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 Post subject: Motorola VT-71 tv (need help!)
PostPosted: May Sat 04, 2019 3:27 am 
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Location: California
Hi all,

I recently bought a Motorola VT-71 tv, from I think 1948. It has a TS-4H chassis. This is only the second antique tv I'm going to try to fix, so I'm still pretty new at this. (The last one was an Olympic model 755 I restored over a year ago, which is still working well). Right now I'm trying to replace the electrolytic filter capacitors, but I'm having a really hard time with it. I looked for schematics, but it's incredibly confusing because there are conflicting diagrams that don't match up. I first found a "Sam's Photofact" diagram, but some of the connections are different. 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! For one thing, 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! There are so many wires connected to the capacitor cans and trying to trace them to components and compare them to schematics that might be different is giving me a headache! This is so aggravating. I've never had this much trouble just trying to replace filter capacitors before!

Can anyone help me make sense of the capacitors? :(

- Chris


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sat 04, 2019 3:57 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Rockaway , nj 07866
The electrolytics with wires running from one can to the other means they are connected to each other but not to the chassis ground , be careful of this , if in doubt do what I've done connect the new electrolytic ground to the can ground and the wire / wires going to the plus side of the can to the plus side of the the new cap , there's room to do this.
The 140mfd I thought was connected to a selenium rectifier , don't be shocked if you have to replace those selenium rectifiers , a 1N4007 will work ok , so you will need two .
Mike


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sat 04, 2019 6:16 am 
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My VT-71 schematics are hiding somewhere where I can't find them. But I think that I can give you one hint. The ballast is just a bunch of resistors inside of a metal or glass enclosure. The Sams might be showing it as separate resistors instead of a ballast tube.

I seem to remember that there were 2 different ballast tubes used. There were almost the same except one had one resister omitted and one connection moved to a different pin.

There may in fact be three different types of grounds or commons. One would be the chassis which is mostly just used as a shield and a high frequency ground.

I will see if I can find my schematics.

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Tom


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sat 04, 2019 7:24 am 
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Joined: Jan Tue 03, 2017 6:43 am
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Location: California
Thanks for the replies so far.

Tom, I hope you are able to find your schematics. That would be a huge help! Maybe yours will make some sense! I'm getting nowhere.

And here I thought this tv would be easier to fix because it's smaller. Ha! Yeah, right!


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sat 04, 2019 6:03 pm 
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I can't find the schematics yet, so the following is from memory.

The chassis is not used for the main circuit ground. The chassis is connected to the main circuit ground through a capacitor and probably a fairly high value resistor. The capacitor bonds them together at high frequencies but separates them at power line frequencies. Most sets with this arrangement would have nothing connected to the chassis, but I think that this set has small high frequency bypass caps connected from the filament string to the chassis.

I believe that this set has a voltage doubler power supply with circuit ground connected to one side of the power line. The other side of the power line is connected to the rectifiers through a small value resistor and an electrolytic cap. One rectifier is connected between the output end of the electrolytic cap and circuit ground. When the power line swings negative, the capacitor charges up to about 140 volts because the rectifier to ground does not let the output end of the cap swing negative. When the power line swings positive to about 140 volts, the output end of the cap gets to about 280 volts. That is 140 volts from the power line plus 140 volts from the charge on the cap. The 280 volts flows through the other rectifier and charges up the first filter cap.

I hope this helps instead of confusing you.

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Tom


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sat 04, 2019 9:16 pm 
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Location: 06475
Early Television Foundation has lots of info on this set. Here is the Riders manual:

http://www.earlytelevision.org/pdf/moto ... er_tv1.pdf

They also have the Sams photofact and service manual for this set.

It's possible to cut the multisection can capacitors off and re stuff them without disturbing the wiring. Phil's Old Radios has an article on how to do this:

https://antiqueradio.org/RCACTC-7ColorT ... urbedBases

Hope this helps!


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sat 04, 2019 11:15 pm 
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Location: St. Louis, MO, USA
Take pictures of what you have and put labels on the wires. This has saved me more times than I can admit to.

