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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Tue 27, 2018 6:41 am 
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Location: Belrose, NSW, Australia
Well done!

You're about 40% there in my experience. Now to get the rest of it going properly.

But at least you have that big oscilloscope screen to guide you!

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Mar Tue 27, 2018 5:21 pm 
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Location: Dallas, TX
WOW! That's quite a turn around. Congratulations!
Raster also!
Glad you didn't chuck the tube.

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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 Thu 29, 2018 12:26 am 
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Location: Belrose, NSW, Australia
Thinking.....

There must have been a reason the bad pin was already full of solder. Someone has been there before!!

You might have to suck all the solder out and re-do it with new flux, if the joint turns out to be intermittent.

Such things are the details you need to observe so as to get a vintage TV that will continue to work reliably.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Apr Sun 15, 2018 10:09 pm 
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Location: Chicago
Ok. Now I'm over the high of getting power to the CRT and getting a raster. Now I'm turning my attention to getting the picture on the tube. One item that I need a little advice on is this part of the schematic:

Image

I'm confused by the connection to C43 and R43. Is that from Pins 3 and 9? That "3-9" part isn't clear to me.

Thanks for your help!

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Apr Mon 16, 2018 1:23 am 
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Location: Belrose, NSW, Australia
Pins 3 and 9 are connected together inside the tube, They should also be connected to pin 1, the cathode.

You'll find this site to be a useful reference:

https://frank.pocnet.net/sheets1.html

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jul Sun 15, 2018 9:59 pm 
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I'm still messing around with this, although not too much since April due to other projects and work travel.

I lost the raster during testing with my oscilloscope, but believe that this is some issue I can find. The Pin 10 voltage is down to 260V from 340V, so I believe that there something wrong with that circuit.

Hope to have this done this fall.

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jul Sat 21, 2018 1:37 am 
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yes check all components in the 340 volt line. replace any bad ones. i dont have a schematic in front of me. i would say the problem has to be in the 340 volt line.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jul Sat 21, 2018 10:37 pm 
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Ok. Got the raster back. There were a few resistors near the horizontal drive that had floated just beyond the 20% tolerance and I found one wire that might or might not have been disconnected at the horizontal drive. I reattached and am back to a good space.

Now to hunt down the signal problem....

cheers,

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jul Mon 23, 2018 4:10 am 
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ok your no picture sound working points to a problem in the video section. testing tubes and voltage measurements at the tube sockets in the video section should point to a problem.

here is where I would use the b&k television analyst and signal injection to narrow it down further. this is invaluable in locating the defective stage.

since you already had trouble with pins not soldered in the crt socket you could try touching up the solder on the rest of the pins just incase.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jul Mon 23, 2018 4:48 am 
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Location: Belrose, NSW, Australia
Raster shows brightly with no retrace lines so CRT connections are OK.
If it's not broken, don't fix it.

I'd be looking for B+ and AGC voltages on the vision IF and the tuner before I'd even consider pulling a tube. If there is a component fault in the AGC it will develop a large negative voltage and cut the VIF stages completely off. With gear of this age, tubes in the small signal stages are not the most common cause of trouble. They can be down to 30 or 40% emission and still work just fine on the signal from your modulator.

As a general strategy I normally start with the cathode voltages on each if the IF stages. If you get maybe 1 volt on the cathode of a stage is is most likely OK. Move on and check the next one.
If you get zero volts, check the AGC on G1 next. If that's also zero or close to, check G2 and plate voltages. If zero or close to, you have an open circuit resistor most likely. If G2 and plate voltages read the same as B+ you have a dead tube (or an O/C heater connection or a bad socket).

The other thing to remember is that resistors can be more than 20% off their marked value without stopping it from working. So don't sweat the small stuff.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Jul Sat 28, 2018 5:26 pm 
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Got it working and it may have been a bad tube that I replaced. There was also a small spark happening in a wire so I placed a piece of wire sheathing in there and no longer experience the problem. Thanks for all your help everyone!

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Nov Sun 04, 2018 3:03 am 
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I've finally had some time to go through and check the waveforms using my analog oscilloscope to try to get the picture back on this tv. Here's where things stand:

I currently have a strong raster, but no picture signal to the CRT. I don't really see any of the squiggly lines that seem to indicate the tv signal:

Image

So I went in to test the tubes and all seems good, but then I found this broken coil that plugs into a cable from the tv tuner to the chassis:

Image

I'm pretty sure that I did this when I was testing all the resistors because I thought that there was a problem there.

According to the schematic, the 10k resistor seen in the above picture should connect with the coil, but it no longer does:

Image

I've never seen what the coil looks like on the inside and wondering if there's something that I can do to see if this is the issue or not.

Any thoughts are appreciated!

craig.


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Nov Sun 04, 2018 6:26 am 
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Location: Belrose, NSW, Australia
That coil certainly looks cooked. My guess is the tuner's had 6.3v and ground reversed at some stage and the lower part is now open circuit. There's been a heavy current flow through that coil. It would certainly explain why you have no signal from the tuner. I'd be checking what's at the other end of the IF feed cable too....

