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 Post subject: Admiral 12x12 - Newbie
PostPosted: Jun Fri 07, 2019 4:02 pm 
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Posts: 42
Location: St. Louis, MO
Hi All,

I've restored several dozen antique broadcast radios and communications receivers. This week a friend of mine came across an Admiral 12x12 similar to what he had when he was a kid. The picture tube is supposedly good, and we are taking this as an opportunity to learn some details about NTSC and television circuitry.

1. Any good general advice for working on this type of set? I'm aware of how the flyback transformer works and the safety issues surrounding the 9 kV supply to the picture tube. Aside from the usual recapping and checking the various resistors and inductors, are there any other technical issues specific to a TV that we should be aware of?

2. Any good reading material or other references for understanding the circuitry involved? I've got the Sams for this set, which has enough information to do the restoration. That said, part of our goal is to understand the circuitry.

3. Can anyone give a general explanation of how the vertical sync works? The horizontal looks simple enough- the amplifier tube will invert the signal 180 degrees, and the next tube in the series is biased so that it only switches on when the now positive horizontal sync pulse comes through. The output of the tube controls the horizontal oscillator. (it also supplies the pulses to the flyback transformer to generate the high voltage for the tube anode) That said, I'm not sure I understand how the vertical sync tells the difference between the horizontal and vertical sync pulses. References to good books or other information would be appreciated.

Thanks for the help!

Roger


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Fri 07, 2019 5:25 pm 
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rbc7snc wrote:
Can anyone give a general explanation of how the vertical sync works?

The vertical sync is passed through a filter commonly called the vertical integrator. You should see several stages of the sync passing through a series resistor with a cap to ground. That removes the horizontal sync and leaves just the vertical sync.

Note that in newer sets the horizontal sync is much fancier. With a simple horizontal sync, a noisy received signal will cause the horizontal sync to be poor. Newer sets used a circuit that we would now call a phased lock loop. One common circuit looks somewhat like an FM radio detector with a filter cap added.

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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Fri 07, 2019 6:05 pm 
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Welcome to the deep end of the pool, Rodger.
That's a good question about reference material for TV systems.
Here is a good starting point, of course the color info doesn't apply, that's a whole other level of complications.
http://datagenetics.com/blog/april12018/index.html

Also, if you didn't already, check out the Wiki page. It actually explains more than NTSC for some reason. Note the section "Transmission Modulation Method".
https://en.wikipedia.org/wiki/NTSC

I think the horizontal is more complex than the vertical.

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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Fri 07, 2019 6:20 pm 
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RCA and some others were using the synchroguide PLL circuit from their first post WWII models onwards. Basically the H AFC diodes would compare sync phase with H sweep phase and send a correction voltage to the H osc to adjust it's frequency slightly to hold it in phase.

When solid state sets started becoming the norm they designed digital counter circuits that would count down to the expected time of H and V synch look for the pulse sync to it if found and repeat. They were so stable that the video and sound could be indestinguishable and the sync hard to see on a scope but still hold sync with only a few pulses a frame being decent.

A TV is a wide band AM radio (driving a video monitor?) and an FM radio driving an audio amp. Don't attempt to adjust the video alignment you need a calibrated sweep/marker generator, an oscilloscope, the service alignment procedure and a lot of patience to make it better. Don't change any mica ceramic or other NON-paper dielectric caps in the tuner and IF stages... doing so will mess with alignment...also try and maintain lead dress there.

Sync is basically a non-linear amp that clips video and anything below the sync threshold. The sweep synchs being separated by filters.

AGC is a circuit that checks video level and tries to hold sync level constant by sending a feedback voltage to tuner and or IF to throttle their gain...Keyed AGC takes a horizontal output pulse and gates only the horizontal sync to look at to track video level, non-keyed (cheap AGC) averages the whole video signal over more than 1 sync cycle.

Paper caps should be replaced with more critical eye for capacitance value. Using .0022 and .0033 for .002 and .003 is fine but using a .0047 (instead of a .0039) to replace a .004 might not be best.


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Fri 07, 2019 8:48 pm 
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Location: New Hampshire
Not to start a food fight :D Get the power supply then the HV up before recapping.
Once you do that you know if 3 of the big four parts are OK ( flyback, yoke, CRT ).
After that work stage by stage starting with any with problems.
Recap a few at a time rechecking as you go. The hardest problems are self induced !

