I noticed that some of the vendors listed overall end-end tcr and at least
one that listed the wiper tcr too. Hopefully you can get the datasheets
with both.

Todd

On Saturday, September 26, 2015, David Garrido d.garrido@me.com wrote:

David:

Run the numbers.  Assuming that the 5K-20 turn pot R1005 is set in the

middle of it's travel (2.5K), that all of the other resistors, in series and
parallel ( R1006,R1007,R1008 and R302), that make up one step of the "A"
decade have a "0" TC, only R1005 has a maximum of +/- 100 ppm per degree C,
then the maximum change on the total resistance between step 0.9 and 1.0 of
the "A" decade would only be 0.000000410 % change over the entire +/- TC of
R1005.  That is 0.00410 ppm.  The "Absolute Linearity" of the "A" decade is
specified at 0.1 ppm at +/- 1 degree C, without a temp correction chart.  Of
course all of the other resistors mentioned have a TC also, which could be +
or - to the total nominal of 10,000.  Therefore, unless I am mistaken, the
less expensive +/- 100ppm pots would work just fine and you could not
observe or measure the changes.  This is also why I didn't worry about my
"trimming" R1008, the "factory selected" resistor, slightly to get the 20
turn pots to adjust to the middle of their ranges, when necessary.

IMHO

Bill

----- Original Message -----
From: "David Garrido" d.garrido@me.com
To: volt-nuts@febo.com
Sent: Saturday, September 26, 2015 5:05 AM
Subject: Re: [volt-nuts] Fluke 720a self calibration repair

3800P-502?

tempco and I found info that indicated 20ppm/C on one website, but that was
not a DS, just a listing of the info.  If they can be 50ppm/C or even
100ppm/C, then the price drops exponentially.

legendary performance.

https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts

Dave wrote:

Horse puckey.  See the current Bourns 3005 datasheet (below).  As I
posted previously, it is WW.

Charles

Yes, I see.
Please excuse my stupidity.

Dave M

Charles Steinmetz wrote:

David:

I just looked inside my 720A at the 20 turn 5K trimpots.  They are

Bourns 3005P-502 printed on them, cermets, standard 3/4", tempco 50 ppm.  So
the replacement is available directly from stock and nothing fancy.  Mine
are date code 8412 so my unit was built somewhere in the mid '80s.  Not too
sure where the idea of a wirewound pot came from here.  They are more than a
little pricey at $13.98 ea from Allied.  Looks like you need 24 to replace
them all in the "A" and "B" decades.  So that would be $335.52 unless you
buy 25 or more at which time the price drops to $12.51 which totals $312.75.
They come in tubes of 25 each which accounts for the price drop.

Digikey is $14.35 and Mouser is $14.35.  Wow!  Expensive little devils.

On the other hand (left) Digikey shows a unit from "TT Electronics/BI
Technologies" for $1.40 ea.  Interesting.  Why such a massive difference in
price?  These are 100 ppm but as I have noted in another posting the tempco
isn't very important.  It's the resistors in the oil filled can that set the
overall stability.

Bill

----- Original Message -----
From: "David Garrido" d.garrido@me.com
To: volt-nuts@febo.com
Sent: Saturday, September 26, 2015 5:05 AM
Subject: Re: [volt-nuts] Fluke 720a self calibration repair

3800P-502?

tempco and I found info that indicated 20ppm/C on one website, but that was
not a DS, just a listing of the info.  If they can be 50ppm/C or even
100ppm/C, then the price drops exponentially.

legendary performance.

https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts

Bill wrote:

Perhaps from the Bourns 3005 DATASHEET I posted a couple of days ago.

See https://www.febo.com/pipermail/volt-nuts/2015-September/004504.html

Cermet ($1.40) vs wirewound ($14).

Charles

Charles:

Mia Culpa!  I had the datasheet also but got caught by the description

that Allied used when I looked them up.  It showed "Cermet" but obviously
Allied & Mouser has these shown incorrectly.  Digikey shows these as
Wirewound.  How interesting.

I stand corrected.

Bill

----- Original Message -----
From: "Charles Steinmetz" csteinmetz@yandex.com
To: "Discussion of precise voltage measurement" volt-nuts@febo.com
Sent: Tuesday, September 29, 2015 8:46 AM
Subject: Re: [volt-nuts] Fluke 720a self calibration repair

in

https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts

Hi David,

My 720A's trimpots were original. I have them somewhere but they are a
bit scarce right now. Anyway they were 20 turn wire wound trimpots. I
suggest that you use wire wound ones and if 20 turn pots are not within
your budget then 10 turn will not be too much of an issue. On one or two
of my trimpots were very sensitive to adjustment so 20 turn is best.
Wire wound trimpots are far more reliable than the others. If they do
have a problem with the contact resistance going up it is usually fixed
by just giving the adjustment screw a little turn. Properly fixed my
720A tracks my 3458A very closely.

The circuit board was cleaned and clear coated after manufacture. Be
careful about creating leakage paths. Never spray anything on the
circuitry. I used to use Krylon to paint repaired circuit boards but
decided to just clean each connection with alcohol swab and call it a
day. I forgot to mention that all the decades need many revolutions to
condition the contacts before calibration and before each important use.

Charlie

On 9/29/2015 12:08 AM, Bill Gold wrote: