[R-390] URM-25 replacement

[DJED1 at aol.com]

For the last 5 years I've beeen doing my alignments on my R-390A with a 
URM-25 I got from Fair Radio.  It's worked OK, as long as I put a frequency counter 
on it to get on frequency.  But as I was repairing it a couple of weeks ago I 
saw that it was from 1952, and decided it was time for an upgrade after 50+ 
years of service.  I looked at the HP signal generators on the e-place, and 
picked up an HP 8660A for only $300 (not bad for something that cost $25,000 when 
it was new).  The R-390s certainly hold their value better- I think they only 
cost about $2000 new and are worth over $500+ now.  Anyway, it's quite a 
change from the URM-25.  The 8660 was one of the first synthesized generators 
(mine has a binary display!), and can be set to the nearest cycle.  The one I got 
has a plug-in that covers .01 to 110 MHz, so it matches nicely the 
capabilities of the URM-25. 
I struggled with how to test this thing- I expected that I couldn't zero beat 
to better than 10s of cycles because the receiver audio will cut off at very 
low frequencies, so how to tell if it worked as specified?  I decided to try 
and measure with an offset so that I had a beat note of a few hundred cycles, 
then put the frequency counter on the audio output and measure the beat to a 
cycle.  It worked very well- I could measure an offset against WWV down to 
about+- 1 cycle when the modulation was off.  After warming up the generator, I set 
it to 15.000300, fed it into the R-390A  along with WWV, and measured the 
audio beat as exactly 300 cycles.  So it looks like the old 8660 still is working 
well.  I checked the attenuator and it seems to be working OK, so tomorrow I 
will measure the sensitivity of the receiver, which the URM-25 measures as 0.3 
microvolts or so.  The 8660 goes down to 0.03 microvolts, so I should be able 
to tweak the receiver as never before.  The only drawback is that the 8660, 
unlike the URM-25 is definitely not portable- it's about 65 pounds.  It may be 
easier to carry the receiver to it than vice-versa.
Ed   WB2LHI