For reading check big libraries first. They may still have the old books.
Learn the block diagram of a B&W set. What each block does & the symptoms
when it does not work. I can ( and have ) taken a few younguns with just very basic
electronics & made them techs. Everything was block diagrams & divide and conquer
as Sencore used to say.
Last get a good scope & use your set as a guinea pig after its running. Nothing like
seeing things to understand what a good wave form looks like.

GL & keep us posted on your progress.
73 Zeno 8)
LFOD !


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Sat 08, 2019 1:42 am 
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You should check out Bob Andersen's channel on YouTube, he has brought several Admirals of that era back to life and knows a ton about them.


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Sat 08, 2019 4:13 am 
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I don't happen to have any good books on the single subject of TV circuits. I've got a lot of general electronics books and many magazine articles about TVs that I gathered over the years. I went to a HAM convention today and there was someone there selling a lot of older books. I found one that would be good, I am sure there are others. "Television Simplified" by Kiver, 1948. I think that it is interesting that this "Simplified" book is over 400 pages long. It does cover TV cameras and transmission some however.

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


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Sat 08, 2019 9:11 pm 
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Location: Dallas, TX
I just remembered something else I happened to see at the Ham convention.
A copy of a magazine, "Radio & TV News" from October of 1958. It has an interesting article on
page 61, "Key Test Points in TV sets.". The article was written a good many years after your set
and circuits do change over the years. I had printed a copy of that article some time ago.
https://www.americanradiohistory.com/Ar ... 958-10.pdf

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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Mon 10, 2019 5:14 am 
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Joined: Mar Fri 28, 2014 10:04 pm
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Location: St. Louis, MO
Thanks for the advice.

We started the restoration today.

Electrolytics are on order, so we started with the tuner block. The section was removed, disassembled, cleaned, and lubricated. It was interesting to see how they handled the lead dressing given the high frequencies involved. 300 ohm transmission line right up until the last possible point, and then minuscule leads with tons of shielding. You could tell they put a lot of thought into problems with stray capacitance and transmission line issues. The oscillator and mixer were built directly into the tuner section to help with these issues. Fortunately, all of the components were in spec. Replacing parts on the tuner section would have been a real chore.

Once the signal had been mixed down to the video IF frequency, it went into a separate high frequency subassembly. This included the 1st, 2nd, and 3rd video IF amps, the video dector, video amp, audio IF amp, and ratio detector for FM audio. You could clearly see that they were still paying a lot of attention to lead dressing and shielding given the frequencies involved. We replaced the wax paper caps, and found several resistors that were out of tolerance. There were a couple of ceramic disc caps that looked overheated, and those will be replaced as well. After the video amp and ratio detectors, everything looks to be standard low frequency construction methods often see in radios of that era.

Question so far: How does the video detector work? My understanding is that the NTSC signal is modulated onto the carrier RF (and IF) using a vestigial sideband. At first glance, the video detector looks like a generic diode detector, and it's not clear to me how this happens. The earliest SSB receivers often used a BFO to reconstruct the missing side band. How exactly do you get the missing portion of the vestigial sideband with NTSC?


Thanks.

Roger


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Mon 10, 2019 3:33 pm 
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Location: Dallas, TX
rbc7snc wrote:
Thanks for the advice.




Question so far: How does the video detector work? My understanding is that the NTSC signal is modulated onto the carrier RF (and IF) using a vestigial sideband. At first glance, the video detector looks like a generic diode detector, and it's not clear to me how this happens. The earliest SSB receivers often used a BFO to reconstruct the missing side band. How exactly do you get the missing portion of the vestigial sideband with NTSC?


Thanks.

Roger

Sounds like you know your way around pretty good.
Yep, there seldom any paper cap in the tuner area, sometimes drifted resistors, usual advice is limit things to a careful cleaning.
The video processing before transmission involves reducing one sideband leaving only a vestigial on that side. The other sideband and carrier is left, so it isn't Single Sideband Suppressed Carrier. Just an AM detector is required. The sound is on a 4.5MHz subcarrier and is FM. They did a lot of clever things to get a lot of info into a 6MHz total spectrum space. Getting color and even stereo sound added in later years (and backward compatible) took things even further!

