[Hallicrafters] URM-25D Help Needed

[roy.morgan at nist.gov]

Quoting richard.arland at verizon.net: 
> >From: Craig Roberts <crgrbrts at verizon.net>
> >Subject: Re: [Hallicrafters] urm-25d rf probe
> 
> >I bought a full set of original AN/URM-25D accessories (test leads, 
> >dummy antenna, outboard attenuators, etc.) from Fair Radio Sales about a 
> >year ago.  They may still have some in stock.
> 
> 
> 1. Where does one find out what these accessories are? I don't have the
> TM/TO, are the aforementioned accessories the total list of "accessories" for
> the URM-25D?

Here's what you do:

1) Download a copy of the manual. See BAMA

> > 2. Several years ago I seem to remember that there was someone on this list
> (or possibly the "Boatanchors" list) that had a laundry list of modifications
> and alignment instructions for this RF gen. 

That *might* have been me.  I have a list of documents that I own or have heard
of.  BUT: The most useful theing beyond the manual is a set of overhaul notes
written by Dallas Lankford and offered (for $1, as I remember) by the editors
of Hollow State News (HSN).  HSN is in haiatis condition now but the folks who
have the back copies and related publications are still around.

Dallas's overhaul notes are hand-written (though there may be a transliteration
into digital text) and were about 11 pages in length. He updated them once.
They were  announced in HSN but were never published in the newsletter.  In a
later HSN, there appeared a note which gave the physical and parts layout of
one circuit board.

Note: The URM-25F is very much the same electrically as the D version, if not
identical, but is NOT the same physically.  The mechanical layout and
construction is enough to puzzle you if you are following Dallas's notes for
the D version while trying to overhaul an F.  He never did a version of the
notes for the F version.

Another particularly important item is the article in HSN about the modulation
oscillator and how to fix it. In summary, the resistors drift in value and
cause balky or failure to start when you select modulation.  The cure is to
replace the faulty parts, or parallel the resistors with others to bring the
value back down (not a good idea in my opinion.)

You can do a quick check of the attenuator by measuring the DC resistance at the
attenuator output.  The value you measure should be the same except for maybe
the first and last step.  The attenuator is susceptable to being "transmitted
into" by a transceiver. This burns at least one of the tiny, odd-valued,
precision resistors in the attentuaor.  This is not a hopeless situation, but
it does mean acquiring some unusual resistors and being very careful with the
repair.

Lastly, I have collected a number of email list postings and notes on the
URM-25.  I'll be happy to forward a copy to those interested. These are
basically unedited posts from whatever mail list I found them on.

        Anybody remember this article,
> who wrote it, and where one can find a copy for myself? 

See above.  The Lankford overhaul notes are the best thing you can have, once
you have a manual.  I can send my list of related publications and manuals on
Monday.
 
> ...URM-25D I bought at the Virginia Beach Hamfest, I would like to perform
the
> mods, and make it into a useful piece of test gear. Right now it has a few
> "issues".  

I'd be glad to hear what the issues are. I may be able to point toward a
suitable fix.  

Here are some notes from memory:

The only "modification" anyone has recommended that I remember is to move the
line input bypass caps from the rear of the housing they are in to the sides -
this gets them out of the way of possible danger when you remove the unit from
the case.

The nearly essential repairs include replacing all, repeat all, paper tubular
capactiors, including the ones in the audio bridged Tee oscillator, replacing
any drifted resistors, and checking the filter caps (though they *may* be oil
types and not need replacement).

If you should have the blocking capacitor accessory, you MUST replace it's cap,
because it is almost certaily leaky, and applying the test lead to a plate in
the radio under test will allow current to overheat the resistors in the
attenuator. You do NOT want to do this.

Nolan Lee found some original test lead coax to be so lossy as to foul up the
output levels, and replaced all the coax with new.

The output level indicated on the panel meter and attenuator will only be
correct if the output is terminated by 50 ohms. One supplied accessory does
this, but HP, Tektronics and others made gadgets that do the same thing.  You
can easily fabricate a substitute if you don't have one.  At higher
frequencies, the load should go at the output end of coax used to connect the
generator with the radio under test.

The voltage divider is useful when making low level sensitivity measurements.
The URM is moderately good on leakage, IF you do not use the high level output
to run a counter or whatever, but should not be counted on at the bottom end of
its attenuator range.  Reports of R-390's that have 0.07 uV sensitivity
measured with the URM and no special methods are probably bogus.

The "Dummy Antenna" is a simple network arrived at by the RMA (Radio 
Manufacturers Association) and standardized in the 1930's.  It had at least two
purposes: 1) to simulate an average antenna and facilitate alignment of the
front end of receivers intended for us with wire antennas, and 2) to create a
more standardized method of measuring receiver sensitivity.  The manuals for
many older military radios call for this thing to be used. You can very well
get along with out it, but again, you can fabricate one fairly easily if you
care to.

Roy

Roy Morgan, K1LKY
7130 Panorama Dr.
Derwood MD 20855