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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 27, 2018 2:28 am 
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Here's the legend:

Image

Seems to say measure from Pin 1 of tube V4. So I put the negative lead on pin one and then alternate the positive on the other pins? Interesting. Ok. Will try.

And I can email the schematic if you are interested to look, but I will look at schematics between V3 and V7.

The wire extensions are professionally made one foot long wires with alligator clips on each end and protective plastic sheaths over the clip.

Thanks to all the quick ideas already!

craig.

PS. And here's schematic between those two tubes:

Image


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 27, 2018 2:34 am 
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Notimetolooz wrote:
Just glancing at a few things, I see stuff that doesn't make sense.
V5 pin 6 (screen) voltage if anything should be higher than pin 6 (plate).

Yes, I don't see how pin 6 could be higher than pin 5. Perhaps putting the probe on pin 6 is causing the tube to oscillate and the meter is reacting to the resultant AC voltage. This is where the old vacuum tube voltmeters with their isolating 1 meg ohm resistor in the probe would perhaps work better.
Or perhaps some stage in the IF strip is oscillating without regard to the probe being on that pin. That might explain the negative voltage on pin 1 of V3.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 27, 2018 2:46 am 
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Hmm.. I wonder what voltage is on pin 4 of the LV power supply and if R27 as installed is 2.2 meg ohms.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 27, 2018 2:51 am 
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The professional-ness of the clip leads will have nothing to do with whether the circuit oscillates or not. Just connecting an extra wire (or even
a meter) to some circuits means adding a random capacitance and inductance (the wire and anything attached or near to it) which the engineer
didn't intend. Oscillations can occur in such situations. One possible fix for that is to use a 1Megohm resistor in series with the attached wire,
with the resistor located at the tube pin. That often will help avoid oscillation, but the resistor has to be taken into account along with the input
resistance of your meter. It can be tricky.

There sure are a bunch of wacky voltages in your list.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 27, 2018 2:56 am 
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I'll double check on R27 once the kid goes to bed. And pin 4 of the LV power supply is the left side of R27, right? That's what you are trying to see? Just trying to make sure I measure the right thing.

Update: R27 is 2.2M as expected. Pin 4 leading to R27 is -13.8. I'm no electronics expert, but that probably isn't right, is it.


Last edited by cvsolfari on Dec Thu 27, 2018 4:30 am, edited 1 time in total.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 27, 2018 4:26 am 
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cvsolfari wrote:
I'll double check on R27 once the kid goes to bed. And pin 4 of the LV power supply is the left side of R27, right? That's what you are trying to see? Just trying to make sure I measure the right thing.

Yes, it is on the left.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Fri 28, 2018 4:22 am 
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So I checked a number of things today. I did find one resistor (R55) that was floated to 295k instead of the correct 220k. I also found R54 was connected to the wrong spot, but even after changing it back, all the readings on tube 3 are still the same.

I did test changing the dials on the front and the horizontal drive would take the V3 Pin 1 from -18V to -16V. One other thing that I noticed is that the brightness dial only turns about a quarter of the way, then the picture raster disappears. Does that indicate that the voltage to the brightness pot is not s high as planned? Or is it possible that the pot is bad? Should the range be 0k to 200k ohms? Schematic shows 200k.

One other thing that I noticed is that the sound has a hum (I haven't yet adjusted the buzz control), but when I change from testing one pin to another, there's the typical "buzzing" sound like there's a short. I haven't spent much time on the audio circuit because I was so focused on the video, but maybe that's an area I need to review much closer to see if it's impacting the anything else like the significant negative voltage on V3 or V7.

thoughts?

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Fri 28, 2018 4:45 am 
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cvsolfari wrote:
So I checked a number of things today. I did find one resistor (R55) that was floated to 295k instead of the correct 220k. I also found R54 was connected to the wrong spot, but even after changing it back, all the readings on tube 3 are still the same.

I did test changing the dials on the front and the horizontal drive would take the V3 Pin 1 from -18V to -16V. One other thing that I noticed is that the brightness dial only turns about a quarter of the way, then the picture raster disappears. Does that indicate that the voltage to the brightness pot is not s high as planned? Or is it possible that the pot is bad? Should the range be 0k to 200k ohms? Schematic shows 200k.

