Bulk tape eraser

I don't know if my experience with tapes for the TK50 / TK70 applies, but for what it is worth, here is what was the problem. When a tape has been written by a TK50 drive, the TK70 drive will no longer be able to write on the tape until the tape have been BULK erased. Since I had a number of tapes written by a TK50 and I wanted to use the tapes for producing backups using a TK70, I obviously needed to be able to BULK erase those tapes. FORTUNATELY, I had an old voice coil fram a speaker available. It must have been a large speaker because the coil was a cylinder about 3" in diameter and about 3" long. The permanent magnet seems to have a few pounds of pull on any iron object, so I thought it might be useful. It probably takes about 30 seconds of slowly rotating the voice coil over the surface of the TK50 / TK70 tape to sufficiently BULK erase the tape such that the TK70 drive would write on the tape. Since I believe that the TK50 / TK70 tapes are early DLT tapes, this method may also be successful, if a bit slow, for a small quantity of tapes. Jerome Fine