[time-nuts] tracor 308a rubidium oscillator needs mag field to come up to 5 MHz

[Christopher Hoover]

i repaired the power supply in the tracor 308a that i acquired.   a tant cap
had shorted and blew out the rest of the main power circuit.  
 
btw, i used a trick to fix this power supply that i've used several times
very successfully with other gear.  it may be of interest to others.  most
of the supplies in older equipment are comprised of a pass transistor and a
regulator section.  the regulator section is typically either a regulator ic
(e.g. lm[123]11) or an op amp ic with a reference.  the pass transistor is
usually in a to-3 case and mounted on a heat sink, often on the rear.  the
trick is to replace the pass transistor with an lt1038, a high current
three-terminal positive regulator, and to rip out most of the regulator
section, keeping/adding as need only a trim pot and appropriate bypass caps.
the lt1038 in to-3 with proper sinking gives you 10 A current (peak to 24A)
and up to 35V input-ouput differential.  this often exceeds the abs max
ratings of the older pnp power in the power circuit. 
 
back to the tracor 308a ....  it lights up now and locks.  it takes a while,
but it stays that way.  
 
i've started taking some long t.i. runs to check the stability.  early
unscrutinized results indicate an adev of 5.6e-9 at tau=1s and 1.4e-9 at
tau=10s.
 
one issue remains:   i have to crank the magnetic field setting almost to
its high limit (9.91/10.00) to get 5 MHz out; lower settings give a
frequency that is too low.   i presume this is unusual.    
 
i have a rudimentary understanding of the rubidium oscillator physics, but i
do not understand what would cause this.  can i buy a clue?
 
 
cheers,
-- chritopher.