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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Tue 01, 2019 6:54 am 
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Set up the scope as if you were going to measure the signal at W2. And then back up to pin 7 of V6 and see if the signal is there. If it is then use the scope to follow the signal toward W2 and see where it is lost.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Tue 01, 2019 9:41 pm 
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Not to distract from the the train of thought here, but I have a few questions and suggestions.
The waveform at W1, pin 2, the grid of the Video Output (V6) looks odd. If the signal here is incorrect
then the Sync Separator won't have the signal it needs to work.
What kind of video are you feeding to the set when you are looking at the scope? Is it a still picture or is
it a moving image? If it is just snow then that isn't a good start, you should be using a good video image.
From the scope display it looks very uniform, maybe with a stripe down far left or right.
If you want to more closely match the waveform pictures in the SAMS you should change the time setting
on the scope. For the ones labeled 30 cycles try setting it to 5 mS/div, with the triggering set to TV Frame.
You also should be using AC coupling on the vertical input. It also looks like the trace is out of focus, you
can't see much detail.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Tue 01, 2019 11:19 pm 
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It's pretty clear that there is no video at W1, only a sync pulse. Typically sync pulses will appear (from the free-running vertical and horizontal oscillators) in the absence of any signal, but this is not normal operation. As mentioned, you will need a good picture signal coming through the tuner and IF stages and out of the detector diode to get 'scope pictures to match the Sams illustrations.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Tue 01, 2019 11:25 pm 
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I did notice that there was no picture information in the signal at W1. Still, if there is something at W1 there should be a bigger something at W2.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Wed 02, 2019 2:26 am 
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Tom Schulz wrote:
I did notice that there was no picture information in the signal at W1. Still, if there is something at W1 there should be a bigger something at W2.

EDIT: Incorrect grid bias might explain that. It only measures slightly negative. The Fringe Lock might effect that.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Wed 02, 2019 3:25 am 
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the current image is a chromecast screen, not a moving picture. I will cast something to the chromecast and if that doesn't work for some reason, I will switch over to an HD antenna with a digital to analog converter that works on another tv.

I will also test at W1 and W2 when changing the fringe lock to see if the signal waveform changes.

All for tomorrow. Thanks for the ideas.

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Wed 02, 2019 3:38 am 
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Notimetolooz wrote:
Tom Schulz wrote:
I did notice that there was no picture information in the signal at W1. Still, if there is something at W1 there should be a bigger something at W2.

EDIT: Incorrect grid bias might explain that. It only measures slightly negative. The Fringe Lock might effect that.

The schematic shows that the grid of V6 should be -0.3 volts. Given the cathode resistor, the exact grid voltage is not critical. The plate voltage of V6 is low but is in the range that the tube should be amplifying. I suspect that the signal will be found at the plate of V6 (pin 7) and will disappear somewhere between there and W2.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Thu 03, 2019 4:12 am 
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Ok. So I tested pin 7 of V6 and only found an approximately 2V signal.

This was after confirming that I have a good tv signal going to the TV. I removed the chromecast and connected my digital antenna to an D2A converter.

The Fringe Lock did modify the waveform up to 20%, but off of the 2V height didn't really do much to solve the problem.

Here's what the waveform at W1 looks like with 5 ms/div and the tv form set. Seems like I'm possibly doing something wrong with the scope:

Image

Is there an article that you can point me to about "AC coupling on the vertical input"? Not sure what that means.

I changed the V6 tube just to make sure that it was not the problem and I went from 110V at pin 7 of V6 to about 118.

So if the signal is getting to V6, it's not being multiplied as expected. However, given my waveform, there is doubt that the signal is getting to V6, right?

Thanks for all your patience!


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Thu 03, 2019 4:50 am 
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For AC coupling, right below the volts/div knob there is a switch labeled AC gnd DC. Move it to AC. I am not sure how much that is important.

To make the display look somewhat better, reduce the brightness somewhat and find the knob labeled focus. Adjust the focus.

