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Old 2nd May 2019, 3:25 am   #1
ct92404
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Default 1940's Motorola VT 71 tv

Hi all,

Well, I said earlier that I probably wouldn't get another antique tv...but I did! This is the second tv I'm going to try to restore! The Olympic model 755 tv from 1952 that I fixed over a year ago is still working great. Here is the thread about that one:

https://www.vintage-radio.net/forum/...d.php?t=140224

Recently, I bought a Motorola VT-71 tv. I looked it up, and I think it's from 1948. It needs a lot of work, but it's a really small tv and so I think it will be easier to work on. (I'll post some pictures soon). So far, I've just cleaned it. But I had a couple of questions that I was hoping someone might be able to help with.

I'm going to replace the filter capacitors before I even dare try to plug this thing in. But my question is if it's okay to replace electrolytic capacitors with a slightly lower capacitance value? One of the capacitors I need to replace is 60 microfarads, 150 volts, and I was going to replace it with a 47 microfarad 250 volt capacitor. The only other option I saw at the electronics shop was 100 microfarads and I thought that would be too high.

The tv also has selenium rectifiers, and I've read that they can go bad. Should I also replace them before I try to power the tv up? I'm anxious to test the tv to at least see if the CRT is still good, but I don't want to cause any damage.

Thanks for any help and suggestions!

- Chris
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Old 2nd May 2019, 4:30 am   #2
Argus25
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Default Re: 1940's Motorola VT 71 tv

Hi,
Don't power it without new electros fitted. All the original twist lock types are available from AES (antique electronic supplies USA, they have good mail order).

Your selenium rectifier may well be ok. If it has to be replaced, it will require a rectifier with a power resistor that drops about the same voltage (and generates about the same heat) as the original rectifier.

I'm pretty sure you have landed a model set (like my Admiral) that has/uses the amazing electrostatic H scan system invented by Faudel & White. Its very difficult to understand how this works from the schematic, this article will help you understand it:

http://worldphaco.com/uploads/Admiral_TV..pdf

I'd have to check the VT-71's schematic, but I think when I looked at it some years ago it was the same in this area...just checked, it is.
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Old 2nd May 2019, 8:23 am   #3
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Default Re: 1940's Motorola VT 71 tv

68uF (Microfarad) is a better option for your capacitor.
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Old 2nd May 2019, 8:26 am   #4
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Default Re: 1940's Motorola VT 71 tv

Perhaps use two 32uF, space permitting.
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Old 2nd May 2019, 11:51 pm   #5
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Default Re: 1940's Motorola VT 71 tv

When it comes to the electrolytics, its better to replace the twist lock ones with new ones than hanging axial caps under the chassis and cutting connections to the existing capacitors which sometimes act as supporting tags for components. You don't want to leave them connected as they will be leaky. If you remove them, then there are obvious chassis holes. So its best to order the correct replacement twist lock can style capacitors for a proper repair. It also improves the chassis appearance with the new capacitors.
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Old 3rd May 2019, 12:33 am   #6
ct92404
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Default Re: 1940's Motorola VT 71 tv

Thanks for the replies so far everyone.

I talked with another guy I know at an electronics surplus shop and he said that I really should use a higher value capacitor, and that 100 mfd would be fine. He also said that it would be okay to test the tv with the original selenium rectifiers. I was kind of hesitant about that, but he has helped me a lot with antique radios I've fixed in the past and the Olympic tv, so I generally trust his advice.

The capacitor cans have cardboard tubes covering them, and I was able to take them off. So what I was thinking of doing was to glue the new capacitors inside the tubes, then cutting the cans but leaving part of the bottom still attached to the chassis so that I could just fit the cardboard tubes back on. It will look original from the outside. I usually don't care too much about how the chassis looks, my main concern is just that it works and it's safe. But now that I have a little bit more experience, I'm going to try to take more care this time and make it a little neater. Well, I'll do my best anyway! I'm definitely not a professional!

I attached some pictures. I took them with my cell phone, I hope they're clear.
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Old 3rd May 2019, 1:29 pm   #7
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Default Re: 1940's Motorola VT 71 tv

A couple of points. Increasing the value of capacitors above what the original design had, doesn't often help anything.

It can result in higher peak currents in the region of the rectifier/s or higher voltages in the set, and more charge storage when the set is turned off.

It is possible that this can result in issues like CRT beam afterglow and phosphor burn at turn off, especially if the scan collapses prior to the crt EHT supply and or the CRT grid goes more positive with respect to the cathode.

