[R-390] URM-25 replacement
Roy Morgan
roy.morgan at nist.gov
Mon Feb 28 17:50:04 EST 2005
At 11:28 AM 2/26/2005, Bob Camp wrote:
>If you have a generator that uses a "stacked attenuator" rather than the
>old sliding coax attenuator it's a good idea to check the attenuator.
URM-25 folks,
I got both a URM-25 and a URM-26 this weeekend. The URM-26 (20 to 440 mc
or so) uses a waveguide beyond cutoff attenuator, so there is only one
single resisor in the attenuator.
The URM-25 is a switched ladder attenuator. I checked mine by measureing
the dc resistance at the output jack. One setting (1 uV) was way above 50
ohms - something like 700. Crispy critter resistor at that spot! Another
was modestly off and all the rest were close to 50 ohms.
Gotta find some proper resistors to repair that attenuator!
Roy
> They switch in a variety of fixed attenuators to make up your final
> attenuation value. At any given setting you may have a fairly small
> number of pads in the circuit. With an external attenuator and a receiver
> (R390 recommended) you can step through the entire range of the
> generator. Since the attenuator is the part most likely to fail in a
> non-obvious way it's worth doing on a "new to you" generator.
>
>Checking the pads you use is probably a good idea. It turns out you can do
>a quick and dirty check on *any* pad with the same technique. Terminate
>one end of the pad with 50 ohms. Use your DVM to measure the resistance on
>the other end. It should be 50 ohms. Reverse the pad and repeat the
>procedure. Pads tend to die a lot more often from RF or DC overload. The
>thing that usually goes is the part would be a resistor to ground in a PI pad.
>
>If you want to go whole hog on your new pad you can put a *small* DC
>voltage on the input and check the DC on the output. The key issue here is
>keeping the DC low enough that you don't blow up the pad. Depending on
>your voltmeter you can check pads up into the 30 db range this way. Past
>that an AC signal and an AC voltmeter probably are a better idea.
>
>Both of these techniques depend on the fact that the DC and RF performance
>of the pads are nearly identical. This is normally a good bet at R390
>frequencies. It may not be as good a bet at microwaves. I have certainly
>seen a number of dummy loads that are AC coupled. I have not seen any wide
>band attenuators that are AC blocked though.
>
> Take Care!
>
> Bob Camp
> KB8TQ
>
>
>On Feb 26, 2005, at 11:04 AM, Cecil Acuff wrote:
>
>>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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>>
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>
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- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
Work: Voice: 301-975-3254, Fax: 301-948-6213
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