[Milsurplus] AN/TNH-21 tape deck manual needed - NSA?

[Brooke Clarke]

Hi Nick:

I don't have a manual but have an idea.
The TNH-11 can be used either in fixed locations or mobile and the tape 
speed can be changed +20% to -30% meaning that it uses an adjustable 
speed motor.  This from NAVEDTRA10251, see:
http://www.prc68.com/P/NAVEDTRA10251.html
So your unit is a newer model that also can be used fixed or mobil hence 
the servo system for speed control rather than synchronous motors like 
for AC line power.

The reel motors are torque type and will do as you describe if the 
machine is turned on with no tape.  The Magnacord 1024 did this.
http://www.prc68.com/I/M1024.html  It's normal operation.  The supply 
reel tries to turn in such a direction that it's fighting the capstan 
and the take up reel tries to pull the tape when recording or playing.  
This way the tape speed is set by the capstan drive.  There are tension 
arms on either side of the capstan to take up the slop when the tape 
started or stopped.

If you had tape properly threaded (it needs to go between the capstan 
and it's pinch wheel) and it's the capstan that's accelerating, then 
there's probably something wrong with the speed sensor circuitry that's 
part of the capstan mechanics.  For example if there's a optical 
tachometer maybe it's lamp has burned out.

Have Fun,

Brooke Clarke
http://www.PRC68.com