So I would initially at least keep the uF values close to standard and if it turns out the values need increasing for some particular reason later, do it then. Go for higher voltage versions if they are a similar size.

Twist lock capacitors are a type I do not bother to rebuild, because new parts are available from AES (re-manufactured new stock) and the re-build won't be as good. I rebuild many other types that have not been re-manufactured though.
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Old 3rd May 2019, 2:25 pm   #8
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Default Re: 1940's Motorola VT 71 tv

Chris, out of interest, what TV channels does the set cover just A2 - A6 ? Or does it cover A7 - A13 as well?
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Old 3rd May 2019, 3:21 pm   #9
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Default Re: 1940's Motorola VT 71 tv

Nice Tv

I’ve got the Bakelite cased 7VT2, very similar inside to your set! Before I could get anything sensible from mine I had to replace the wax paper caps rated 6KV, the ones connected to the centering controls and the EHT smoother. I still have the selenium rectifiers fitted in mine, and although they are working they do occasionally let off a bit of a bad smell! Normally when the set is switched on from cold. Might be worth pulling the ballast tube out and checking that all the sections are OK. What I did with the filter caps was to cut them open carefully and remove the innards and replace them with a new capacitor, then stick the can back on. Although if new twist lock cans are available, I’d go with that instead!

Just Radios have all the 6KV rated capacitors for sale, good quality film caps too.

Regards
Lloyd
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Old 3rd May 2019, 11:47 pm   #10
ct92404
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Default Re: 1940's Motorola VT 71 tv

Yikes. I'm having kind of a hard time with the schematics. I've been doing some research and apparently there were a lot of revisions to the circuit. Some things in the schematic I found are different from my tv, so I had to look up another one. I'm having a hard time figuring out exactly how the filter capacitors are supposed to connect. It's so confusing!

The Olympic tv was tedious, but at least I was able to follow the diagrams fairly easily. This one is going to take me a lot longer! Even if I'm able to fix the electronics, the cabinet needs some work too. This is going to be a long project, that's for sure!

Lloyd, did you have any trouble with conflicting schematics on your tv?

Argus25, I'll definitely keep what you said under advisement.

Steve, to be honest I'm not sure exactly how the channels are set up. I think it's technically supposed to go up to channel 13, but I read somewhere that because of FCC regulations back then you had to adjust the tv depending on what area you lived in. I'm a little confused about it. I'll just be happy if I can get the tv to work at all! As long as I can get it tuned on Channel 3, then I hook up a DVD player to it or an RF modulator. I got an HDMI to video converter and an RF modulator so I can hook up my computer to the Olympic tv. I like look up old movies on YouTube and watching them on it for the authentic experience!

- Chris
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Old 4th May 2019, 1:00 am   #11
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Default Re: 1940's Motorola VT 71 tv

Quote:
Originally Posted by ct92404 View Post
I like look up old movies on YouTube and watching them on it for the authentic experience!
Chris, you hit the nail on the head there. That is the whole fun side of vintage TV restoration. I watch vintage shows on my vintage TV's and video monitors just for fun.

Some really great shows to watch, are Dr Who, Time Tunnel, Voyage to the Bottom of the Sea, Lost in Space and the brilliant Sci-Fi from the mid 1950's, Forbidden Planet. Then there are all the old Bogart movies and all the early Tarzan movies are pretty amusing. And there are the old Test patterns too.
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Old 4th May 2019, 2:24 am   #12
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Default Re: 1940's Motorola VT 71 tv

Quote:
Originally Posted by Lloyd 1985 View Post
I’ve got the Bakelite cased 7VT2, very similar inside to your set! Before I could get anything sensible from mine I had to replace the wax paper caps rated 6KV, the ones connected to the centering controls and the EHT smoother. I still have the selenium rectifiers fitted in mine, and although they are working they do occasionally let off a bit of a bad smell! Normally when the set is switched on from cold. Might be worth pulling the ballast tube out and checking that all the sections are OK. What I did with the filter caps was to cut them open carefully and remove the innards and replace them with a new capacitor, then stick the can back on. Although if new twist lock cans are available, I’d go with that instead!

