[AMRadio] Test Equipment Recommendation


Edward B Richards zuu6k at juno.com
Tue Aug 24 15:53:37 EDT 2004


I think you guys are getting carried away with technicalities. We are not
space laboratories, we are amateurs. Look at your license. Nearly all
VTVM's use a 12AU7 and a 6AL5. They are available in abundance. I have
about 50 of each. Any good used tube will be aged. Even if recalibration
is required we are talking about removing 2 screws and adjusting 3 pots.
Not an all day job. Re calibration, Heathkit used a fresh zink-carbon
battery for a terminal voltage of 1.55 volts. I use a fresh alkaline
battery with a terminal voltage of 1.62 volts. It works for me. For ACV
if you don't have another meter or a friend that does, wait for early
morning when the electrical grid is lightly loaded and calibrate your ACV
for 122 VAC. You will be in the 5% accuracy of the VTVM.

73, Ed Richards K6UUZ


On Tue, 24 Aug 2004 11:43:39 -0500 "Jim Wilhite" <w5jo at brightok.net>
writes:
> 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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