Dennis


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sun 05, 2019 12:48 am 
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I think Tom has it right. I happen to have my Wallace's Telaid book for Motorolas (1946-53), and its schematic matches what he says. There are two ground symbolss that I see: the triangle of horizontal lines (chassis) and a triangle outline (common). Filled black triangles, also pointing down, are DC supplies (B+ and B++), but other schematics may have other symbols. Certainly they have other part designators.

I've scanned my schematic, or at least the power supply part: http://tv-boxes.com/tvs/TS-4H_power_supply.jpg
It seems pretty straightforward on the drawing, but I haven't yet tackled my VT-71 restoration, so it's sorta theoretical for me at the moment!

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


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sun 05, 2019 2:10 am 
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Ok, I think I'm finally starting to make a tiny bit of progress! It turns out that the negative side of the capacitor I was confused about (C1 in the Sams schematic) does in fact connect to the ballast tube! I just noticed an area labeled M5, which in the parts list is the ballast tube. In the diagram, it looked to me like C1 was connected to a separate resistor. But it's the ballast. So at least now that makes a little bit more sense! Now that part matches both the Sams and Riders diagram. I wish that was the only thing I was confused about. I have a long way to go!

Mark, unfortunately I can't find any symbols at all on the cans or chassis showing which capacitor is which, or the polarities. I know the cans themselves - C1, C2, C3, but I can't see anything that labels the sections. I tried looking with a flashlight but just don't see anything. The only way I've been able to figure anything out is just trying to trace the wires to components and match it up with diagrams. So far, the only ones I've figured out are C1 and the positive side of C2A. I don't know where the negative side of C2A is supposed to go. I guess I'm going to have to trace each wire and compare and cross-check it with the diagrams. eek!

I don't know enough about how these circuits work to really be able to tell how parts are supposed to be connected. I'm pretty much relying on schematics.

Why did they have to use multi-section capacitors?! :roll:

- Chris


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sun 05, 2019 3:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Crystal Bay, NV
The cans are stamped on the side of the aluminum, but the cardboard cover may be hiding that. Sometimes the cardboard is embossed with the ratings too.
Looking at the underside where the wires attach, you can see either a tringle, a half round or a square cut in the base. This tells you which section is which.
Often the schematics will show this symbol, other times you can refer to the parts list.
Of course, tracing the circuits from the schematic will do the job, but with more effort.
======
Ron


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sun 05, 2019 3:56 pm 
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TahoeTV wrote:
The cans are stamped on the side of the aluminum, but the cardboard cover may be hiding that. Sometimes the cardboard is embossed with the ratings too....
Often the schematics will show this symbol, other times you can refer to the parts list.
Of course, tracing the circuits from the schematic will do the job, but with more effort.
======
Ron


Of course, Ron is correct. I just looked at the Sams online and unfortunately, it doesn't show the symbol on the schematic of the low voltage power supply. Darn...

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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sun 05, 2019 5:39 pm 
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The cardboard cover on the cap cans came to mind last night after it was too late to post.
You can often remove the cardboard cover non-destructively if you heat the cardboard with a heat gun. The cover is often held on with tar.

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Tom


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Mon 06, 2019 5:20 am 
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Ok, 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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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Mon 06, 2019 7:58 am 
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...I just took another look at the Rider's schematic I have and it DOES show a .05 mfd capacitor connected from the negative to the chassis! (If I'm reading the connection symbols right).

Maybe I'm not losing my mind after all! :lol:

- Chris


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Mon 06, 2019 5:20 pm 
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One thing that can be confusing is when you find things connected to tube socket pins that are not shown in the schematic.
Many tubes have pins that are not used by the tube. Some manufactures use those unused lugs on the tube sockets as tie points instead of installing terminal strips. When you look at the schematic you will see what pin numbers are used by the tube. All pin numbers not shown are candidates for use as tie points.

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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Wed 08, 2019 5:18 am 
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Tonight, I decided to do a quick test with the tv. I was anxious to see if I at least connected the capacitors right.

