[R-390] URM-25 replacement

[Cecil Acuff]

Hi Bob and group.

Another suggestion someone mentioned at some time in the past to determine 
if your setup is causing you to THINK you have that ultimately sensitive 
receiver is to add 10 or 20db inline pad to the output of the generator at 
the front panel of the generator and see if your measurements move down the 
appropriate amount or stay the same. Could be a leakage problem.  If there 
is little change it's probable that the generator is leaking more signal 
into the air than your attenuator setting indicates and the radio is 
detecting that instead.  I guess it's the same principal as what Bob is 
describing it just keeps the generator and radio terminated into a 
controlled load.

Part of the problem with getting a signal into the R-390 series is the 
darned balanced input and some of the silly things we may do to get a signal 
into the radio at that port.  That is where it needs to be inputted but it 
should be shielded from connector to connector, radio to generator.  That's 
not always easy to do.

Just my 2 cents worth added on to the good info Bob has posted....

Cecil...
----- Original Message ----- 
From: "Bob Camp" <ham at cq.nu>
To: <DJED1 at aol.com>; "R-390 HF Receiver List" <r-390 at mailman.qth.net>
Sent: Friday, February 25, 2005 10:04 PM
Subject: Re: [R-390] URM-25 replacement


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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