[AMRadio] Test Equipment Recommendation


Jim Wilhite w5jo at brightok.net
Tue Aug 24 12:43:39 EDT 2004


When considering a VTVM one should look at one thing in particular, tube
replacement.  The Senior Voltohmyst uses a 6AL5 and a 12AU7 .  Finding tubes
is not a problem, but for reasonable accuracy the right tube is.

According to the manual for the Voltohmyst, the tubes must be thoroughly
aged.  For example, to age the 12AU7 the grids and cathodes are connected to
B- and the heathers be operated at 6.3 volts for approximately 12 hours
before installation.  The tube can be installed then left turned on for
about 36 hours then the instrument must be re-calibrated.  If the second
method does not provide proper results, the first method should be followed.
The 6AL5 can just be aged by leaving it on for about 36 hours, but then
re-calibration must follow.

While neither of these methods are particularly difficult, how many of us
have standard sources to calibrate our meters?  It seems to me a good modern
Fluke meter might be the best choice.  They are not easily destroyed nor go
out of calibration easy.  Some are sensitive to high RF fields which blow
the input protector.  I have had that trouble with a Fluke 77.  Most, if not
all provide a high impedance input and are rugged.

Granted using them to tuned for peak resonance is difficult, Sears sells a
dual digital/analog meter with high impedance.  They are not terribly
expensive and can provide both digital and analog readings.

73  Jim
de W5JO

----- Original Message ----- 
From: "Merz Donald S" <merz.ds at mellon.com>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Tuesday, August 24, 2004 9:21 AM
Subject: RE: [AMRadio] Test Equipment Recommendation


> Really? Is this the best recommendation that we can come up with? Every
one of these that I see is awfully beat up after 30-50 years. And these were
not especially high quality units to begin with.
>
> On the other hand, I don't know what else to recommend. I am using an old
Radio Shack Micronta VTVM, probably from 1970. I've had a Simpson 303 (these
do not age well--to be avoided) and the RCA VoltOhmyst in several different
flavors, plus several other brands. The Micronta has delivered the most
reliable, accurate service of any of them. But I can't really recommend that
because nobody could ever find one.
>
> Given the advanced age of the VTVMs that survive today, this actually is a
harder question than it first seems.
>
> 73, Don Merz, N3RHT
>
>
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net]On Behalf Of Edward B Richards
> Sent: Monday, August 23, 2004 11:27 PM
> To: bpancake at mindspring.com; amradio at mailman.qth.net
> Subject: Re: [AMRadio] Test Equipment Recommendation
>
>
> Bill;
>
> Go to that auction place and buy a Heathkit VTVM. There are usually a lot
> of them listed. All are 11 megohm input impedance. Then buy an RF probe.
> The later VTVM's have a phone jack input and so do most RF probes. Also
> known a demodulator or scope demodulator. Good luck
>
> 73, Ed Richards K6UUZ
>
>
> On Mon, 23 Aug 2004 19:30:22 -0600 (GMT-06:00) Bill Pancake
> <bpancake at mindspring.com> writes:
> > Would appreciate a recommendation as to make and model of a VTVM that
> > will come with an rf probe that is suitable for using when working
> > on boatanchor gear.  I just need an idea of what to look for.
> > Thanks, Bill, ab0zz at arrl.net
> >
> >
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