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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: May Sun 28, 2017 4:08 am 
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cvsolfari wrote:
Ok everyone. Thanks for all the great tips. I've been working on this since my last posting and here is what happened:

1. Changed the C1 and had raster. Quickly shut it off and posted on the site.

2. Changed the other 3 electrolytic caps (C2, C3, C4) and tested again. Received a raster and plugged in my digital antenna, d2a converter and started dialing in the picture (changing the picture settings from the front). Was receiving audio, then after 10 minutes, the picture went noticably darker so I quickly shut off the tv. Light on screen went to a center beam that took about 2 minutes to dim down to nothing.

3. I assumed that this was due to an old cap that failed so I went through and changed out all the old caps, including bumblebees, and now get a strong audio signal, but no picture. Based upon the comments earlier and some research on B+ voltage, that seems to be the likely culprit.

4. All tubes that can be checked, have been and I even have replacements for most of the tubes because I wasn't sure where the problem was.

So I'm back to the initial problem with audio, no raster, but have recapped the tv and know that the CRT works (or was working).

I've been looking at testing the tube voltages, which is what I can see in the schematics. Is it better to first test the resistance measurements from the docs:


craig.


Checking the resistances from the tube sockets might show something, at any rate it does no harm. If you are getting good audio you can assume the circuits around tubes V1 to V6, V8 to V12 and V20 are OK.
So when you say 'no picture' do you mean no raster? If so, I like the trick of holding a neon or fluorescent lamp near the Horizontal Output tube. If it lights then the Horizontal Oscillator and Output are working, maybe off frequency though. That also means the B+ (340V) for those is there. You could measure that with a meter also.
You could also measure the voltages on the CRT socket.

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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: Jun Tue 06, 2017 4:29 am 
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Hi Tim,

Thanks for the ideas. Here's where I'm at:

1. When I say no picture, I mean no light appears on the CRT. So if the appropriate term for that is raster (in order to differentiate from an actual picture), then yes, no raster.

2. I tested the voltages at the electrolytics and found C1A, C1B, C2A and C2B all very close to the expected values. However, C3A should be 110V and mine showed about 150V. Here's the image with my markings on it:

Image

3. I tested the horizontal output with a neon light and was able to see the neon light up. Really cool to see.

What would you suggest that I test next? It feels like this is just going to be a little sleuthing to identify the bad part/tube because so much seems to be working properly... (not to jinx it or anything...)

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jun Tue 06, 2017 7:16 pm 
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Is your brightness control turned up? Check voltages/waveforms at the pins on the socket base of the CRT. Is the CRT filament lighted?


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jun Tue 06, 2017 11:49 pm 
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brightness control was turned up, but I turned it to normal before my next round of tests. The CRT filament was not lighted in any recent test.

The voltages at the CRT are as follows:

Pin 1 - 0V measured - 0V expected
Pin 2 - 33V measured - 25V expected
Pin 10 - 339V measured - 340V expected
Pin 11 - 130V measured - 50 V expected
Pin 12 - 0V measured - 0V expected.

I don't have a way to test the waveforms.

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jun Wed 07, 2017 4:05 am 
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Ok. I might have some additional information that explains a lot of this.

When I was originally replacing the electrolytics, a resistor lead (R48, I believe) connected to the C3 cap broke off when. Fortunately, I had a set of radio shack resistors lying around and one of them was the right one. What I didn't notice at the time (or think about) was the wattage rating of the resistor, which is 1/4 W, where most of the original resistors are rated 1/2 W or greater. So with a lower wattage resistor in the system, I saw everything working, but it might have failed or drifted due to an "overload".

Tonight I zeroed in on the resistors (R51, R49) around pin 11 and found two of them had drifted about 20%. Seems reasonable to think that since I tested a number of other resistors in other sections and they were all basically right on, my low wattage resistor may have caused some damage and throw off the voltage. Is this reasonable to think?

