[R-390] URM-25D recap.

Mon, 24 Jun 2002 12:18:12 -0700 (PDT)

On Mon, 24 Jun 2002, JM/CO wrote:

> Yes, an "RF" voltmeter is required here. The signal IS alternating current,
> not DC. All AC voltmeters have a frequency passband. The typical "panel"
> meter, used to measure, say, 0 to 150 volts AC will be accurate only at the
> 60 Hz AC power line frequency. A voltmeter used to measure, say 10 Mhz, must
> have a passband going up to at least that frequency. An excellent meter for
> this purpose is the Hewlett Packard model 410-C, which goes up to ( I
> think ) around 700 Mhz. The older ( and much cheaper ) model 410-B will work
> just fine too, if you can find one that still works properly.
> Chuck N1LNH

HP 410's are general purpose VTVM's, measuring AC and DC volts
and resistance. They are fine for measuring high voltage levels
such as transmitter outputs. They don't measure real low voltages
however. The later models of the HP 400 series of AC voltmeters,
400E, etc, are pretty flat to above 5 MHz, and go down to 1 mV
full scale on the most sensitive ranges. The other choice for
low levels is the Boonton 91 or 92 series of RF voltmeters, which
are both wide freq range and sensitive.

If the URM-25 output level is only set measured at a single
freq, I'd use the 400E and measure it at about 1 MHz. Using
a sensitive meter such as the 400E would let you check the
attenuator output at several points, not just max output.

John

> -----Original Message-----
> From: Jim Temple <[email protected]>
> To: [email protected] <[email protected]>
> Date: Monday, June 24, 2002 12:22 PM
> Subject: [R-390] URM-25D recap.
>
>
> >Hello to the list,
> >
> >I picked up a URM-25D at Dayton this year.  This unit had the typical loss
> >of modulation described in many of the messages posted in the archives.
> >Upon inspection, ALL the large paper caps located on the modulation board
> >were physcially wet with a slick discharge from the innards of the caps.
> So
> >I obtained replacements and installed new caps as recommended in the
> >archives.  I even broke open the case and replaced all the similar paper
> >caps located on the various internal modules, which were also wet with a
> >discharge.  Upon reassembly, all functions work as intended, and after a
> few
> >hours warm up, the stability of the unit is rock solid.  Now on to my
> >question.
> >
> >Question:
> >
> >In the lists experience, is a recalibration necessary after recapping this
> >unit?  The frequency alignment seems to be quite good.  But I am uncertain
> >about the calibration of the voltmeter function.  I have the Navy manual
> for
> >the URM-25D, but am interested in tips on calibrating the voltmeter
> >function.  For instance, 2.0 volts are mentioned as an approximate value at
> >the RF output jack.  Is a RF voltmeter required for this measurement?  Is
> >this voltage a DC voltage?  Any help here?
> >
> >Thanks for your past support.
> >
> >Jim
> >
> >
> >
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