Some good news, some bad news...

The good news is that nothing violently blew up or caught fire. :mrgreen: The ballast tube seems to be working and got warm. I saw the tubes glowing (although I couldn't tell if the CRT did anything). The rectifiers also didn't smoke or do anything else weird so they might be okay for now. But after a few seconds, I smelled burning and saw thin smoke coming from somewhere under the chassis. I found a resistor had burned...and wow, it really got cooked! I looked in the Sam's schematic and figured out that it's R87. It connects to pins 8 and 6 on the V14 tube. (It looks like they're using pin 6 as a tie point). I don't know what would cause the resistor to burn up like that, but the only thing I can think of is that there are some wax capacitors that are connected to the tube and I'm wondering if one of them is bad, maybe that caused excessive current to pass through the resistor. I think one of the high voltage capacitors also connects to it. I haven't replaced any of those capacitors yet because I didn't want to replace too many at once. I only replaced C1-C3 in the cans.

Tom, you were definitely right about them using unused pins as tie points. They really liked doing that in this tv!

I didn't see anything else that seemed to be burned or otherwise damaged. None of the other resitors connected to tube V14 were burned, and I carefully looked over the entire chassis. I even powered it up quickly one more time while I had the chassis turned over and I confirmed that R87 was the only thing burning. I'm glad that at least the area I worked on seemed to be okay! No smoke or boiling electrolytic capacitors over there!

Any suggestions on what might cause this one resistor to burn like this? Could maybe one of the wax capacitors or a high voltage cap be shorted? I'm thinking the problem has to be limited to somewhere in that spot because otherwise other components further up the circuit would have been damaged too.

- Chris


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Wed 08, 2019 4:21 pm 
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I'm too lazy to look up the schematic, but I can offer you some general advice. Your basic tool is your ohmmeter and voltmeter. When I power up a restored set, I hook my voltmeter to the B+ line. If all is well, the B+ will come up quickly (to more than the normal value), then sink back as the other tubes draw power. Of course, be on the lookout for any smoke or sparks. All the while, my hand is on the cheater cords, ready to yank the power.

In your case, I would use the ohmmeter to see if any shorts can be detected that way. If you have connected an electrolytic cap in the wrong direction, it can look like a short when voltage is applied.

======
Ron


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Thu 09, 2019 3:28 pm 
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Forget the Sams - it's for the TS-4J chassis.

You can get the TS-4H schematic here: https://www.earlytelevision.org/images/ ... 1_ts4h.jpg

and the entire original Motorola service manual covering all the revisions here: https://www.earlytelevision.org/pdf/mot ... manual.pdf

I believe you are talking about the 100 ohm cathode resistor on the 25L6 HV oscillator tube ? Likely the bias is wrong on the tube or it's not oscillating and drawing too much current.

Replace the paper caps around it. If the HV caps are the original paper ones, replace those too. If they are leaky they will load down the HV supply.


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Fri 10, 2019 1:25 am 
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Hey, thanks for reading my comment on YouTube!

Yeah, that's the same resistor. 100 ohms, 1 Watt, according to the parts list. (Though I'm going to get a higher wattage one when I replace it to be on the safe side).

The "bias" is a voltage applied to the grid of the tube? I'm trying to remember everything I learned from working on the Olympic tv. I'm still a bit new to all this. So would bad paper caps or high voltage caps cause a wrong bias?

Is there any possibility I could have connected the electrolytic caps wrong? I hope not! I deciphered the schematics the best I could! :shock:
Normally, I have no problems reading schematics, but the way this tv is connected is so insanely confusing!

I'll replace all the capacitors connected to that tube and go from there. I guess that's my next step. But if you can think of anything else, please let me know. Thanks for your help!

- Chris


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Fri 10, 2019 2:57 pm 
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Bad paper caps, out of tolerance resistors or power supply issues could throw the bias off. I suspect it's not oscillating because of bad paper caps. A power tube like that that is biased to be "on" but not oscillating will get really hot possibly even to the point where the plate glows red.


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