Re: {Collins} 30L1 meter shunt continued - wrong value for R10



Jim,

You lost me. I am not sure where all these leads you are suggesting are actually connected to the meters and which meter goes where on the resistor.

As Pete mentioned, I guess you could measure current on an ohm meter from a supply with a given load and then match a shunt resistor to duplicate the ohm meter reading on the shunted meter.

Lee, w0vt



-----Original Message----- From: Jim Garland
Sent: Friday, January 06, 2012 10:39 PM
To: 'Collins Collectors Association'
Subject: Re: {Collins} 30L1 meter shunt continued - wrong value for R10

The best way to measure low resistances is using a so-called 4-probe method.
Instead of using a common 2-lead ohmmeter, one uses a sensitive DC voltmeter
and ammeter. Two of the leads go to each end of the unknown resistor. One
lead at each end of the resistor supplies a known current through the
resistor, and the other two leads go to the measuring voltmeter. Because no
current (or a negligible current) flows through the voltmeter leads,
presumed to have a very high imput impedance, the resistance of the leads
doesn't contribute to the reading. The technique can be used to measure
microohms with great precision, although doing so requires lab grade
voltmeters and ammeters.
Many bench voltmeters (Fluke, Keithley, HP, etc.)  have built-in provisions
for making 4-probe resistance measurements, and these can easily resolve .01
ohms with very good accuracy.
73,
Jim Garland W8ZR

-----Original Message-----
From: lee [mailto:pulsarxp@xxxxxxxxxxxxxx]
Sent: Friday, January 06, 2012 9:28 PM
Cc: Collins
Subject: Re: {Collins} 30L1 meter shunt continued - wrong value for R10

Talking about substituting meter shunt resistors, I'd like to know what
others do for substituting them in all kinds of rigs.  In the past I have
always purchased 1% values from Mouser and then added them in series and
parallel to get the exact value needed.  of course this sometimes means
using three of four resistors to come up with the value and then do you
wind
up with 1% tolerance?  Maybe not with just a small sample.  In rare
instances I have wound them with small copper wire when a very small value
is needed using tables to tell me how long a certain wire should be with a
given wire size.  What bugs me is a very small values I don't trust any
volt
ohm gear around the shack to measure my finished product.  (I wish I had a
super low resistance meter, but then, without yearly calibration, how
accurate would a meter like this be and could you really trust it without
constant lab certification).  Larger shunts are not as great a problem to
duplicate when needed.

That said,  I am just curious what others do when faced with this problem.

Lee, w0vt

=================


On Jan 6, 2012, at 3:23 AM, Dick Weber wrote:

> Smith and Gentlemen:
>
> The value for R10 shown in earlier 30L-1 manuals is listed as 1960 Ohms.
> This is wrong. The correct value listed in later manuals is 1780 Ohms.
In
> my two 30L-1s I've found that 1740 Ohms gives the most accurate meter
> reading when reading plate current. In either case the resistor should
be
> a 1% resistor.
>
> I found out about this error in the manuals the hard way after R10
burned
> out due to a power supply problem. The resistor was so burned I couldn't
> read its value. So I checked my manual to find the listed value to be
1960
> Ohms. After putting it in, it seemed that the plate current meter was
> reading too low. Then with some testing, I found R10 should be more like
> 1740 Ohms, which then lead me to find out that later manuals list R10 as
> 1780 Ohms.
>
> 73,
> Dick, K5IU.
>
>
>
> post to the group!

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