[NJARC] Old Battery Question

[Jim Whartenby]

Greetings Scott
You express very well why most of us like to repair and collect old vacuum tube equipment.

There are several reasons why your "True RMS" DVM is giving you readings that may not seem right.  The first thing to remember is the difference between accuracy and precision.  A good tutorial is:

http://www.mathsisfun.com/accuracy-precision.html

The second item to consider is how much the meter loads the circuit under test.  A good explanation is found here:

http://www.allaboutcircuits.com/vol_1/chpt_8/3.html

As I understand the issues that you bring up; you prefer to use period test equipment to repair equipment of roughly the same age.  There is no problem with this as many of us have done just that.  You can also use modern test equipment and just ignore the numbers past the first two or three significant digits.  This should give you the same precision that reading an analog meter will give you.

The precision of the measurement when you are measuring B+ with your DVM is most likely caused by the AC ripple in the power supply.  Your DVM is so good, it is trying to measure a fluctuating voltage.  An analog meter would average this fluctuation out because of the mass of the meter pointer.

An accuracy issue is caused by the meter loading the circuit.  This changes the actual voltage at the measuring point.  The old VOMs are 1000 ohms per volt, the better VOMs like the Simpson 260, Triplet and the like are 20,000 ohms per volt and some are 50,000 ohms per volt.  This means that the Simpson will read a bit higher then the former since it loads the circuit less.

Your DVM is most likely 10,000,000 ohms per volt with 4 1/2 digits and is highly accurate.  The DVM is comparable to a VTVM but the precision is at least another digit or so because of the issue of reading an analog scale better then three significant digits.

You can do a test with the battery you are assembling.  Measure the cell voltage with as many meters as you have.  All should agree at the two most significant digits with the DVM able to tell the actual voltage to  better then four digits when measuring a single cell.  In other words, all of your meters should tell you that the cell voltage is 1.5 volts.  The DVM should tell you that the cell is 1.5123 volts.

Hope your Thanksgiving was as good as mine,
Jim



--- On Thu, 11/27/08, Scott Roberts <ng19delta at yahoo.com> wrote:

> From: Scott Roberts <ng19delta at yahoo.com>
> Subject: Re: [NJARC] Old Battery Question
> To: "New Jersey Antique Radio Club" <njarc at mailman.qth.net>
> Date: Thursday, November 27, 2008, 1:24 PM
> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Hi Guys-
> As I mentioned, the 75% seemed suspect to me-  my DVM
> drives me nuts: I know the VTVM in my Precision set tester
> is reasonably accurate, enough for my needs. The set tester
> I am working on is one where it plugs into the tube socket,
> and the tube goes in the other end, and you get reading on
> all the voltages & resistances, etc for the tube socket.
> 
> I would enjoy working with newer test equipment. I
> haven't any much beyond the oscilloscope, but even that
> is old. I can't afford the newer stuff, and am quite
> happy getting the older stuff to work. It is less
> complicated, and the digital "True RMS" I have
> just annoys the daylights out of me- I like a readout which
> stabilizes- not spends all its time wandering about.
> 
> Anyway, I'm building a battery pack, actually two, and
> hope to have the set in operation tonight sometime.  Then I
> can finish building a test adapter(the set had all but one
> of its adapters- the one for octal. Figures. However, it has
> a standard 7 pin base, and I had one on a tube which was
> suspect, so I carefully removed it, and am mating it to an
> octal socket, having devined the proper pin-out for the
> octal adapter, based on the socket which clips on the end to
> go into the set. Once it is all together, I can probably get
> some reasonable readings on what the BC-348 is doing. Sorry
> it seems like I do things the hard way, but I have to work
> with what I have.
> 
> Scott
> 
> 
> --- On Thu, 11/27/08, Marty Friedman
> <radiomf at comcast.net> wrote:
> 
> > From: Marty Friedman <radiomf at comcast.net>
> > Subject: Re: [NJARC] Old Battery Question
> > To: "New Jersey Antique Radio Club"
> <njarc at mailman.qth.net>
> > Date: Thursday, November 27, 2008, 9:33 AM
> > Visit our web site - See http://www.njarc.org
> > _______________________________________________
> > Hi Scott,
> > 75% CHARGE might be ok on a  battery, but if it is 75%
> > VOLTAGE consider it dead for all practical purposes. I
> agree
> > with John that you should restore one item at a time.
> RMS
> > voltage should be of no consequence with DC voltages.
> > Marty
> > 
> > 
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