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jun Wed 07, 2017 5:29 am 
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cvsolfari wrote:
The CRT filament was not lighted in any recent test.

The voltages at the CRT are as follows:

Pin 1 - 0V measured - 0V expected
Pin 2 - 33V measured - 25V expected
Pin 10 - 339V measured - 340V expected
Pin 11 - 130V measured - 50 V expected
Pin 12 - 0V measured - 0V expected.

If the CRT filament is not lighting, you will certainly not get anything on the screen. Pins 1 and 12 are most likely the filament pins. There should be about 6.3 volts AC across them.

Resistors being 20% off value will not stop the set from working, but they may well fail more completely in the future. You should get the right wattage resistors.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jun Wed 07, 2017 5:47 pm 
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cvsolfari wrote:
brightness control was turned up, but I turned it to normal before my next round of tests. The CRT filament was not lighted in any recent test.

The voltages at the CRT are as follows:

Pin 1 - 0V measured - 0V expected
Pin 2 - 33V measured - 25V expected
Pin 10 - 339V measured - 340V expected
Pin 11 - 130V measured - 50 V expected
Pin 12 - 0V measured - 0V expected.

I don't have a way to test the waveforms.

cheers,

craig.


Tube biasing 101:
Referencing the schematic pin 2 is grid and 11 is cathode. The sams voltages show the grid being 25V negative with reference to the cathode. Your sets measurements show the grid being 93V negative with reference to the cathode (because the cathode voltage is WAY off)....As things stand the cathode-grid voltage differential is cutting off the emission substantially more than desired. You probably want to get the cathode down around 50V to get raster.

Now all of this assumes three things The CRT heater lights, The CRT has been tested good on a CRT checker, and the HV is present and close to designed value.

I forget if you've tested the CRT or not, but if it is defective it could cause bias shift. Also if there is NO HV that could cause no raster. One check you may want to do for high voltage is: with the set running take a well insulated plastic handle screw driver and put it's tip within 1/4" of the HV connector on the side of the CRT (if the connector has a rubber suction cup then slip the driver under it) you should hear an arc, and be able to draw 1/4-1" of arc there if you can not draw that much then your HV is probably low. I know you drew an arc off the HV rect top cap, but if the HV rect is bad it's heater winding on the fly open or shorted, or the HV wire is open there will be no HV at the CRT where it counts most.

To light a CRT (assuming it was not designed to need an ion trap) you generally need three things: correct gun bias voltage, enough HV, and a CRT that has enough emission capability (ie. a good testing CRT).


Last edited by Electronic Memory on Jun Thu 08, 2017 2:27 pm, edited 1 time in total.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jun Thu 08, 2017 5:41 am 
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I noticed on the schematic that there is a TV/Phono switch. Your particular unit might not have the switch, but it looks like if it is set to Phono then it cuts the filament voltage to the CRT. Have you checked that switch?

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jun Tue 27, 2017 5:08 am 
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A [maybe] dumb question.....whose bright idea was that to have those 5U4s on the top of
that transformer?? That had to be a pain in the butt to take that thing apart to work on it.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 14, 2017 2:17 am 
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Ok. I'm back. I had some other projects and a different tv to get recapped and working and those are all done now so I am back to this one.

Thanks for all the suggestions. I will be working on this on Saturday. Now I have one more question:

What is the best way to handle this capacitor/coil combination? This is what it looks like:

Image

Here's the identifier on the schematic:

Image

and here's the reference in the coil section of the documentation:

Image

Do I need to have the coil wrapped around the .01 cap? Should I disassemble to replace the cap with a new one and then reattach? Will be quite a bit of work to ensure that the wire is right.

Thanks in advance!