One other thing that I noticed is that the sound has a hum (I haven't yet adjusted the buzz control), but when I change from testing one pin to another, there's the typical "buzzing" sound like there's a short. I haven't spent much time on the audio circuit because I was so focused on the video, but maybe that's an area I need to review much closer to see if it's impacting the anything else like the significant negative voltage on V3 or V7.

thoughts?

cheers,

craig.

Last thing first. the audio circuit should not interact with anything else. So don't worry about that now.

If the raster disappears abruptly then the brightness pot might be bad. But worry about that later.

The AGC tube is fed a sample of the horizontal signal to match it with the received sync pulse. If varying the horizontal control makes a change in the voltage at pin 1 of V3, it sounds ke the AGC tube is starting to think about working. Perhaps moving R54 did something. Try adjusting the AGC control and see if it does anything to the voltage on pin 1 of V3.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Fri 28, 2018 5:25 am 
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Ok, Tom. Great call.

Changing the AGC has a dramatic impact on the V3 pin 1 voltage. It went all the way down to -30 and up to +10, at least. I didn't go both ways to the end, but I was shooting for 6.5V and was easily able to achieve that. I also checked the Pin 2 and was able to see around 8.5 to 9, which is right where it should be. Pin 5 didn't change from 160+, but pin 6 did drop to 135V. I'll check the other ones tomorrow to see how those values may or may not change, but I feel good about that.

I still didn't see a visual change when switching the tuner dial, but assume there's some more work to do before getting there.

Until tomrorow!

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Fri 28, 2018 5:45 am 
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cvsolfari wrote:
I still didn't see a visual change when switching the tuner dial, but assume there's some more work to do before getting there.

Until tomrorow!

craig.

Try feeding a signal into the tuner (antenna input) anyway.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Fri 28, 2018 6:45 am 
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Also, If you still have all those test clip leads attached to all the IF tubes, you will most likely not be able to get a signal through the IF until they are removed.
They will seriously detune all the tuned circuits.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Fri 28, 2018 8:37 pm 
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I feel like I'm making a lot of progress. Here are the latest voltage measurements:

Image

Overall, a big improvement. I think that the R54 change and adjusting the AGC has moved the voltages to mostly the right range.

It seems like there is a signal going to the screen now because it flickers and has those horizontal lines that are not like just a raster. With the voltages as they are above, I think that the signal could/should be going through now.

With that assumption, I started looking at why the horizontal hold didn't seem to be able to flatten out the picture like I could do on other tvs. I noticed that V16 pin 2 was about half the planned output while the other pin voltages were dead on so I searched a bit and found a resistor that was also mis-connected. R115 was connected to the horizontal drive, like R112 is. I moved that and am going to test it out this afternoon.

Image

Fingers crossed!


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Fri 28, 2018 9:07 pm 
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cvsolfari wrote:

With that assumption, I started looking at why the horizontal hold didn't seem to be able to flatten out the picture like I could do on other tvs. I noticed that V16 pin 2 was about half the planned output while the other pin voltages were dead on so I searched a bit and found a resistor that was also mis-connected. R115 was connected to the horizontal drive, like R112 is. I moved that and am going to test it out this afternoon.

Image


You lost me. R115 is supposed to be connected to B1 (Horizontal Drive) where R112 is.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Fri 28, 2018 10:17 pm 
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Yeah you are right. I guess that it was right before. Thanks for catching that....


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Sat 29, 2018 1:12 am 
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So upon further investigation, the 47 Ohm resistor was not connected to ground as required. The end that was supposed to connect to ground was also connected to the horizontal drive. It's correctly connected now.

Does that Horizontal Drive do anything important?

The V17 tube measures correct voltages, but still not getting a picture on the CRT. What concerns me is that when changing the tv tuner dial to different channels, there's no flash or any visible change on the tube. Therefore, it seems that the signal is still not getting to the CRT.

I also validated that my HD antenna and digital to analog converter are working by testing them on my other working Zenith tv.

Based upon the voltages that I have, what could be the next areas to look at? If there's no visual flash when changing channels, that seems to indicate that there's still no signal or at least not a powerful one enough, right?

cheers.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Sat 29, 2018 1:24 am 
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With the 47 ohm resistor not connected to ground, V17 will not work at all. Therefore, you have no horizontal sweep and no high voltage to the CRT anode. No hope of seeing anything on the screen without those.