There's not that many turns on it so you should be able to re-create it. Tuning of that coil falls within IF alignment, it affects the tilt of the IF passband. You may be able to re-tune it visually while displaying a test card and rocking the fine tuning back and forth - that's how I used to do it back in the day when fitting an exchange tuner to a chassis.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Nov Sun 04, 2018 8:55 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 1437
Location: Dallas, TX
Somethings are easier to fix than others. This may be a tough one.
Maybe the coil (L10 I think) is just dirty, looks like maybe the terminal tore off when you were
testing the 10K. If that is the case then you need to reattach the terminal and find the end of
the coil winding wire and reattach it to the terminal. You might have to unwind a turn for the
coil. A delicate job in a tight spot. Then you will have to re-adjust L10. There is a ferrite slug
inside the coil that has threads like a setscrew. You probably will need a non-metalic hex tool
to turn it.
You need a high temperature non-conductive glue to attach the terminal, epoxy might work.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Nov Mon 05, 2018 12:14 am 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 462
Location: New Hampshire
Not sure where to start but I assume the symptom is........
Bright raster with nothing else seen AND no sound ?

Now lets talk tuners & IF's.
You need both tuner & IF's working to get a pix & sound. A crude check of the IF's is
change the channels ! You should see all those tuner contacts making & breaking in the
raster. You should also hear it in the sound. If you dont your IF is dead !
If you hear / see all that crashing while changing channel your tuner is dead !

A tuner needs
Antenna input
B+ ( typ 150 VDC )
AGC voltage ( a few volts +- )
Filament voltage
And it outputs at the IF jack.........

Tuner repair........ Try tubes, carefully clean, fix mechanical things like fine tune gears.
ANYTHING else consider last resort. Just moving things can throw it off. Same with the wrong
type of part. Besides its crowded in there & not fun to deal with !

I think the 10K has been changed at some point. It doesnt have the grey patina of original work.
The scope pattern has a "ringing " look to it, ignore it for now.

gotta go eat
73 Zeno 8)
LFOD !


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Nov Mon 05, 2018 3:28 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 552
Location: Belrose, NSW, Australia
To me, it looks like the bottom part of the winding, from the tap to ground, has been burned away.

Can you measure something less than 1 ohm from the tuner output (inner of the RCA socket) to ground? If not, that confirms my suspicion. I have seen this happen before.....

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 27, 2018 1:30 am 
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Well I've had some time pre and post Christmas to look at this in more detail.

Fortunately, I have a working tuner from another Zenith tv and to eliminate the variable of the bad coil in the original tuner, I swapped the working one into this tv. I checked the schematics of both tuners and they are replicas of each other so I feel this should work.

Next I tested the tv and still receive the raster, but no picture. When I turn the dial, still no change in the tuner visual, which indicates to me that the signal is not getting to the screen.

So next I took quite a few voltage measurements around the IF and AGC stage, based upon the recommendations of irob2345 and zeno. Here are my list of voltage measurements and schematics for the IF and AGC stages:

Left voltage is expected, right is actual, colors are for the attached wire extensions to know which pin is which.

Image

and here are the schematics:

Image

Image

Image

The V4 has a special designation and I'm not sure if those voltage measurements are valid due to that black dot next to the tube pin numbers.

I also tested these tubes today to ensure that they are all in the acceptable range and aren't affecting the voltage readings.

There's a lot of the pin voltages that seem off so I hope that someone out there has some ideas of what to check next. Thank you all!


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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 27, 2018 2:13 am 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
There should be a key (footnote) somewhere on the schematic that explains the meaning of the black dots.
many times it indicated that the negative lead of the meter needs to go to somewhere else than ground
or B-.
The devil can be in the details, how long are the wire extensions? Attaching long wires to high frequency
circuits could cause them to oscillate.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 27, 2018 2:13 am 
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Location: Ann Arbor, MI
cvsolfari wrote:
The V4 has a special designation and I'm not sure if those voltage measurements are valid due to that black dot next to the tube pin numbers.

look around the schematic for a note that tells you what that black dot means. It will probably tell you to put the negative lead of the meter somewhere else than ground.

Two voltages stand out and could be the cause of the other voltages being off. Pin 1 of V3 being -18.8 volts and pin 6 of V7 being -16.1 volts. I expect that those 2 pins are linked through a few resistors with perhaps a bypass cap. The partial schematics posted don't show that section of the circuit.
Try adjusting the AGC control and see if those voltages will change.

Edit: posting at the same time as Notimetolooz.
Edit2: That negative voltage on pin 1 of V3 would turn the tube off so that no signal would pass through it.

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 Post subject: Re: 1955 Zenith TV No Sound
PostPosted: Dec Thu 27, 2018 2:21 am 
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Location: Dallas, TX
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).
Whoops! Pin 5 is the plate.

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


Last edited by Notimetolooz on Dec Fri 28, 2018 8:51 pm, edited 1 time in total.

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