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 14, 2017 9:44 pm 
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Clip one lead of the cap, leave the coil connected, leave the cliped cap in as a coil form, hook a new cap in parallel with the coil and call it done


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 14, 2017 10:33 pm 
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Ok thanks. I wasn't sure if the coil around the cap was a bug or a feature. Seems to be an unnecessary design.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Fri 15, 2017 2:37 am 
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cvsolfari wrote:
Hi Tim,

2. I tested the voltages at the electrolytics and found C1A, C1B, C2A and C2B all very close to the expected values. However, C3A should be 110V and mine showed about 150V. Here's the image with my markings on it:

Image

craig.

I've been distracted by other projects. As far as the 110V source measuring 150V, that just indicates that the 110V circuits
are not pulling the normal current. A clue perhaps.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Fri 15, 2017 6:28 am 
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cvsolfari wrote:
Ok thanks. I wasn't sure if the coil around the cap was a bug or a feature. Seems to be an unnecessary design.


It made for a good cheap coil form. Most small coils were wound on resistors 1000+ times the resistance of the wire in the coil....The parallel resistor had almost no electrical effect on the components impedance and made for a cheap coil form.

I'd say the same for the capacitor, but unlike resistor forms, it likely did serve a purpose forming an L-C resonant circuit with the inductor wound on it. Best to keep the cap as an (electrically disconnected) coil form and install a new cap. If it had a wax-cardboard shell instead of plastic I'd advise you to restuff the cap (for neatness/simplicity), but with a plastic cap, clipping it and adding a new one in next to it is the most practical method.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Mon 01, 2018 4:19 am 
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Hi Everyone,

I've been doing some more testing to try and narrow down the problem and found that the 12AX7 tube has pin 6 at -46.6V when it should be 6.6V. All other pins on that tube are within about 20% of expected, except for Pin 8, which should be 9.6V and was tested at 5.5V, probably because pin 6 is so far negative to expected.

When I trace the connection to 6SN7GTA pin 1, it should be -76V but it's -57.7V. In the right ballpark. See the schematic of these two tubes:

Image

When I trace the 12AX7 pin 6 the other direction, one place it connects to is Pin 1 of 6CB6. Should be 6.5V, but it's also -46.7V. See the following schematic:

Image

I'm fairly certain that I've tested and replaced any tubes that were bad, but certainly I might have misses something. Any thoughts on what I can look at next?

cheers and Happy New Year!

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Mon 01, 2018 6:26 pm 
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That sounds like C46 may be leaky. Try replacing it even if it is a new cap.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Fri 05, 2018 8:45 pm 
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I replaced the C46 but no luck. Same negative values. Could it be the tube?


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sat 06, 2018 2:52 am 
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You can rule out the tube being bad with the test detailed in my earlier topic:

viewtopic.php?f=3&t=331576

This allows you to test a tube with no connection to the TV chassis, i.e. it's a standalone test.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sat 06, 2018 4:21 am 
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Tubes can develop internal shorts, but if none of the other pins on those tubes have negative voltages on them then no internal short could produce the negative voltage. About the only source of negative voltage is the grid of the horizontal output tube. That tube creates it's negative voltage by rectifying the signal from the horizontal oscillator. The grid and cathode act as a diode.

I wonder if there could be some carbonized (burnt) solder flux on one of the terminal strips that is conductive. Check to see if there is any way some burnt flux of other junk could be conducting some of the voltage from the horizontal output grid to the 12AX7.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jan Sat 06, 2018 11:06 am 
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Sorry! Didn't read the whole story! To me, a 12AX7 is a valve, not a tube! A CRT is a tube!
I assume you did get the CRT heater working. Do you have a raster yet? No point probing the AGC until you do.

Anyway.....

Such a high negative voltage on the grid sounds like you've got some oscillation happening there.
I would suggest that you put the ORIGINAL 12AX7 back in. Those new ones you buy today are made for audio, not TVs. I've had the same trouble with a Sovtek bottle (tube!) recently. The old one WILL be OK in this circuit.

If that doesn't affect it, try removing the last video IF amplifier tube. What voltage do you get then?


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