Motorola VT-71 Composite/s-video in?

[Electronic M]

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.