View Full Version : Yet Another Motorola VT-71 Restoration


kvflyer
09-01-2010, 07:08 PM
This is my first attempt on a restoration of an Electrostatic Deflection TV. I remember Dad having one of these on the floor in the basement when I was a kid. I have his old Sams Photofact with voltage readings written in. That is what I used for this restoration. I got the set from the second owner, the son of the owner. Coming from eBay, I paid about $100 for it. So why so much? It was 100% complete with the knobs, bottom and back. The other VT-71 that I have is missing the back but has all of the knobs.


So as you know, the CRT, a 7JP4 is often bad. Over the years, I bought a spare from one of the members of this forum thinking that most likely, the set that I get will be bad. I have no way of testing the CRT.

So, lets get started. Here is the front of the cabinet.

http://antiqueradios.com/gallery/d/121082-2/Front_011.jpg


I has the usual rotted picture tube surround. I had to carefully rock the CRT back and forth to free it. And, as you guys have said, that stuff just came off with water. I was so happy that it was that easy.

I have a new reproduction mask that I got from an ARF member. So, that issue is solved. As for the cabinet, it is in fair condition. I will concentrate on the electronics first and go for the case later. Maybe even in Florida when I move there.

Note: This thread is also running on another forum so if it looks familiar, well, you know why. I am further ahead than this appears so bear with me and I will catch this thread up to the original one for those who have not seen it.

kvflyer
09-01-2010, 07:09 PM
Here is the top of the chassis. Nice and clean with a good copper plating. It should clean up very nicely.

http://antiqueradios.com/gallery/d/121089-2/Chassis_top.jpg

And here it is underneath before I start.

http://antiqueradios.com/gallery/d/121095-2/Chassis_under.jpg

More to follow!

kvflyer
09-01-2010, 07:10 PM
As you know, the first thing to go is the electrolytic capacitors. Here is the first one, a two section after the lip has been pried up. This set has a voltage doubler power supply and a "Hot Chassis" so all of the capacitors have cardboard sleeves.

http://antiqueradios.com/gallery/d/121105-2/DSCN2374.JPG

Here is how it looks close up prying up that lip.

http://antiqueradios.com/gallery/d/121108-2/DSCN2375.JPG

And after you yank the guts out, this is what is left. Note the 62 year old rubber band. It was still elastic!

http://antiqueradios.com/gallery/d/121111-2/Guts_Out.JPG

And here is what is now going inside of the aluminum can.



http://antiqueradios.com/gallery/d/121114-2/New_Caps_001.jpg

Almost ready to crimp the can back together. BTW, I tested each can before crimp. I didn't need any more surprises!

http://antiqueradios.com/gallery/d/121117-2/New_Parts.JPG

More to come...

kvflyer
09-01-2010, 07:11 PM
Looks like an original doesn't it? Mind you, there are three of these caps and some pictures are of one cap and the other pictures are of one of the other caps. But you get the idea. After I did two of these I got to thinking, "Why did I remove the wafer? I had to drill out the rivets. And then, I had to use screws and nuts to put them back. I could have just removed the capacitor from the wafer. I got smart after the second one...

http://antiqueradios.com/gallery/d/121120-2/Finished_Cap.JPG

kvflyer
09-01-2010, 07:13 PM
Here is an earlier picture that I didn't have time to upload yet. I will take close-up photos later and post them as well.

For all, these are about 1/2 of the capacitors replaced so far. All capacitors were carefully unsoldered and totally removed from the socket or lug. I left no pigtails behind. And, this time did not break a single lug!

http://antiqueradios.com/gallery/d/121202-2/Some_caps.JPG

kvflyer
09-01-2010, 07:16 PM
And here you can see the 6,000 WVDC capacitor that an electrostatic set needs. These little guys are available from Dave of Just Radios in Canada. They are not expensive. A lot smaller than those huge wax condensers.

http://antiqueradios.com/gallery/d/121205-2/DSCN2388.JPG

kvflyer
09-01-2010, 07:17 PM
More progress. Coming down the home stretch... as far as capacitors. Every resistor that I randomly checked was pretty close. I paid attention to the high values and they were still within 10%, some even 5% when marked with the silver stripe.

