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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sat 11, 2019 2:34 am 
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Location: Ann Arbor, MI
On this set there should be a spring wrapped around the 1B3 high voltage rectifier. There should be a wire attached to the spring running to pin 5 of the 25L6 tube. The spring should be positioned about half way up the 1B3.

The spring and the plate of the 1B3 form a capacitor that provides the feedback needed to make the tube into an oscillator.

In general, any two conductors separated by any insulator form a capacitor. In some circuits the capacitance between different parts and wires has a significant effect on the circuit and in other circuits the effect is not important. The possibility of this effect being important is why people will tell you to put replacement parts in exactly the same location and position as the old part. The effect is most important in high frequency circuits. You have to be very careful if you do any work in the tuner. In general, don't do anything to the tuner unless you really have to.

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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sat 11, 2019 4:47 am 
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Joined: Jan Tue 03, 2017 6:43 am
Posts: 32
Location: California
Last night, I double checked the work I did on the main electrolytic capacitors. I carefully compared the connections with the schematics, tracing each line on the diagram with a highlighter marker as I verified them. It looks like I did connect the capacitors right. Also, the resistance measurements on the ballast tube match up perfectly with what's shown in the diagram, so it should be ok.

I talked with the owner of the electronics surplus shop where I usually get parts, and he agreed that the problem could be caused by bad capacitors connected with the 25L6 tube. He said the most likely culprit is one of the high voltage filter caps. I'm going to replace all the paper caps anyway though.

Tom, I saw the spring you're talking about on the rectifier tube. I had read earlier that the placement is very critical, so I was careful to not touch it at all! It looks like nothing in the high voltage box has ever been messed with, so I left everything there alone.

- Chris


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Sat 18, 2019 5:11 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1524
Location: Phoenix, Arizona
You should assume all the high voltage caps are bad and just replace them. The other paper caps are likely bad also. Replacements are available.

Be careful with the Ballast as I believe replacements are extinct. People do replace the resistance elements with wire wound resistors but the original offered some voltage regulation.

The B+ supply is a doubler and this set should be considered as a "Hot Chassis" as there is no transformer.

Jim


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Mon 20, 2019 12:54 am 
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Joined: Jan Tue 03, 2017 6:43 am
Posts: 32
Location: California
Yeah, I'm planning to replace all the capacitors. I've already ordered them, and they should arrive tomorrow, in fact! :mrgreen:

I just really hope the CRT is still good!


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Wed 22, 2019 2:47 am 
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Joined: Jan Tue 03, 2017 6:43 am
Posts: 32
Location: California
I did it! I got the CRT to light up! :D

No picture yet, just "snow," but the screen looks really nice and bright, even in a well lit room. It seems to be scanning ok, and it responds well to the brightness control. I'm trying to connect my laptop to the tv, using an HDMI to video converter and an RF modulator, then hooking up a coaxial cable to the antenna screws with a 75 to 300 ohm converter. It's the same way I hook up the computer to my 1952 Olympic tv. I'm not sure yet where to start to try to get a picture. When I turn the knob to change stations, the tv does seem to respond. The screen flickers as you'd expext. But it's out of tune somewhere. I have the speaker disconnected, so I don't know if it's receiving sound either.

I'm just so happy that the cathode ray tube is working! :mrgreen:
It looks really strong and bright. And no more burning resistors! Ha!

I guess I'm going to have to do a little more research to see what I should do to get it to receive a picture. I see rows of adjustment screws on top of the chassis. know the general advice is to not mess with inductors, but I'm wondering if I need to adjust the one for channel 3.


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Wed 22, 2019 4:49 am 
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Joined: Mar Sun 01, 2009 10:27 pm
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Location: Ann Arbor, MI
As I understand it, these sets were adjusted by the installer for the channels in use in the customers area. So the position labeled '2 - 3' would have been set up for channel 2 in areas that had channel 2 and would have been set up for channel 3 in areas that had channel 3. A given area would never have had both channels in use.
Can you set your modulator to use channel 4? Many modulators have a switch to switch between between 3 and 4. That would be easier than adjusting the tuner in the set.
The Sams for this set would have the adjustment procedure for the tuner. But that procedure would probably require you to have a signal generator. I still can't find my schematics for this set, so I can't look and see.

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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Thu 23, 2019 12:01 am 
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Joined: Jan Tue 03, 2017 6:43 am
Posts: 32
Location: California
I'm getting closer! The sound is working now! :D
It's loud and clear. The tv still doesn't have a picture, but I feel like it's almost there. The raster (if I'm using the term correctly) is holding steady and instead of being just random static, it's actually forming lines like it's trying to form a picture. I guess the horizontal synchronization is off? I think I'm getting really, really close now though.

Tom, unfortunately I can't change the channel on the RF modulator. It's preset for channel 3.


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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Thu 23, 2019 1:35 am 
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Joined: Mar Sun 01, 2009 10:27 pm
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Location: Ann Arbor, MI
If you are getting something that looks like slanted bars, try adjusting the horizontal hold control.

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 Post subject: Re: Motorola VT-71 tv (need help!)
PostPosted: May Fri 24, 2019 12:39 am 
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Joined: Jan Tue 03, 2017 6:43 am
Posts: 32
Location: California
I've tried adjusting the horizontal hold. It's responding, and I was able to get a bit of an image, but there are wavy lines distorting the image. It looks very similar to the problem I had with the Olympic tv, and in that case I was able to fix it by carefully adjusting a couple of the coils. But I've tried that and I'm not really getting very far. The sound is working, and this is the closest I've been to getting a picture. I'm kind of hesitant to mess with it too aggressively, because I don't have an oscilloscope or any of the equipment to do an alignment. With the Olympic tv, I just went purely by sight and ear, making adustments until the picture cleared up.

I'm not at home right now, but when I get a chance I'll get a picture of the tv.

I also did notice that there are a couple of paper/wax capacitors I missed. So maybe replacing those will improve things.


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