HP 7970E - interest to split?

Jon Elson elson at pico-systems.com
Sat Sep 30 20:29:13 CDT 2017

On 09/30/2017 12:47 PM, Chuck Guzis via cctalk wrote:
> What I found a little surprising is that on a 7970 head assembly, the DC
> resistance of the 9 track coils is about 76 ohms, but on the 7 track
> side, the resistance measures out to about 27 ohms.
> Yet, both feed into the same read amplifiers.  On the standard 7970
> dual-mode drives, the switching between 7 and 9 track, is performed with
> a JFET on the input.  I use small-signal DIP relays.
> I would have expected a big difference in the signal levels between the two.
Ahh, but the genius is built into the heads!  Note that 
nobody recorded 800 BPI on 7-track tapes.
I think they supported 200 and 556 BPI, only.  There were 
some low-density 9-track tapes, but
generally most 9-tk tapes were 800 BPI.  So, this means that 
a transition spans less longitudinal distance along the tape.
So, you get less output from the head for an 800-BPI 
transition. For 800 BPI, they might make the head poles a 
bit shorter, but they HAD to make the gap narrower!  The gap 
must be no more than half the length of a bit, or the
signal output rapidly drops.  I'm guessing the smaller head 
poles and narrower gap forced them to use more turns of much 
smaller wire to get the signal back up to what the read amps 


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