[volt-nuts] Wanted wirewound precision resistors and ESI DB877 help

[Electronics and Books]

A source are old Fluke DC potentio meters. These are calibrators with a null detector. On ham fares they are dirt cheap and full of precision resistors for the decades.

 

ElectronicsAndBooks at Yahoo dot com

From: Fabio Eboli <FabioEb at quipo.it>

To: volt-nuts at febo.com 
Sent: Monday, June 18, 2012 1:33 PM
Subject: [volt-nuts] Wanted wirewound precision resistors and ESI DB877 help
 
Hello, I have an ESI DB877, when it arrived
it was electrically open.

In short:
I need to find or build replacement for one each
20, 200, 20k and for 4 of the 5 2Meg resistors.
Any advice where to find suitable resistors without
going bankrupt is welcome.
If anyone has precision wirewound resistors to
sell can contact me directly.

The full story:
The unit is in a not good condition, with signs of
resistor overloading, corrosion due to humidity
and wet of oil.
The selectors are ok, the contacts measure about
6-8mOhm and are very repeteable. The working
resistors seem very good, measuring values few
ppm far from the "should be". I'm using a multimeter
calibrated in 2004 so probably the DB877 resistors are
better than the keithley :)

The unit contains 8 decades, each built using one
"1" resistor and 5 "2" resistors, for example the
kohm decade is made using 1 1kohm and 5 2kohm
resistors, soldered around a selector. The
selector connects one 2khom in series per
each even position of 2,4,6,8,10 and puts the 1kohm
in series to the group in odd positions of
1,3,5,7,9
The resistors are wirewound on mica sheet the
lower values, and on ceramic formers the 100k
200k 1Meg and 2Meg resistors.


The 20Ohm was replaced by a wirewound pot,
corroded and unusable, the 200ohm was burned
and the 20kohm was open circuit due to a dent
in the wire.

The 2Meg resistors measured reasonable values,
seem there was some leackage, so I decided to
test them near rated current.
I connected the voltage source and an ammeter
so to have 100uA in the resistors
(are rated to 130uA nominal).
After few minutes the current flowing started
dropping and the resistors mesured open circuit :(

Now I fear I've killed some good 2Meg precision resistor!
What happened? The voltage and current stress was
lower than the nominal values and far less than maximum.

The resistors are leacky: once open circuit the
current still flows trough them but less than
should be and unstable. The only thing I can
imagine is some electrolythic corrosion but
really I dont know.

On the resistors here is written by hand 2Meg +-0.003
Any advice where to find replacement is welcome.

Fabio.

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