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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sun 06, 2019 7:58 pm 
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cvsolfari wrote:
What would a coil leaking look like? How can I diagnose that?

I can think of 2 ways to see if L20 has leakage between the primary and secondary.
One way is to see if it gets warm or hot after the set is running a few minutes.
The other way is to disconnect the ground connection at the bottom of the secondary. If the plate voltage on V6 goes up and the signal level goes up then L20 would have leakage. That would disable the sound, but this would just be for testing the video.
Such a problem in L20 would be unusual, but your lack of signal at V6 is unusual.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sun 06, 2019 8:11 pm 
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I haven't seen the schematic section that has R58.
Since the screen isn't in sync, (because the signal is so low going to the deflection
oscillators), any faint video getting to the screen may be scrambled so much it isn't
visible.
A VTVM has a 1 Meg resistor in the DC probe near the tip that prevents the wire lead
capacitance from loading down the AC part of the signal. This difference may effect the
reading between the two types of meters.
Yes, I know you checked the winding resistance of L20, but I don't think you measured the
resistance between the windings. Some of the small caps like C48 might be leaking.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sun 06, 2019 10:59 pm 
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Notimetolooz wrote:
I haven't seen the schematic section that has R58.
...................
Yes, I know you checked the winding resistance of L20, but I don't think you measured the
resistance between the windings. Some of the small caps like C48 might be leaking.

R58 is a 100K resistor from the grid of V8 (the sound IF tube) to ground.
It would seem impossible for a 3.3 pf cap to be leaky, but this whole problem seems impossible. Ungrounding the grounded end of the secondary of L20 would also eliminate any possibility that C58 could cause the low voltage and signal. If C48 were leaking then it would probably get warm or hot.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sun 06, 2019 11:57 pm 
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Location: New Hampshire
I have watched this tread on & off. What exactly is the symptom at this point ?
From the last 2 pages it seems to be "have raster but no audio or video".
That symptom is not rocket science. At worse there are 2 separate "major" problems.
One audio & one video. Also is there a link to a complete schematic, I would rather not go through
8 pages to maybe find it.
BTW I know you have a scope & meter. Do you have any other test equipment ??

73 Zeno 8)
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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Mon 07, 2019 2:41 am 
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Hi Zeno,

I don't have any other testing equipment besides what you have seen.

I sent you a message about the schematic. I can also send it to anyone else that would like to help.

The basic issue, as you say, is has raster, no video or audio.

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Mon 07, 2019 6:04 pm 
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Location: New Hampshire
OK Craig I replied to msg.
If you havnt change the video detector diode. If it shorts it will kill everything as the sound is
taken off AFTER the video out tube & those diodes gave trouble. BTW if you test it one
end HAS to be lifted. A few actually plug in.
Since the symptoms can be caused by the tuner, IFs, vid det or video out you will need to
divide it to isolate the bad stage. We have a few tricks we can try without equipment.

73 Zeno 8)
LFOD !

cvsolfari wrote:
Hi Zeno,

I don't have any other testing equipment besides what you have seen.

I sent you a message about the schematic. I can also send it to anyone else that would like to help.

The basic issue, as you say, is has raster, no video or audio.

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Mon 07, 2019 6:17 pm 
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Zeno, it seems he has OK video up to the grid of the Video Output. The signal at the plate has about the
same amplitude as the grid. He has changed the Video Output tube I believe. The problem looks to be
incorrect as far as the plate load, however he has check or replaced nearly everything there. The plate
voltage seems to be a bit low also. Maybe there is a problem with the tube socket, or something odd
like that.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Mon 07, 2019 10:24 pm 
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Location: New Hampshire
Tnx Tim
The only thing I saw with a quick look was a waveform taken at I assume the G-1 of the video out. ( pg 7 ?)
There was zero video at that point, just what looks like the blanking pulse. Chasing an audio problem is at this
point not the way to go IMHO. His scope looks good enuf to see undetected video from the tuner IF out on.
Thinking was to divide & conquer at 3rd IF to narrow things down.
73 Zeno 8)
LFOD !

Notimetolooz wrote:
Zeno, it seems he has OK video up to the grid of the Video Output. The signal at the plate has about the
same amplitude as the grid. He has changed the Video Output tube I believe. The problem looks to be
incorrect as far as the plate load, however he has check or replaced nearly everything there. The plate
voltage seems to be a bit low also. Maybe there is a problem with the tube socket, or something odd
like that.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Tue 08, 2019 4:29 am 
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I went and grabbed a couple of shots of the current W1 and W2 waveforms and share them here:

W1: Should be 2V P-to-P

Image

W2: Should be 50V P-to-P

Image

It looks like there is a signal coming from W1 (although it's a bit orderly, which seems somewhat odd to me). These two photos show a 10X increase to W2 over W1. Maybe I did something wrong when measuring that W1 form at Pin 2 V6.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Tue 08, 2019 4:51 am 
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I can't tell what your volts/div settings are in those 2 pictures. So I can't tell what voltage you are measuring.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Tue 08, 2019 5:17 am 
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Location: Dallas, TX
Looks to me that W1 was measured at 0.2 V/div, which would put it at about 0.5 Vpp.?

At any rate, am I correct that R3, R43, R48, R49 and R50 have been checked?
Is the 260 V correct at the brightness control R7?
What is the voltage on the CRT pins 2, 10 and 11?
Looking back on some of the posts, it looks like the signal on CRT pin 2 (W10) is more
like 70 Vpp instead of 45 Vpp.

EDIT: cvsolfari, the red knobs on the vertical range selectors have to be turned all the way to CAL
(clicked) so that the voltage readings are accurate.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Tue 08, 2019 4:09 pm 
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Location: New Hampshire
This link shows what you want to see for composite video. It may be flipped over.
Note it shows 2 lines of video, thats traditionally done & also in Sams that way.
You must adjust the time on the scope to get it that way. Use the red knob to tweak it.
When you get it right make a note or mark the scope TVH ( for TV horz ).
Above the dotted line is the video & on regular programing moves around.
White is up black down.
Below line is called blacker than black. At some point what is there is "blanked out" so
it wont reach the CRT. If you have a generator the pattern will make more sense.
Below the line is the horz sync pulse & the color sync pulse.
Work with that & see what you get......


https://www.google.com/imgres?imgurl=ht ... mrc&uact=8


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Tue 08, 2019 7:21 pm 
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zeno wrote:
. Use the red knob to tweak it.
When you get it right make a note or mark the scope TVH ( for TV horz ).

That is the red knob on the time display control.

Thanks zeno. I had tried to find similar info on YouTube showing a scope display but had no luck.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Wed 09, 2019 3:50 am 
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Notimetolooz wrote:
Looks to me that W1 was measured at 0.2 V/div, which would put it at about 0.5 Vpp.?

Getting out my magnifier, it does look like the waveform at W1 is about 0.5 Vpp. And the waveform at W2 looks to be 6 Vpp. And the signal looks nothing like a video signal. But in an earlier post the signal at W1 does seem to be 2 Vpp and does look like video. So perhaps in these latest photos the digital converter used to source the signal to the antenna input is not set up correctly.

Anyway, about a gain of 10 in the video output tube. But with voltages that low we could be seeing junk signals being picked up by stray coupling. Some of what I see in those waveforms looks like hum from the power supply. The schematic indicates that we should be seeing a gain of about 25.

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