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 <[email protected]> To: [email protected] 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. ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