All caps except mica and disc have been replaced. What I found was any repairs done were done to the HV capacitors. ALL other components are original... or shall we say, "Were" original. Whomever did the repair work just snipped off the component and then tacked it back to the terminal. It made it easy to remove. But I got all traces of the old wire out of that little hole. Two mica capacitors were just tacked in. They measured perfect so they went back in the proper way.

http://antiqueradios.com/gallery/d/121241-2/Finished_Chassis.jpg

And this is the former home of the selenium rectifiers.

http://antiqueradios.com/gallery/d/121244-2/rectifiers_001.jpg

kvflyer
09-01-2010, 07:19 PM
Note: Please remember that this thread was running for about two weeks on another forum. So, please don't think that I work this fast!!!


I did the dim bulb power up (for the first time ever) and connected it to a variac with a Kill-A-Watt connected in series. The lamp glowed dull orange and the watt meter indicated about .56 amp. The Sams said it should be .96 amp. I got B+ on the diodes and since it is a doubler, I got half on one and the full amount on the one downstream.

The main reason for the dim bulb was that I am always unsure of a diode to selenium connection when they use a plus sign. The Sams took that guess work out of it and I mounted and connected them as per the drawing. No dead short so I figured it was OK to step up the power. When I got up to 110 VAC, the set was not working but no smoke.

I took the dim bulb out of the circuit and again started to bring up the voltage and there it was, a faint raster. The B+ was the same as my dad had marked on the Sams 50 years ago. Voltage up to 110 and this is what I got!

http://antiqueradios.com/gallery/d/121247-2/Raster.JPG

I got good focus and could see the scan lines clearly. It appears that the Horiz frequency is way off but remember, no signal. The controls on the back seem to move the trace around on the screen as they should.

Hooked up a B & K TV analyzer and did not get a test pattern but the tone was coming through. So, I have lots more to do. But I am glad to be where I am. I was able to test my 7JP4 tubes and they all work. The one in this picture is my spare. I will put the one out of my second VT-71 in this TV because it seems to be the strongest. The 7JP4 that came with this set has what appears to be a dark spot in the center, not unlike when you have the brightness up too high and burn a line into the phosphorous on the screen. Not bad enough to get rid of though.

Maybe the caps for the Scott 299C will come from Canada tomorrow??? Then back to that to finish.

kvflyer
09-01-2010, 07:20 PM
One of the members of the forum suggested the 1N34 may be bad. Here is how that panned out...

Do these sets have a Germanium diode for video detector? if so maybe it's open?

How about this from the SAMS? I suspect it is located inside of a can next to the video amp. What I can't get over is that they used such small tubes to do the job. I know, it is only a 7" TV!!!

http://antiqueradios.com/gallery/d/121298-2/Detector.jpg

kvflyer
09-01-2010, 07:22 PM
Here is what one of the members suggested. At this time, I did not know where the diode was. I was thinking it was in the round can. To my relief, it was in the rectangular can. Here is how it went...

I opened up one of those cans on a very similar TS-18 chassis. The can was held on by two nuts from below.

Here's what it looked like inside. The diode is the cylinder between the coils on the right and the capacitor to it's left.

Do you have access to a B&K 1077 TV Analyst ? It will allow you to inject a signal at any point in the IF stages modulated at the correct frequency.
http://www.bobandersen.com/images/7VT2%20restoration/23-04.jpg

kvflyer
09-01-2010, 07:23 PM
(I snapped a couple of pictures before I ran out the door that night.)

