[R-390] URM-25 replacement

[Bob Camp]

Hi

There is one problem that you may run into and a fairly easy way to get 
around it.

Almost any signal generator depends on the integrity of it's shielding 
to keep radiated RF from messing up the measurement you are doing. A 
URM-25 is a fairly simple beast in this respect. A 8660 is pretty darn 
complex by comparison.

Our beloved R-390's are fine receivers, but they are not right at the 
top of the class when it comes to shielding. If you have enough RF 
running around the test bench it will get into the radio without going 
through the antenna connector.

If you are going for that 0.03 uV alignment this may be an issue....

The easy way to check what's going on is to simply unplug the coax from 
the antenna connector on the radio and see if the signal goes away. To 
be complete you probably should ground the shield of the coax to the 
radio case just to be sure you don't have a ground loop.

Of course if you fail the test then you have to do a little work. Often 
it's as simple as dressing the power cords on the bench with a little 
more care. Other times you need to clean the finger stock on the plug 
in compartment of the generator. Not a lot of rocket science, but there 
can be some work involved.

If you want to go totally nuts then here's something else you can do. 
Make up a "sniffer coil" on the end of a scrap piece of coax. Four or 
five turns about 3/4 inch in diameter seem to work well. Crank up the 
generator into a shielded 50 ohm load. Hook the coil to a good radio 
(R390 recommended ...). Tune the radio and generator to the same 
frequency. Run the coil all around the generator looking for a signal. 
Meter openings, attenuator knob's and power cords are all good things 
to check. If the generator passes this test at 30 MHz and at 500 KHz 
you should be fine.

	Take Care!

		Bob Camp
		KB8TQ



On Feb 25, 2005, at 10:20 PM, DJED1 at aol.com wrote:

> 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
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