Hi Peter,
Just found this thread and thought I’d chime in…
The PVW2800 has (IIRC) 2 switches to detect record lock-out on the tape. The large format tapes use a tab on the left (looking down on bottom of tape) and the small tape has the tab on the right.
My guess is that the lock-out detector switch for the small cassettes has gone bad.
Hope this helps,
Cheers,
Randy