[Hallicrafters] urm-25d cover parts

[Chris Kepus]

Roy,
I just got a URM-25D and have been looking for more specific info on the
accessories.  It looks to me like you have the key items "nailed".  What is
the chance you would share specific construction and electrical detail on
the "makin's" of these three so Jack, myself and probably a bunch of others
could make up a set to test our Hallicrafters receivers?
Tnx es 73,
Chris
W7JPG

-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net] On Behalf Of
roy.morgan at nist.gov
Sent: Saturday, February 18, 2006 5:19 PM
To: Barry H
Cc: w1jhs at midmaine.com; hallicrafters at mailman.qth.net
Subject: Re: [Hallicrafters] urm-25d cover parts

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
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