Just Radios have all the 6KV rated capacitors for sale, good quality film caps too.
There's a lot of really solid advice in this entry!
First thing, you have to make sure you are using the proper schematic for this chassis, as there is several variations of this chassis. The voltage divider used for the B+ source is different from the various versions.
Another thing to note is the B- is floating and not tied to the chassis.
More later, as I read more entries from this thread.
Dave, Midwest USA.
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Old 4th May 2019, 3:05 am   #13
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Default Re: 1940's Motorola VT 71 tv

Dave, the schematics are driving me crazy! My tv has a TS-4H chassis. I found a "Sams Photofact" schematic for my tv, but some of the connections don't match up. On the actual tv, capacitor C1 (140 mfd) is connected to pin 1 of the "ballast tube." But on the Sams diagram, it's shown connected to a resistor. So I looked up another diagram and found a "Riders" diagram which is supposed to be for the TS-4H chassis. It does seem to show the negative side of C1 connected to the ballast. But the way the rest of the diagram is drawn is really confusing! There are like 3 different type of symbols used for ground or common connections.

I think I have the new capacitor for C1 connected now, but I'm completely lost on the others! This is so aggravating. I didn't have nearly this much trouble just trying to replace the filter capacitors on the Olympic tv!
Can you or anyone else here help me figure out where the heck the capacitors are supposed to connect? I've tried tracing the wires from the cans to the components, but with conflicting schematics I can't make sense of it and there are so many wires connected to the capacitors. It's driving me nuts!
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Old 4th May 2019, 3:32 am   #14
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Default Re: 1940's Motorola VT 71 tv

Quote:
I can't make sense of it and there are so many wires connected to the capacitors. It's driving me nuts!
That reminds me of a scene in a movie were there is a severed wiring loom with dozens of wires hanging out and the guy trying to fix it, with a horrified look on his face, remarks there are too many wires and he doesn't know what to do.

The thing to remember here, don't get overwhelmed by all the connections and wires.

Focus on only one at a time, be sure you know where it connects at each end and what the purpose of the connection is. Compare to the schematics, if it doesn't match, just document what you find and build up the schematic of the actual set you do have in front of you. Only then, move to the next wire to study. That way it is not so overwhelming.

Also note, depending on the design, if you see any of those twist lock electrolytics on a thin brown phenolic wafer mount, usually that is done because the negative of the capacitor does not connect to the chassis and its connection is elsewhere and often this is why a cardboard sleeve is put over the can on the chassis top as it avoids shocks/shorts.
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Old 4th May 2019, 10:30 am   #15
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Default Re: 1940's Motorola VT 71 tv

I did have some conflicts of schematics, my set uses the TS-18 chassis, and there were slight differences from the Sams that I used. I mostly just used the schematic as a rough guide, and when replacing components just do them one at a time, that way I only replaced like for like, which came back to bite me when I replaced a cap in the discriminator, only to find later it had previously been replaced with the wrong value to try and fix a fault with no sound sometime in the past! That’s when the schematic was really useful. You might find it useful to have several schematics to hand, as one part of the circuit will match one schematic and another will match a different one, there are still components in my set that I haven’t found on the schematic at all!

Oh, one thing to be careful of, there is 6KV or there abouts on the centering controls and a few other bits in that general area when the set is powered up, make sure your solder joints are not spiky here, keep fingers and multimeter probes away!

Regards
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Old 4th May 2019, 4:33 pm   #16
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Default Re: 1940's Motorola VT 71 tv

Quote:
Originally Posted by ct92404 View Post
Dave, the schematics are driving me crazy! My tv has a TS-4H chassis. I found a "Sams Photofact" schematic for my tv, but some of the connections don't match up. On the actual tv, capacitor C1 (140 mfd) is connected to pin 1 of the "ballast tube." But on the Sams diagram, it's shown connected to a resistor. So I looked up another diagram and found a "Riders" diagram which is supposed to be for the TS-4H chassis. It does seem to show the negative side of C1 connected to the ballast. But the way the rest of the diagram is drawn is really confusing! There are like 3 different type of symbols used for ground or common connections.

