[Hallicrafters] urm-25d cover parts

[roy.morgan at nist.gov]

Quoting Barry H <barry_hauser at juno.com>:
.. The accessories disappeared from many of them before they were
> surplussed out or later on....
> They're fairly easy to replicate ...
> I'm told that the components in the originals probably need replacing anyway.

All true:

- I hve made terminations ( 50 ohm resistor to ground) from: two 3.4 inch copper
pipe caps, one resistor, one short wpiece of pipe, two screws and two chassis
mount BNC connectors. You can make a voltage divider or "Standard Antenna" from
the same plus a few parts.

>  ... The schematics and parts values for most if not all the accessories
> are in the URM-25 manuals.  You might be better off rolling your own.

Yes, I agree. Pomona makes VERY nice little boxes with BNC connectors on each
end - may be pricy bought new. 

TWO gotchas: Nolan Lee, now absent for all these lists, discovered that 
1) the original BNC coax cables that came with a URM-25 he had were VERY lossy. 
He simply threw them out and got or made new ones.  So beware of those original
cables.
(I have some RG-8 Coax made in 1944 and look forward to measuring its loss one
of these days.)
2) The capacitor in the "Isoloation" unit, was LEAKY, thus endangering the URM
output attentuator if you connect to a point carrying B+.  (You do NOT want to
sart a search for the right resistors to replace burned out ones in the
attentuator.)  I strongly suggest replacing that cap in any old/original unit
you may have. Do not use it until you do.


IN short, Make your own accessories and be sure that they will work right.  The
three I suggest are:
 - the 50 ohm termination: essential to make the thing read output voltages
correctly
 - the 5:1 or 10:1 voltage divider: useful when making very low signals for
sensitivity measurements.
 - The "dummy antenna": useful for testing boatanchor receivers whose manual
call for it.

Roy Morgan
7130 Panorama Dr.
Derwood MD 20855