ive come across a problem on another machine i recently ran into. this particular unit (otair 5050 8 track) seems to partially erase on tracks 1 and 3.
First, find out what circumstances this happens in. Record a long test tone on all 8 tracks, say 5 minutes or so, at 0vu.
Then play it back a couple of times, just to make sure that there isn't something magnetized in the transport, or that it's not physical wear on the tape over those tracks, or crap on the heads or something.
Next step - play back the whole five minutes and try recording each track in turn. Say, 30 seconds on track 1, then switch over to track 2, track 3 etc. Make sure you do these ONE AT A TIME - do NOT arm more than one track at once or you'll confuse the results.
When you've recorded it, play it back and check the levels to see if anything unexpected is happening. If it was a straight misalignment I'd have expected it to affect all the tracks to some degree. I'd also expect an incomplete erasure of the actual, target track.
After that, I'd suggest you try recording each track in turn, on a VIRGIN stretch of tape - not something that's ever been recorded on previously.
Test each track in turn, and only that track. E.g 30 seconds of 0vu test tone on track 1, followed by 30 seconds on track 2 etc. Then, erase each one in turn and see if it works. If you have a residual signal afterwards, you may well have an alignment problem.
If this doesn't make sense, let me know and I'll try and draw some diagrams