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  #1  
Old 12-11-2017, 08:39 PM
mbates14 mbates14 is offline
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Motorola VT-71 Composite/s-video in?

In a ditch effort to get my VT-71 back into full operation again, instead of using the tuner and attempting an IF alignment which uses gear I do not own, has anyone attempted to modify the set to allow a composite/audio input?

Audio is fairly easy and straight forward. But the video side, I am not sure. I was thinking about using A/V extenders. Put one at the DirecTV box, and the toher one at the TV. it has S-Video which is nice because I could get rid of the chroma signal, stick strictly with luma.

Problem is, Not sure where to inject that signal, its not as straight forward. Detector diode? would it be strong enough? or would I need serious modifications?
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Old 12-11-2017, 09:03 PM
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SwizzyMan SwizzyMan is offline
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Inject your composite source to pin one of the video amp tube (make sure to disconnect existing connections going to pin 1 first!). Make sure to add a .1uf say 630v cap in series to isolate from any dc feedback. Then of course connect the shield of the wire to ground!
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Old 12-11-2017, 09:22 PM
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Electronic M Electronic M is offline
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First off if you don't have a line isolation transformer to power the VT-71 with, don't try this...If you connect a hot chassis set to a grounded chassis video source something WILL be damaged.

The detector diode probably is your best bet for an injection point. If polarity or amplitude is wrong an OP-AMP circuit can be made to deal with that.

Personally, unless you know that the tuner or if is badly damaged, I recommend just trying the tuner and IF circuits after a recap and seeing how they do...Most sets don't NEED an alignment to work decently. Bear in mind the detector diode and video peaking coils in these can fail and give odd symptoms that look like alignment issues.
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Old 12-12-2017, 10:34 AM
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bandersen bandersen is offline
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The VT-71 B- bus is routed underneath the chassis and isolated. The chassis is floating and can be grounded safely.
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Old 12-12-2017, 10:45 AM
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One of my vt71 sets I used the ant wire to the 1n34 and got sound and pic but it's been fixed since and don't remember why I did it to begin with.
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