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 Post subject: Update-Pictures of the Working Magnavox CT-220
PostPosted: Apr Sat 04, 2020 2:44 pm 
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Location: Hamilton Square, New Jersey
More pictures taken just now. I bought this whole unit ten years ago in Manhattan for $20. A gentleman had purchased a residential building on East 36th Street. The final tenant had passed on. The building owner’s daughter had listed the contents on Craigslist. I restored the lower unit shortly after bringing it home, but the television sat in my basement for 10 years. This is the Magnavox Windsor Imperial. It has a decal that reads “Liberty Music Shops-New York City. Now you see it playing New York City TV stations picked up off of the antenna just as when it was new.


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Last edited by DarrenWGaransi on Apr Sun 19, 2020 3:05 pm, edited 3 times in total.
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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sat 04, 2020 6:22 pm 
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Posts: 5332
Location: Woodinville, WA USA
If you're getting a decent picture on the screen, then your HV is likely in the right ballpark, but measuring is always useful.

I am looking at the Riders service manual available from the ETF website (http://www.earlytelevision.org/pdf/magn ... ider_3.pdf). That schematic shows 9KV for the picture tube second anode, which is normal for a common CRT like the 10BP4. The manual lists four possible CRTs (10BP4, etc.). You can look up the data sheet for the particular CRT used in your set (go to https://frank.pocnet.net/ ) and find the maximum HV and typical HV levels for that tube. For instance, the 10BP4 CRT has a typical operating voltage of 9KV at the 2nd anode, just as shown in your schematic (https://frank.pocnet.net/sheets/049/1/10BP4.pdf).

When measuring HV at the 2nd anode, you can temporarily unplug the HV lead and secure it safely to a spot where it won't short against anything, and measure the HV there. You may also be able to slip the probe under the cap without removing the lead completely.

As you guessed, ALL of the remaining paper caps and electrolytics need to be replaced. It's often a waste of time to trouble-shoot specific symptoms (loss of horizontal lock, whatever) until that's done. If you need help identifying which caps to replace, see the recapping article at https://antiqueradio.org/recap.htm . For instance, there is a type of flat molded paper cap (made by Micamold and other companies) that looks similar to a flat mica capacitor, but is simply a paper (i.e., bad) cap inside a plastic shell. Paper caps typically fail by leaking, not by drifting in capacitance value or going open.

It sounds like you're off to a great start! If you still have issues after you finish recapping, they should be straightforward to diagnose. That should be an awesome set to use and enjoy, once it is fully restored.

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sat 04, 2020 7:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Lafayette, CO
All of the early Magnavox televisions l've seen were of top-notch quality. Didn't take much to bring back to life, that is, just the usual suspects. Nice piece you have there, let's see pictures when you're done. Craig


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sat 04, 2020 8:57 pm 
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Joined: May Tue 01, 2007 4:01 am
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Location: Hamilton Square, New Jersey
Thanks for the feedback guys. I have another question. The paperwork states "all voltage readings taken at 1 megohm. Does that mean I should place a 1 meg resistor in series with the multimeter?


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sat 04, 2020 10:15 pm 
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No, the 1 megohm means the impedance of the meter itself. In this case, probably a VTVM (Vacuum Tube VoltMeter), rather than a VOM (Volt-Ohm-Milliameter). Nowadays the better digital meters replace a VTVM with more than 1 meg input resistance, but cheap ones may not.

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A collector of TV signal boosters and UHF converters -- God help me!
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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sat 04, 2020 11:55 pm 
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Location: Dallas, TX
You do know the 2nd anode of the CRT can store a charge very well, I hope.
Best to measure the HV with one hand in your pocket or do it completely hand free if possible.

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


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sun 05, 2020 12:26 am 
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Location: Hamilton Square, New Jersey
Well now the horizontal won’t lock at all. I can get the picture to become stationary but it won’t lock into place. I guess I have to wait until the rest of the caps arrive.


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sun 05, 2020 1:10 am 
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Location: Belrose, NSW, Australia
Always wise to replace caps a few at a time and check if it still works. Easier to find your mistakes that way!

Also take lots of pictures.

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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sun 05, 2020 3:41 pm 
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Joined: May Thu 14, 2015 4:15 pm
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Location: Dallas, TX
Horizontal sync is one of the circuits that is easily effected by weak components. It almost works but not well enough. A capacitor or two, resistors, tube, maladjusted coil, or sometimes diodes can all cause sync problems. Everything has to be in good shape to get the sync solid. Some set designs were better than others. By the 1980s or so, the circuits were designed such that the sync was very robust because different methods were used since integrated circuits were available.

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


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sun 05, 2020 4:16 pm 
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Joined: May Tue 01, 2007 4:01 am
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Location: Hamilton Square, New Jersey
I know I'm kind of stabbing in the dark until I replace all of the caps. I've ordered them from JustRadios, but I'm so impatient being cooped up at home. I figured it would be fun and aid in my learning to figure out how each failing cap or resistor affects the slippery slope. I have learned that if I connect a random piece of wire to the other side of a 180pf cap, connected to the grid of the horizontal oscillator tube, that the horizontal sync comes back. I think that the piece of wire is picking up the sync signal somewhere since the cheap converter box is right next to the chassis, and supplying it to the horizontal drive circuit. I will check some of the caps and resistors that affect that area today. I know that when I do a complete cap replacement, I might solve the problem but I will never know which actual part caused it. Fun to learn. I wish I had the brains of the Shango066 gentleman on Youtube.


