sony SVO-1630 hi-fi stereo vhs stereo recording/playback problem

[neazoi]

Hello,
I received a sony SVO-1630 hi-fi stereo vhs today. It seems it functions ok apart from the fact that when I record stereo audio it records on the tape, but when I play it back, it plays stereo for a few seconds, then switches to mono for a few more seconds, then back again to stereo, then back again to mono etc... I also tried to record video as well, at the same time stereo audio was recorded, because some vhs do not allow writing audio without a video carrier.
Do you know why this is happening? Please give me a hint.
Could it be a head misalignment problem?
Could it be a bad tape problem (the tape I tried was a bit old from an ordinary vhs, not hi-fi)?
I appreciate your answer.

[Mooly]

That's a typical scenario and could be many things.

The definitive checks would be to look at the FM envelope of both video and audio to check tape path alignment. Also check the levels of each field relative to the other as well as absolute level.

It could be a worn head drum or even lower drum assembly that has lost "goodness" for want of a better word.

I used to work as a bench tech where this kind of problem was seen daily... tbh hifi VHS decks were a real problem at the best of times. Head life was often poor with the audio the first thing to suffer.

In the first instance you need to make sure the mechanics and tape path alignment are perfect. The tape also has to be perfect. If there are any wrinkles or edge damage then that will cause problems too.