I think that the signal is getting to V6 about right. If you could actually see a picture on the CRT, you would probably be adjusting the fine tuning at the tuner for a good picture. Right now, you are just trying to get something to the CRT.

It seems as if something is loading down the signal at the plate of V6. I see that the sound is taken off from there and coupled into the sound section by C49. Could that be shorted? It seems somewhat unlikely, but you could just disconnect it and see if that changes anything. Let me look at the schematic a little more.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Thu 03, 2019 5:18 am 
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Check coils L20 and L21 for the right resistance. Check the resistors around the contrast control. Check that C66 is not shorted. Check that everything around this area is connected correctly and that there are no leads touching anything.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Fri 04, 2019 4:40 am 
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Hi Tom,

I checked all the resistors around the sound tubes (V8-V12) and didn't see anything out of the ordinary. Nonetheless, I disconnected the sound connection just past C49. Actually, I disconnected the cable that connects to V8 pin 1 and the R58 100kOhm grounded resistor. Didn't change the output of the different voltages or waveforms around V6. I left the audio part disconnected for future testing to eliminate some potential errors.

I tested C66 with my digital multimeter and a known good capacitor at .0033 and it seems fine and not shorted. Is it possible that V13 is bad and causing this? Could it be shorted inside the tube? I might not have tested that lately.

I tested the resistance of L20 and L21. For L20, it was about 15 Ohms on the left side and 1.5 Ohms on the right side. Very close to expected 12 and 1.3 respectively.

L21 is a little more interested. From the schematic, it shows a 5 Ohm inductor and a 5.6 kOhm resistor. I disconnected the combined piece and received a reading of about 5.6 Ohm. I don't know how to measure that 5.6 kOhm resistor. Here's what that part looks like:

Image

The only other interesting thing that saw was when I measured the voltage at pin 11 that goes through the CRT. This is basically where I measure waveform W2. The voltage measurements are usually fairly stable after a minute or two, but this was the only one that had a strange bouncing of the voltage in the third significant digit, i.e. just after the decimal point (50.3 for example). It started around 55V and slowly bounced around and dropped to the mid-40s over about 7-10 minutes. I don't know if this is significant because I haven't seen it happen elsewhere, but wanted to mention it.

I want to close with a picture of the CRT. At the top left, you can see what I believe to be the picture trying to come through, but not with a high enough voltage to get where it needs to be.

Image

So I feel that I'm seeing what you say that the signal is coming through the IF, but there's something still causing this to not quite get there.

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Fri 04, 2019 6:12 am 
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V13 could not affect weather the signal gets to the CRT. What it controls is the ability of the vertical and horizontal to lock in correctly. Have you ever adjusted the vertical or horizontal hold on a working TV? If the set were working except for V13, you could adjust the vertical hold to almost stop the rolling but it would never lock in. The picture would just slowly drift up and down.

There is no good way to test the 5.6 K resistor inside of L21, but it is not critical. If everything were working except that that resistor was open, the picture would be just a little funny. For any sharp vertical line in the picture you would see the line repeat a few times close to the original line. That is known as ringing.

The DC voltage at pin 11 of the CRT is controlled only by the brightness control unless there is some leakage in C45. Controls such as the brightness control can sometimes be a little erratic.