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sun 05, 2020 5:26 pm 
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DarrenWGaransi wrote:
I have learned that if I connect a random piece of wire to the other side of a 180pf cap, connected to the grid of the horizontal oscillator tube, that the horizontal sync comes back. I think that the piece of wire is picking up the sync signal somewhere since the cheap converter box is right next to the chassis, and supplying it to the horizontal drive circuit.

It is more likely that you are slightly increasing the capacitance at that point. A capacitor is just two conductors separated by an insulator. So that wire and the rest of everything form a small capacitor. Note that there is an adjustable coil attached there. You very likely will end up having to slightly adjust that coil.

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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Sun 05, 2020 9:33 pm 
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Joined: May Tue 01, 2007 4:01 am
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Location: Hamilton Square, New Jersey
Well it was a 12AU7 video amp tube causing the problem. I had an extra one in my stash. The picture has stayed locked for over an hour now. It will be good to see how much things improve further when all the replacement caps are in.


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Mon 06, 2020 12:15 am 
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Good sleuthing (though it sounds like a bunch of luck too). Spend some time reading up on those old TV circuits so you'll have a better idea where to look as you troubleshoot. Your audio tube knowledge will help a lot, but only to a certain point, since TV circuits are much more varied in function and configuration than audio. The subtleties in various sets' designs can trip up or baffle even experienced servicers.

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A collector of TV signal boosters and UHF converters -- God help me!
tv-boxes.com


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Mon 06, 2020 5:03 pm 
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DarrenWGaransi wrote:
I know I'm kind of stabbing in the dark until I replace all of the caps. I've ordered them from JustRadios, but I'm so impatient being cooped up at home.

I avoid buying from just radios unless I absolutely have to. Because they are in Canada when they send to the US things can get stuck in customs for around a month in normal times (I don't want to know what covid19 does to customs times).

Most parts you could need can be ordered cheaper off digikey or mousers websites ( especially if you work on enough stuff and buy a years worth of cap stock at a time to hit the 10-100 pieces per part volume discount).
Both places can overnight you your order which is expensive, but their budget shipping option usually takes less than a week which is always faster than just radios in my experience.

I'm glad I keep stock and made my regular stock up recently. My family insists everything new stay in the garage a few days to decontaminate it...it would drive me nuts having parts I need on hand but not being able to use them.


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Mon 06, 2020 9:44 pm 
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Electronic Memory wrote:
DarrenWGaransi wrote:
I know I'm kind of stabbing in the dark until I replace all of the caps. I've ordered them from JustRadios, but I'm so impatient being cooped up at home.

I avoid buying from just radios unless I absolutely have to. Because they are in Canada when they send to the US things can get stuck in customs for around a month in normal times (I don't want to know what covid19 does to customs times).....


I understand. But I just bought capacitors from them and they came quickly and exactly as ordered. I just guess YMMV...

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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Tue 07, 2020 4:36 am 
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Joined: May Sun 07, 2017 11:35 am
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Location: Belrose, NSW, Australia
A supplier in the US I've used for vintage and NOS stuff is Tube Depot. Under a week to ship to Australia, should be even quicker in the States.

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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Tue 07, 2020 12:29 pm 
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Joined: May Tue 01, 2007 4:01 am
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Location: Hamilton Square, New Jersey
I agree that Justradios takes a little longer, but for me I usually receive my order within a week of shipping. I’ve really liked their service over the years.


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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Tue 07, 2020 3:28 pm 
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Don't forget to check the mica capacitors in the horizontal AFC and oscillator circuits, as they are often a source of trouble. When you finish replacement of the components in there, perform the alignment. If everything is OK, that circuit works very well.

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 Post subject: Re: My first TV project-1948 Magnavox CT-220
PostPosted: Apr Tue 07, 2020 6:31 pm 
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Joined: May Tue 01, 2007 4:01 am
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Location: Hamilton Square, New Jersey
Thanks Tim. I ordered micas as I know that the micamolds were awful.


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 Post subject: 1948 Magnavox CT-220 Low Anode Voltage?
PostPosted: Apr Mon 13, 2020 11:35 pm 
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Joined: May Tue 01, 2007 4:01 am
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Location: Hamilton Square, New Jersey
Hi Everyone,

I just received a high voltage test probe in the mail. The anode voltage is 7kv. Is this low? I've read the spec sheet for the 12KP4 tube and the operating voltage is supposed to be 11kv. I have replaced the high voltage rectifier tube. I am still awaiting the capacitor delivery. I must note that the AC out of the transformer is 60 volts higher than the SAMS sheet, as is the DC B+. I figured it was a typo though. Any thoughts?


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