Fluke 731B DC Reference Standard
The 731B is a laboratory reference based on a zener plus NPN transistor combination on a common substrate. It has fixed 10 V and 1 V ranges, and three variable ranges, deltaE (zero to 999 µV), 1.018 V + deltaE and 1.019 V + deltaE. The absolute accuracy for the fixed ranges according to the data sheet is 30 ppm within 1 year. When introduced around 1974 the front panel was black, but later models have a light grey/green front panel.
The 731B was originally delivered with an internal battery pack for field use or simply to allow total isolation from the mains. However, the battery pack will eventually wear out and has to be replaced or to be removed alltogether. If you decide to remove the battery pack for one reason or another, be aware that the output noise ripple will increase unless you carry out a minor change. Take a glance at the diagram in the manual (available on the net), and you will see why: The diode CR5 and the resistor R30 used to charge the battery pack inject ripple current to the summing node of CR8 and CR9. Another consequence of removing the battery pack is that the front meter meter pegs and may emit audible noise when the 731B is turned on. The previous owner of my 731B had removed the battery pack, and I decided to operate the 731B from the mains only. In order to reduce the ripple I simply removed the diodes CR5 and CR8. For a sensible meter reading, R32 was removed, and 1.5 kOhm resistor was put in series with the meter.
The 731B was originally delivered with an internal battery pack for field use or simply to allow total isolation from the mains. However, the battery pack will eventually wear out and has to be replaced or to be removed alltogether. If you decide to remove the battery pack for one reason or another, be aware that the output noise ripple will increase unless you carry out a minor change. Take a glance at the diagram in the manual (available on the net), and you will see why: The diode CR5 and the resistor R30 used to charge the battery pack inject ripple current to the summing node of CR8 and CR9. Another consequence of removing the battery pack is that the front meter meter pegs and may emit audible noise when the 731B is turned on. The previous owner of my 731B had removed the battery pack, and I decided to operate the 731B from the mains only. In order to reduce the ripple I simply removed the diodes CR5 and CR8. For a sensible meter reading, R32 was removed, and 1.5 kOhm resistor was put in series with the meter.
Above: When the battery pack is removed from the 731B, the mains noise is increased significantly, The graph shows the noise when the 731B is set to 10 V output. The noise level in a 10 kHz bandwidth was measured to 324 µV.
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Above: After modification for mains only operation, the noise level in a 10 kHz bandwidth drops from 324 µV to 5.7 µV RMS. Note the different y scale.
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