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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Mon 10, 2019 4:59 pm 
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Don't change ceramic capacitors unless they check open on a capacitor tester or leak current at rated voltage.... Unless those caps are in audio or sweep they are probably critical to alignment... only change paper dielectric and electrolytic caps until you know a part has failed.

I've seen roasted looking ceramic caps work perfectly and perfect looking ones be defective...


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Mon 10, 2019 5:36 pm 
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rbc7snc wrote:
How exactly do you get the missing portion of the vestigial sideband with NTSC?

The carrier is there at full strength and you have one complete sideband on one side of the carrier and a partial (vestigial) sideband on the other side of the carrier. The frequency response of the IF strip is wideband but not symmetrical. The set is tuned so that the carrier is at the 50% point of a slope. The vestigial sideband is on the slope from the 50% point down to zero. The matching part of the full sideband is on the slope from the 50% point up to 100%. The rest of the full sideband is on the flat part of the frequency response of the IF strip at 100%. So when things are added up you get a flat response for the whole video signal. But as you can now see, the fine tuning of the set has to be correct as things only come out correct when the carrier is at the 50% point of the slope.

If you now look at the alignment instructions for the IF strip, the weird alignment procedure will make sense.

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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Mon 10, 2019 7:34 pm 
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Tom,

Your comment just clarified a lot of questions that work percolating through my head.

A friend of mine has a B&K 415 sweep and marker generator. I plan to at least hook it up and take a look at the video IF response curve on my scope. I had been wondering why the shape of the curve was so wonky and why the markers were so important in the video IF alignment. This makes much more sense now. Without them, you would never get the carrier at the 50% point of the slope, and you would have either too much or too little RF energy in the portion with the vestigial side band relative to the portion that is single side band. It also lets you make sure the audio subcarrier is getting passed through as well. I don't have to worry about the chrominance subcarrier on this set, but I can understand why a marker would be necessary for that as well.

I think I'll take the advice I'm seeing here and leave the ceramic caps alone for the time being. There are a couple of resistors that were way out of tolerance on the high frequency section, otherwise it looked pretty good. Hopefully my friend and I will be able to finish up the high frequency section this evening and start moving through the rest of the set.

This has already been a great learning exercise, and I appreciate the helpful discussion.


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Mon 10, 2019 8:25 pm 
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The markers and RF/IF response curve also help define the video frequency response. Different horizontal switching rates define different video detail in the horizontal plane and if either end rolls off too fast or a center section of the bandwidth is notched out the video will take on strange image specific blurring. The video amps after the detector also shape the response so the 2 may be designed to counter each other's deficiencies.

A good way to see video response and see if the alignment is functionally correct is to take a multi burst pattern from a sencore VA 62 or test pattern disc and feed it in to the TV set...there are pictures of that burried here and or on Videokarma.org. Sometimes a set will look worse after an alignment because even though the response curve is (or appears to be right) the video response is somehow not very good.
Sets made after 1954 tend to roll off response curve at around 3MHz to avoid passing the 3.58MHz color carrier, while earlier sets (especially higher end models) tend to try and give as close to 4.5MHz video band width as they can.


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Thu 20, 2019 3:55 am 
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Hi All,

I finished recapping the Admiral this evening. About 40 new caps in all, as well as a handful of resistors that were out of tolerance.

I didn't have an NTSC signal source immediately on hand, but I decided to bring it up on the variac and see what happened. As it sits now, I'm getting a single horizontal band across the center of the picture tube. I'm also getting very little in the way of sound.

There is a 6SN7 tube that is part of both the audio circuit and the vertical oscillator circuit. I think I need to check my work around this 6SN7 (trying a different 6SN7 didn't change anything). My plan right now is to get a proper test source and have another look tomorrow.

Can anyone recommend a good reference that would show the expected waveforms at different portions of the vertical circuit?

I'm thinking that I should be able to take my scope and find the vertical sync signal at each tube in the vertical sync circuit. As long as I pull the horizontal output tube I shouldn't have to worry about voltages beyond the usual B+.

Thanks for the advice.


