I have now been disappointed three times with Ebay 720a's. All were DOA completely or failed self-calibration horribly. These 720a's were sold by individuals at around the $1000USD mark. I can purchase a refurbished and guaranteed 720a for around $3000.00. If one is establishing a modest, and easy to keep in repair home metrology lab, maybe an older Fluke 5000 series calibrator is the way to go? The older calibrators are being offered, again by individuals, or surplus businesses, for around $950USD. The guaranteed working 5440b's go for $2000USD without calibration. There may be more hope, that a used calibrator, with its built in protective circuitry, can be purchased surplus with more success than the 720a. The 720a divider resistors are too easily cooked by idiots. In addition, from my own sad experience, the sealed "A" switch on the 720a has a much shorter lifetime than the open wafer switches. Thus, even the refurbished 720a may suffer an "A" switch failure shortly after the warranty period expires. IMHO, a 720a only makes sense if you buy new for business purposes. Why have I felt the need for a 720a? 1. My 731B calibration procedure calls out a 720a. 2. I have a Fluke 750 divider that I acquired to calibrate my HP3456A voltmeter. The 750 divider manual calls out a 720a for its calibration. 3. Procedure for calibrating my HP735A to 1v +/- 2ppm from my Fluke corp. calibrated 731b calls out a Kelvin-Varley Divider. Can a Fluke 5000 series calibrator take the place of a Kelvin-Varley divider in all of the above procedures? Suggestions Welcome, Stan

A fluke 5440B would come close if you had a way to calibrate it every 30 days. You can come close but you will need 10 volts and a good null meter to calibrate it at 10 volts on the 10 volt and 20 volt ranges. You can stop here but you should really use a divider to get the 1000, 100, 2 and .1 volt points it would really like calibrated. You can use an HP 3458A 10K ohms and 10 volts to get about the same thing. Use the 3458A to replace the 720A divider and null meter. The transfer standard / linearity specs are close to a 720A The 5100 is more versatile but not as stable as the 5440 and a lot harder to calibrate. On Wed, Apr 22, 2015 at 9:24 PM, Stan Katz <stan.katz.hk@gmail.com> wrote: > I have now been disappointed three times with Ebay 720a's. All were DOA > completely or failed self-calibration horribly. These 720a's were sold by > individuals at around the $1000USD mark. > > I can purchase a refurbished and guaranteed 720a for around $3000.00. If > one is establishing a modest, and easy to keep in repair home metrology > lab, maybe an older Fluke 5000 series calibrator is the way to go? The > older calibrators are being offered, again by individuals, or surplus > businesses, for around $950USD. The guaranteed working 5440b's go for > $2000USD without calibration. There may be more hope, that a used > calibrator, with its built in protective circuitry, can be purchased > surplus with more success than the 720a. The 720a divider resistors are too > easily cooked by idiots. In addition, from my own sad experience, the > sealed "A" switch on the 720a has a much shorter lifetime than the open > wafer switches. Thus, even the refurbished 720a may suffer an "A" switch > failure shortly after the warranty period expires. > > IMHO, a 720a only makes sense if you buy new for business purposes. > > Why have I felt the need for a 720a? > 1. My 731B calibration procedure calls out a 720a. > 2. I have a Fluke 750 divider that I acquired to calibrate my HP3456A > voltmeter. The 750 divider manual calls out a 720a for its calibration. > 3. Procedure for calibrating my HP735A to 1v +/- 2ppm from my Fluke corp. > calibrated 731b calls out a Kelvin-Varley Divider. > > Can a Fluke 5000 series calibrator take the place of a Kelvin-Varley > divider in all of the above procedures? > > Suggestions Welcome, > Stan > _______________________________________________ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > -- *John Phillips*