Given that the cathode voltage on V6 is correct, it seems that V6 is drawing about the right current. Perhaps check the signals at the grid and plate again with the scope set on AC and see if you get different readings.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Fri 04, 2019 6:24 am 
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Was that photo taken with a signal supplied by the digital converter with a TV station tuned in? If not try that again. Also slightly vary the setting of the ACG control. Mark the current setting somehow so that you can put it back if varying the setting does not seem to do anything.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Fri 04, 2019 6:03 pm 
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Take care with L21, it is called a 'peaking coil', a bit hard to replace. It effects picture
sharpness.
The reason AC coupling is a better idea is that sometimes signals are on large DC voltages
and you would need to adjust the vertical position on the scope to an extreme point. Using
DC coupling on the scope reveals this DC offset but since the SAMS pictures don't show what
the proper amount is, the info is pointless.
Does the picture of the scope screen that you posted look like what it does in person?
Maybe you camera is making it look wrong. It shouldn't have that slant.
So are you now saying that V6 pin 7 only has about 2 V peak to peak, the same amount
as pin 2 ? Have you changed C43? C43 could effect the AC gain without effected the DC
pin voltages much. Double check the contrast control R3 and R50.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sat 05, 2019 8:12 pm 
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Tom Schulz wrote:
V13 could not affect weather the signal gets to the CRT. What it controls is the ability of the vertical and horizontal to lock in correctly. Have you ever adjusted the vertical or horizontal hold on a working TV? If the set were working except for V13, you could adjust the vertical hold to almost stop the rolling but it would never lock in. The picture would just slowly drift up and down.

I should explain why I asked about C66.
A good capacitor will pass a signal or any varying voltage. But it will block a steady voltage (DC voltage). But a leaky cap will pass some amount of current from a steady voltage. I was thinking that if C66 was leaky, that could pull down the voltage available for V6.
Now if somehow there was a short from pin 7 of V13 to ground that would also short out any signal at C66's top end. But as that is fed through a 33K resistor, that would not put much load on V6. Especially since the contrast control is 5K. (Assuming that the contrast control really is 5K.)

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sun 06, 2019 12:50 am 
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Ok. So I spent more time today on a few of the suggestions and reviewing all the components around the brightness and contrast controls. Here's where I'm at:

1.a. I replaced the C66 (ceramic) capacitor. It had been an original one and had measured well on my digital multimeter.
b. I tested some of the tubes again and the V13 tube didn't seem good so I swapped it out with another tube that I had, but also isn't the ideal reading, but still ok. I've ordered a new replacement tube.

Then I tested W2 and, interestingly, it seems that we're making some progress. The Voltage p-to-p is about 5V now (was 2-2.5V). Here's the scope pic:

Image

That's on a 2V/div scale so definitely made an improvement.

2. I also tested the W10 waveform just to see what that side of things looked like and it's in good shape:

Image

Had to slow down the scope refresh to get a decent picture, but voltage and form look good from my perspective.

3. Voltage at V6 pin 7 is still between 110V and 120V, below the expected 160V.

4. C43 has been replaced a while ago with one of those chiclet capacitors.

All scope photos are now taken with the tv tuner on and plugged in.

It seems that there are some other ceramic capacitors that I haven't replaced because Phil's Old Radio says to only replace them with a specific reason: https://antiqueradio.org/recap.htm

I'm back to the real world on Monday and off to Germany for a business trip on Friday so my response is going to slow down dramatically until the end of January.

Thanks for all the help and hope to have this ready by March Madness!

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sun 06, 2019 2:04 am 
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The scope photos look a lot better now.
I guess the signal at V6 pin 7 is pretty much the same as W2? At least with the Contrast control set to maximum.
The DC voltage at V6 pin 7 still looks like a problem. BTW, what meter are you using, VTVM, VOM, DVM ?
There is a chance that if the type of meter is not the type used for the schematic numbers they will be off.
It seems like we have checked all the usual suspects, perhaps you have a leaky tube socket or maybe L20 leaks
or a wire insulation is faulty. I think you said you disconnected C48.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sun 06, 2019 3:02 am 
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I disconnected the audio just before R58.

The multimeter is a Fluke.

What would a coil leaking look like? How can I diagnose that?


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sun 06, 2019 3:17 am 
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One other thought: should I plug in the audio and try to get a clear audio signal or do you think that this isn't useful to try? Might help identify something mis-wired in the audio circuit that is affecting the video...


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sun 06, 2019 6:30 am 
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With that signal at W2, I would expect that you would see a faint picture on the CRT. You might have to reduce the brightness to see it. Do plug in the audio and see if you get anything.

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