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julie_m · 16th Mar 2019, 9:36 pm

Just a thought ..... It might be worth checking if the erase head (a) has continuity and (b) is getting any power in record mode? Because that's something that is both different between recording and playback, and would affect the picture more severely than the sound .....

The sound head in a VCR has to record onto tape that has already had a bit of (unnecessary; the other head still hasn't reached the top of the tape by this stage) picture signal recorded on it, so it puts out enough magnetism to obliterate whatever was there before all by itself. But the video heads can't put out too strong a field, because the lines of flux have a natural tendency to go wide of the head and the diagonal tracks have to be very close together with no guard band between them (a special trick is used to simulate a guard band on playback). It doesn't matter about the sound track going a bit wide; since there is still room designed into the system for a bit of a guard band above it, and on the bottom side there is no tape, just thin air.

If it was trying to record a video signal over the top of another video signal and some of the original is still there for want of erase head functionality, that could cause all manner of problems .....

If you have no way to test the erase head voltage, can you listen for any breakthrough of old audio?

16th Mar 2019, 9:36 pm	#9
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Old Betamax records audio but no picture Just a thought ..... It might be worth checking if the erase head (a) has continuity and (b) is getting any power in record mode? Because that's something that is both different between recording and playback, and would affect the picture more severely than the sound ..... The sound head in a VCR has to record onto tape that has already had a bit of (unnecessary; the other head still hasn't reached the top of the tape by this stage) picture signal recorded on it, so it puts out enough magnetism to obliterate whatever was there before all by itself. But the video heads can't put out too strong a field, because the lines of flux have a natural tendency to go wide of the head and the diagonal tracks have to be very close together with no guard band between them (a special trick is used to simulate a guard band on playback). It doesn't matter about the sound track going a bit wide; since there is still room designed into the system for a bit of a guard band above it, and on the bottom side there is no tape, just thin air. If it was trying to record a video signal over the top of another video signal and some of the original is still there for want of erase head functionality, that could cause all manner of problems ..... If you have no way to test the erase head voltage, can you listen for any breakthrough of old audio? __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.