Here is the Diode just as you all had described:

http://antiqueradios.com/gallery/d/121373-2/Germanium.jpg

And the lead (or tin, it is very, very soft) cover:

http://antiqueradios.com/gallery/d/121370-2/Cover.jpg

"Tune In Tonight At 6 For Breaking News!" Yeah, Right! :wink:

kvflyer
09-01-2010, 07:25 PM
(Those guys are just too much! I asked for help, they came to my rescue and sent me directly to the problem.)



Yes, it was that diode and it was a 1N34. I do remember seeing them as a kid in that same plastic sleeve. It measured "Open" and that was the problem.

I still have work to do. I don't have enough width but that will be another project in about two weeks. I can't tell you how pleased I am to have this little guy working. It was made in the same year that I was born. You can see the dark spot in the center of the CRT. This is my weakest CRT; the one that came with this set. I have another one that is much better. I have been using this one for testing so that Heaven forbid, something should drop on it and destroy it, it would not be the "Good" one.

Now, after refinements, the cosmetics of the cabinet will be addressed. For what it's worth, I really did not have a lot of time into the restoration, only about two days. And the price of the capacitors was very low.

Thanks a million, I really do appreciate it! "You Done It!"

http://www.streitsonline.com/don/First_picture2.jpg

kvflyer
09-01-2010, 07:29 PM
I found this CRT in my assortment of Picture Tubes. So, now I have a "Test Tube" to use. Since I can't see the number on the tube, I didn't know what it is used in. I suspect an Oscilloscope. But I did "Go For It" and tried it. It works. So, I have a green phosphorus tube that I can use for testing purposes.

http://antiqueradios.com/gallery/d/121460-2/Green.jpg



It does have a different current of the filament, something like 150 ma instead of 300 ma. So if I use it, I will have to bridge the filament with maybe an 18-20 ohm 10-watt resistor when I use it for testing.

It was suggested that this CRT would be good for a gardening show or horror movie ;)

bandersen
09-01-2010, 10:06 PM
More progress. Coming down the home stretch... as far as capacitors. Every resistor that I randomly checked was pretty close. I paid attention to the high values and they were still within 10%, some even 5% when marked with the silver stripe....

Interesting. I also found very few (if any) high valued resistors in these sets that were bad. I did however find some low valued ones (< 1K) that were way off.

I found this CRT in my assortment of Picture Tubes. So, now I have a "Test Tube" to use. Since I can't see the number on the tube, I didn't know what it is used in. I suspect an Oscilloscope. But I did "Go For It" and tried it. It works. So, I have a green phosphorus tube that I can use for testing purposes.

http://antiqueradios.com/gallery/d/121460-2/Green.jpg



It does have a different current of the filament, something like 150 ma instead of 300 ma. So if I use it, I will have to bridge the filament with maybe an 18-20 ohm 10-watt resistor when I use it for testing.

It was suggested that this CRT would be good for a gardening show or horror movie ;)

Very cool :D

You might get a kick out of this thread I posted a while ago. I tried hooking up a 7VP1 and a monstrous 12FP7 to a VT71. http://www.videokarma.org/showthread.php?t=247145

kvflyer
09-02-2010, 09:02 AM
This Motorola VT-71. It is working but I have some traveling to do so I have to get back to it. As you can see, the usual deteriorated CRT surround. I have a new repro one so that problem will be resolved.

http://antiqueradios.com/gallery/d/121082-2/Front_011.jpg


All caps except mica and disc have been replaced. What I found was any repairs done were done to the HV capacitors. ALL other components are original... or shall we say, "Were" original. Whomever did the repair work just snipped off the component and then tacked it back to the terminal. It made it easy to remove. But I got all traces of the old wire out of that little hole. Two mica capacitors were just tacked in. They measured perfect so they went back in the proper way.

http://antiqueradios.com/gallery/d/121241-2/Finished_Chassis.jpg

Ad it actually now does work. First electrostatic set that I have worked on. I guess that an oscilloscope does not count!

http://www.streitsonline.com/don/First_picture2.jpg