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Thu 20, 2019 3:47 pm 
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Since the vertical runs at about 60 cycles, why not use the filament circuit as a test source? Tag a .01 capacitor onto the oscillator grid. If the screen changes greatly, the output section is mostly working. No change, output circuit is open. Both transformers in the circuit may be flaky. Good start, Craig


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Thu 20, 2019 4:22 pm 
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Location: St. Louis, MO
That sounds like a good idea. I've got plenty of different signal generators that I could feed into the control grid to do the same thing with a carefully controlled voltage.

In generally, I like to debug circuits with my oscilloscope. It's certainly not the only way to look at a circuit, but I enjoy the visual aspect of seeing the waveform. Sometimes the problem is in an earlier stage, and it's only obvious when I see the waveform.

When I recapped the chassis I couldn't really check the yoke since there are resistors in parallel with the coils. If worse comes to worse, I can fire up my signal generator and measure the impedence in the deflection coils using an AC signal source and compare the results to the DC values. A friend of mine has an actual yoke tester, which may be an option as well. I think the first step is to take a look at my recapping work in that section and make sure there isn't a short or other mistake, and then check signal along the vertical sync pathway.

A friend suggested pulling the SAMS for a later, but similar model, as they often contain signal tracings at various points. I found one that showed the signals and various points in the vertical sync circuit, which should be helpful in debugging. I also came across a SAMS publication from 1949 on troubleshooting televisions which had a nice discussion of the vertical sync circuit as well as the expected waveforms (which match nicely to the SAMS from the later model Admiral)

Thanks for the info. I'll keep everyone posted once we get a little further along in the process. It has been a fun learning experience, but I still have a lot to learn.


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Thu 20, 2019 5:28 pm 
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rbc7snc wrote:
Hi All,


There is a 6SN7 tube that is part of both the audio circuit and the vertical oscillator circuit. I think I need to check my work around this 6SN7 (trying a different 6SN7 didn't change anything). My plan right now is to get a proper test source and have another look tomorrow.


What chassis do you have there? I would think it is a 20Z1, but that uses a 12AU7 for vertical oscillator and output.

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


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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Thu 20, 2019 7:30 pm 
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Notimetolooz wrote:
rbc7snc wrote:
Hi All,


There is a 6SN7 tube that is part of both the audio circuit and the vertical oscillator circuit. I think I need to check my work around this 6SN7 (trying a different 6SN7 didn't change anything). My plan right now is to get a proper test source and have another look tomorrow.


What chassis do you have there? I would think it is a 20Z1, but that uses a 12AU7 for vertical oscillator and output.

I have an Admiral (don't remember the model) with these tubes. One half of the 6SN7 is the first audio amplifier and the other section is the sync separator.

One thing that hasn't been mentioned yet is that many Admirals (and other makes as well) use what is known as a "stacked B+" circuit. The audio output tube, 6AS5 or similar, is fed by the main B+. But it's cathode isn't returned to ground. It is used to power the tuner and/or IF amplifier. So if the audio tube fails, it wipes out the picture. There is nothing wrong with that scheme but it can cause a lot of head scratching if you don't understand it.

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 Post subject: Re: Admiral 12x12 - Newbie
PostPosted: Jun Fri 21, 2019 3:48 am 
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Location: St. Louis, MO
Made another attempt at the TV today.

I found a good reference for signals along the vertical circuit. The waveforms looked correct up to and including the control grid of the vertical oscillator (1st half of the 12AU7), but the signal almost disappeared by the grid of the vertical output tube (2nd half of the 12AU7). Just for fun, I tried a different 12AU7, which produced a little bit of vertical deflection, but not enough to fill the screen and it didn't appear to be syncing correctly. After a few minutes, I noticed that R77 was overheating.

I think the 6SN7 in the vertical circuit is fine. Voltages are correct at each pin, and the signal looks correct going into and coming out of that stage. I also didn't see any obvious wiring errors in the vicinity of the 12AU7.

At first I thought it might be transformer T2, but after a careful check, the primary and secondary resistances seem to be correct.

How exactly does the 12AU7 vertical output tube get it's plate voltage? The plate voltage on the 12AU7 was high, but it's not immediately obvious to me how the plate supply works. Schematic is attached.

Thanks for the suggestions. I knew there would be some growing pains with a project like this.


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