I think I have the new capacitor for C1 connected now, but I'm completely lost on the others! This is so aggravating. I didn't have nearly this much trouble just trying to replace the filter capacitors on the Olympic tv!
Can you or anyone else here help me figure out where the heck the capacitors are supposed to connect? I've tried tracing the wires from the cans to the components, but with conflicting schematics I can't make sense of it and there are so many wires connected to the capacitors. It's driving me nuts!
That set is a real dandy to follow the schematic regarding where the voltages are sourced from.
The 150v source, called B+ is derived from? I spent an hour looking and comparing schematics from all the different versions and still not sure where it is sourced from. I have three of those gems on the docket for restoration.
I keep on forgetting to order those HV coupling caps.
The C1 cap, 140mfd 150 volt 'lytic is the voltage doubling cap that goes to one section of the ballast as a surge limiter.
I'm only going to replace the 'lytics with outboard sections, as the sets aren't that valuable to replace with expensive custom made cans.
More later! Dave, Midwest USA.
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Old 5th May 2019, 4:35 am   #17
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Quote:
I'm only going to replace the 'lytics with outboard sections, as the sets aren't that valuable to replace with expensive custom made cans.
The new twist lock caps at AES are reasonably priced I think. They are not custom made, they are simply manufactured with the same tools as all the originals were, which I think were found at factories in Mexico.
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Old 6th May 2019, 7:07 am   #18
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Default Re: 1940's Motorola VT 71 tv

I'm posting this on another website, so sorry for the cross-posting if anyone is a member of both.

Right now I'm working on C3, using both the Sam's and Rider's schematics. I'm trying to replace the C3C capacitor (in the Sam's diagram). It's 20 mfd. I think I have it figured out, but there are a few things that are confusing me.

The positive side of the capacitor is supposed to connect to pin 8 of V10 (25L6GT tube). It's also supposed to connect across a 330 ohm resistor. So far, that seems to match in the actual tv. But the resistor connects from pin 8 of V10 to pin 6 of V9 and I can't find anything in either the Sam's or Rider's diagrams that show that. According to the diagram, the negative side of C3C and one lead of the resistor are supposed to connect to what seems to be a common negative. But in the tv, they also connect to a .05 mfd 600 volt capacitor, and the other side of that capacitor is connected to the chassis. That doesn't make any sense. There is a .05 mfd capacitor in the diagrams which might be the same one, but it's connected differently.

Another question: Can I connect the negative side of C3C to the negative side of rectifier M1? In the diagrams, it looks like M1 connects to a common negative anyway. And a wire broke loose from the rectifier earlier, which I think was supposed to connect to C3.

I hope the way I described all this isn't too confusing! I took some notes, and I can make a drawing if that would help any. I feel like I'm starting to make a tiny bit of progress with this little tv, but I still feel lost!

- Chris
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Old 6th May 2019, 8:34 am   #19
ct92404
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Default Re: 1940's Motorola VT 71 tv

...Well, it looks like one mystery might be solved. I took another quick look at the Rider's schematic and it actually DOES show another .05 mfd capacitor connected from negative to the chassis. The Sam's diagram doesn't show that at all from what I can tell. Geez I'm really going to have to double check and cross-reference everything! But at least now I know that capacitor is supposed to be there!
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Old 6th May 2019, 5:31 pm   #20
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Default Re: 1940's Motorola VT 71 tv

Quote:
Originally Posted by ct92404 View Post
I'm posting this on another website, so sorry for the cross-posting if anyone is a member of both.

Right now I'm working on C3, using both the Sam's and Rider's schematics. I'm trying to replace the C3C capacitor (in the Sam's diagram). It's 20 mfd. I think I have it figured out, but there are a few things that are confusing me.

The positive side of the capacitor is supposed to connect to pin 8 of V10 (25L6GT tube). It's also supposed to connect across a 330 ohm resistor. So far, that seems to match in the actual tv. But the resistor connects from pin 8 of V10 to pin 6 of V9 and I can't find anything in either the Sam's or Rider's diagrams that show that. According to the diagram, the negative side of C3C and one lead of the resistor are supposed to connect to what seems to be a common negative. But in the tv, they also connect to a .05 mfd 600 volt capacitor, and the other side of that capacitor is connected to the chassis. That doesn't make any sense. There is a .05 mfd capacitor in the diagrams which might be the same one, but it's connected differently.

Another question: Can I connect the negative side of C3C to the negative side of rectifier M1? In the diagrams, it looks like M1 connects to a common negative anyway. And a wire broke loose from the rectifier earlier, which I think was supposed to connect to C3.

I hope the way I described all this isn't too confusing! I took some notes, and I can make a drawing if that would help any. I feel like I'm starting to make a tiny bit of progress with this little tv, but I still feel lost!

- Chris
The negative side of M1 does go to the common negative. Remove those tired selenium rectifiers and replace with silicon diodes. It'll give you a bit more wiring room. Install a multiple terminal strip in their place.
I saw your thread on ARF, a lot of great advice there as well! Not a member, but follow it.
I'm a member of Videocarma. That's how I got to be a member of this fine site.
Dave, Midwest USA.
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