With the 47 ohm connected correctly (how did it get wired wrong?), check the voltages on V17 now.

The Horizontal Drive control does do something important, and it'll change the voltages on V17 somewhat. It's meant to be set for best efficiency, but "close enough" will get you going.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Sat 29, 2018 3:09 am 
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It does seem that the IF strip should now pass a signal. Did you remove all of the test leads from the IF tubes? The voltages on V6 look to be close enough for that to work although it is strange that the plate voltage is low. One possible problem could be that the detector diode is bad. It is hard to read the label, but I think that it is M2. Are you using a digital voltmeter/ohmmeter? If so, does it have a diode check function? If it does then you should be able to test the diode. To get a perfect test you would have to disconnect it from the circuit. But it looks like it is inside of one of the cans with one of the coils. You could get a crude test by testing between the grid of V6 (pin 2) and ground. You would put the ohmmeter into the diode check mode. Then measure between the grid of V6 (pin 2) and ground. Note the reading. Then reverse the leads from the meter and note the new reading. One reading should be noticeably lower than the other.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Sat 29, 2018 3:36 am 
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Hmm... The plate voltage on V6 (pin 7) could be low if there was something wrong with the contrast control or any of the resistors around it. The schematic I have is too blurry to read the R numbers. The first thing to do is turn the contrast control all the way up and then see if you have any sign of a signal on the CRT.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Sun 30, 2018 7:51 pm 
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A note about diodes and ohmmeters.

Diodes conduct when one polarity of voltage is applied but do not conduct when the opposite polarity is applied. But they do not start conducting until a certain voltage is exceeded. Silicon diodes need at least 0.6 volts to conduct and germanium diodes need at least 0.3 volts. (Transistors are much the same.) Selenium rectifiers need several volts per plate. Some digital ohmmeters intentionally use a very small voltage so that they can make in circuit measurements in solid-state circuits without the diodes and transistors affecting the readings. Those meters generally have a diode check function that applies a higher voltage. Analog ohmmeters with a moving pointer generally use a high enough voltage to turn on diodes.

If you have any diodes laying around you can see how your meter behaves. Just measure your test diode twice, take a reading with the leads connected one way and take another reading with the leads reversed. See what happens. If you have enough voltage you will see a reading one way and no reading the other way.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Tue 01, 2019 4:19 am 
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So I haven't had too much time the last 3 days to work on this, but had a few hours today and wanted to try and get some other information that might help in the debugging of this.

First of all, I will test the contrast idea and the corresponding resistors on Wednesday. Tomorrow is the Winter Classic.

Secondly, I hooked up my analog oscilloscope to what I thought were the key signal areas of the tv. I took pictures of the scope readings and the expected results. A few interesting things came up.

W1 - At Pin 2 of tube 6 (Video Output) - Should be 2V peak to peak

Image

I was using a 10X probe so the reading is .5V per division or about 2V on scope. Seems generally in right range to me.

W2 - Near CRT input between contrast and brightness. Should be 50V p-to-p.

Image

Looks like 5V per division, but only one division means 5V. Well below expected.

W5 - Pin 7 of Tube 13 (Sync Sep) - Should be 25V p-to-p.

Image

Again, looks like about 2.5 or 1/10th of what is expected and from schematic, seems directly connected to W2 above. And Pin 7 should read -2.8V and I measured -0.45V.

W6 - Pin 1 of Tube 13 (Sync Sep) - Should be .15V p-to-p.

Image

Seems to be about 2V and the exact reverse of the W5 scan above. Inverted vertically. Like a mirror copy. One other interesting thing is that the expected voltage of the pin is 0.3V and I received a reading of -0.22V.

Is it possible, and I didn't try changing this, that the fringe lock is dialed in too high (or too low)? a 7.5M range seems large compared to all the other resistors in the set. Again, I didn't test if it has an impact on the voltage reading on the scope.

If the signal goes through the IF stage correctly showing up at W1 with 2V, then seems that the problem comes after that stage, somewhere that should be amplifying the signal. I suppose that I should test the other waveforms to see if that helps narrow down where the problem is.

Happy New Year!! See you in 2019!


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