 26, 2018 8:40 pm 
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Location: Chicago
Well, this morning before departing for work, I did a continuity test on the CRT between pin 12 and 1. Failed the test. I checked against a wire and confirmed that my tester works properly. This seems to indicate that the heater filament is broken, right? Like a lightbulb that burns out. Is this the official end of the road? (sniff sniff) :(


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Mon 26, 2018 9:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: dayton oh usa
resolder the heater pins.
i use fine solder that i can easily feed into the end.
idea being to get fresh solder and flux inside it.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Tue 27, 2018 12:04 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 346
I hope resoldering the pins works for you. That 24" CRT is going to be VERY hard to find.

But open circuit heaters in CRTs are very rare in my experience, except where there's a badly soldered pin in the base.

I have noticed that 90 degree chassis will usually drive a 110 degree CRT to full screen. Maybe that might be your solution, a newer type CRT? It is possible, here is a pic of a 90 degree chassis I just finished, driving my 110 degree test CRT with a 90 degree yoke.

Attachment:
90_driving_110.JPG
90_driving_110.JPG [ 35.18 KiB | Viewed 567 times ]


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Tue 27, 2018 1:15 am 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 875
Location: Dallas, TX
kc8adu wrote:
resolder the heater pins.
i use fine solder that i can easily feed into the end.
idea being to get fresh solder and flux inside it.

Definitely worth a try. Is seems to me the emission goes bad on a CRT before the heater burns out usually.
However if at one time the heater voltage was too high that would burn it out much more quickly.

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Tim
It's not the Destination, It's the Journey.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Tue 27, 2018 3:08 am 
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Joined: Feb Mon 27, 2017 4:53 am
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Location: Chicago
So after reading Phil's great article on repairing the leads through a tube cap, my question is twofold:

1. Should I put flux inside the pin, heat the pin and guide solder into the pin? Basically, the thinking is that the solder connection between the pin and the wire has failed and by adding solder, the connection is reattached? For this step, I'm only focusing on pins 12 and 1.

2. If #1 doesn't work, would it be worth it to work the tube cap off to test the leads into the tube, as Phil does?

I feel a bit of hope here so thanks for the guidance!

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Tue 27, 2018 3:12 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 346
Open circuit CRT heaters inside the glass are VERY rare. In 20 years of servicing maybe 40,000 TVs in the field and in the workshop I only ever saw it happen twice, and in both cases it was due to bizarre but predictable external causes on the same model color TV under warranty. That's a story in itself for anyone who'd like me to relate it.

I'd be resoldering those pins!! And, if that doesn't work, I'd pull the entire base off, clean the wires and re-test. If you need to proceed to this phase you'll need a hot-air rework station or similar so you can safely release all the wires at the same time. Plus some solvent to dissolve the glue that might still be holding the base to the glass.

I think your TV has a mains transformer powering the CRT heater so an accident is highly improbable. And CRT heaters are tough, they will generally take double the correct voltage for years for anyone silly enough to try it. They don't "burn out" like filament lamps do. And they generally continue to glow even if the CRT has zero emission.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Tue 27, 2018 3:45 am 
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Well, the night is darkest just before the dawn. I went and heated one of the pins and added some solder. Tested for continuity and nothing. Heated the other pin , but didn't add any solder because it was quite full anyway. Tested for continuity and heard a heartbeat. Tested again. Same thing.

So I went back to the 6.3 VDC test and saw 6.3 on the voltmeter within 1 second. Thought that I had screwed something up with the connections. Checked everything and... same result. Thought that this might be a big moment so plugged in everything to do a real test. I saw the filament light up and then:

Image
Boom!

Very bright picture and no funny sounds.

Now I'll be up for a few more hours dialing in the picture and planning for the next steps to get this completed for my office.

Thank you all for your patience, advice and support! I definitely could not have gotten here without all your help! You can see all my shenanigans on instagram at hificlinic, if you are wondering what I'm doing with the tv.

Have a great evening and week!

cheers,

craig.


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