[Boatanchors] [Milsurplus] URM25

[WA5CAB--- via Boatanchors]

Paul,

AN/URM-25D was the first in a line that included AN/URM-25G, H, J & K.  J 
and K are rare.  The signal generator used is the SG-85, which went through 
four modifications, basic through D.  If you look on the signal generator 
nameplate you should see a suffix letter after the 85, at least a "B".  
NAVSHIPS 0967-187-5010 with 7 changes and 2 temporary changes covers AN/URM-25D, G 
and H.  I have never found anything on J or K except a short calibration 
instruction.  The power supply in AN/URM-25D is PP-977/URM-25D.  This changes 
to PP-977A/URM-25G.  PP-977A has a 12 VAC winding and a regulator circuit for 
the oscillator filament.  SG-85B is used in AN/URM-25H, and has the 
outboard circuit board you found.  The circuit is a voltage double/regulator.  The 
manual and changes do not make it clear whether the power supply went back 
to PP-977, remained PP-977A either modified to remove the regulator or with 
the regulator there but unused, or changed to PP-977B.  You might check that 
out and report and I will add it to Change 7.  The added regulator circuit 
and some rewiring associated with it were added with Change 7, but they did 
not add a modified complete schematic.

One other comment - if the nameplate on your case still says AN/URM-25D but 
the one on the signal generator says SG-85B/URM-25D or later, someone 
swapped case or generator.

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480

n a message dated 10/12/2014 12:20:04 PM Central Daylight Time, 
elespe at lisco.com writes: 
> Got into a couple URM25D with an extra board mounted below the audio 
> section behind the rf attenuator.
> Board has two power resistors, two electrolytics, two top hat diodes and a 
> stud mount diode.
> I have not encountered this option before
> Can anyone enlighten me what it is?  Guessing maybe dc filament supply 
> from size of components?
> I don’t find it documented anyplace I have looked
> Thanks
> Paul K0UYA