Fluke 5440B Direct Volts Calibrator
The figure above shows the result of one of the first tests I made of the 5440B after having checked the basic functionality: The output voltage after power-on (cold state after 24 hours without power) compared to a Fluke 731B, both set to 10 V. It ends at -90 ppm off, after some 4-5 hours. No internal calibration was made, and would be required once the 5440B has settled.
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The Fluke 5440B is a very well-performing DC voltage calibrator, acclaimed for its stability and with only a few other calibrators in its class.
At the heart of the 5440B you find a series combination of two ovenized SZA263 references, and the voltage scaling is done by a dual string PWM. The principles are explained well in the Appendix A of the 5440B service manual. The 5440B I got was in operational condition, though with a rather dim display, which is something to expect from these aging calibrators. Also, the voltage was about 1 mV off on a 10 V setting (before internal calibration), but before pursuing any calibration, internal or external, I decided as a preventive measure to replace a few components in the 5440B that are known to be at the risk of failure. Just check out eevblog, and you'll know. When taking a closer look at the modules I could see that someone had already done a few changes: A small selection of electrolytic capacitors had been replaced, though of dubious make, and there was a resistor mounted in parallel with R1 on the Outside Regulator board. I went through the boards systematically and replaced a wide variety of capacitors with 105 C low-Z models from Rubycon. The few low-leakage variants for timing were replaced with Nichicon UKL. I cleaned up the scorched area around R1 on the Outside Regulator and replaced R1 with 3 resistors in parallel in order to lower the temperature. There's a few other areas on the 5440B's boards that are discolored by heat, though not as much as the area close to R1 on the Outside Regulator. By looking more closely at the components close to the discolored areas I found that a few resistors and zener diodes in the 5440B are too close to their maximum dissipation. For the sake of reliability I replaced these components with models having a slightly higher power rating. The area around the regulator U9 on the Inside Regulator board was also discolored, and I added a small heatsink on the top of the tab of U9. After you have powered up the 5440B you should wait a while before putting it into use, or to calibrate it, for that matter. The user manual states "a minimum of twice the time it was turned off (up to a maximum of 2 hours of warm up)...". However, by comparing the voltage between the 5440B when powered up from a cold state with a constantly powered up Fluke 731B (both devices set to 10 V) it seems that one should wait about 5 hours rather than 2. A nice thing is that the voltage seems to settle without overshoot or instability. |