Looking for.

[Multi-Volti Devices]

Hello All:

Unless I have been out the scene for two long, I remember 11 Megohm input
inpedance being typical of VTVM's, and 10 Megohms fro DVM's when they first
appeared. Why the difference, was my question, and I was told, adda 1 meg R in
the testlead, as apparently that was pretty common.

The other notable thing was that the input impedance (we should probably say
resistance to be safe, as I don't know if it was constant on AC ranges) was
constant without regard to voltage range, whereas the passive multimeters
(i.e., Simpson 260, et al) were voltage- range-dependent, with input
resistances spec'ed as ohms/volt. A good rating back then was 20kohm/volt.
There were some oddball ones I remember RS carrying with 50-100k o/V if I
remember right, due to recognition of th ebenefits of hi impedance.

Then along came the 'disposable' DVM/DMM in the $10-20 range. Who knows why,
but some of these are 1 M input. Maybe it's because the companies making them
have no clue what they're used for and build them by recipe.

Well, no one asked anyway, but I guess it's too late at this point...

Murray

Steve & Anne Ray wrote:

> I use a RCA Voltomyst I bought at a Hamfest, however any VTVM should work
> fine.  I have yet to see a VTVM that was not designed with a 11 Meg Ohm
> impedance.  You can use a DVM, but make sure it has 11 Meg Ohm impedance,
> other wise if it is typical of several inexpensive ones I have seen it will
> only be about 1 M Ohm impedance and can load down the circuits.   You can
> usually find VTVM at hamfests for $10 to $20.    The VTVM also has the
> advantage over a inexpensive DVM of being able to see any changes
> instantly.  Very helpful when aligning the rig.
>
> A far as the RF probe is concerned the ARRL handbook has had one in it for
> years and you can get the diodes at Radio Shack (1N34 or 1N34A) it really
> doesnt matter as long as they are germaniums.
>
> I have had and used my HW-101 for 22 years now and always tweaked it up
> with just a VTVM and the RF probe.   In fact the HW-101 is the only big rig
> I own all the rest are QRP CW.
>
> The best way to replace the finals is to neutralize using the method
> described in the service bulletins, I have used the method in the manual
> (peak output and min plate current) and then gone back and done it per the
> service bulletin (plate and screen voltage disconnected and using a RF
> probe on the output) and found you get a much better neutralization
>
> I suggest you go to the Heath List archives or this page
>
> http://members.accessus.net/~dwentz/kb9jja/heathkit/
>
> and find the service notes on the HW-101/ SB-100 series rigs they are all
> basically the same with only a few more bells and whistles on the SB-100
> series.
>
> Steve Ray    K4JPN   ex K1VKW   HW-101, HW-8,
> SWL 30-40, Pixie II, NC 38S, OHR 100A 30 M and Heath Kit fan
> ----------
> > From: Sean Pratt <mooware at SPRYNET.COM>
> > To:
> > Subject: Looking for.
> > Date: Monday, April 06, 1998 12:12 AM
> >
> > Greenies
> >
> > I noticed in the manual for the HW-101 it says all test results were
> > made with the IM-11 VTVM, and the use of any other might give far
> > different readings..  Well, I guess then I am in search of an
> > inexpensive IM-11.  anyone have one? Also anyone have a photocopy of the
> > manual for the IT-12 Signal tracer?
> >
> > Thanks guys/gals
> >
> > Sean
> >
>
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