recovering cartridge tapes (was: Tek fiches found!

Brent Hilpert hilpert at
Mon Jul 19 18:59:23 CDT 2010

On 2010 Jul 19, at 4:03 PM, Dave McGuire wrote:
>   Every vacuum-column drive I've seen has used pressure sensors to
> determine the position of the tape loops within the columns.  Granted
> I've not seen a HUGE variety of different drives, but...just sayin'.

On the desk beside me, I'm looking at a part from a Honeywell vac-col 
tape drive. It's an aluminum extrusion 1/2-inch wide and about 6 inches 
long, with a masked glass plate on one side. Inside, spread along the 
length are 8 selenium photocell strips.

I think this was located somewhere around the mid-point on the vacuum 
column, that is, the target area in the column to keep the tape loop 
at. Perhaps optical was chosen for servo response time compared to 
mechanical pressure switches.

I haven't examined a large variety of drives either, but at least on 
some drives, the sensors are not distributed evenly along the length of 
the column, but are concentrated around the target area, and become 
sparser towards the extremities. Never seen it, but one could imagine a 
hybrid design with optical sense around the target area and pressure 
sense further out.

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