The HD140 digital multimeter from Beckman Industrial is another in the “heavy-duty” series, very similar to the HD110 in a previous article. Unlike the HD110 however, this is a 20,000-count 4½ digit model with RMS-responding AC capability.
Unfortunately I can find no information on it other than the fact it shares a service manual with the 4410 model. Presumably the internals are the same, except that the HD140 has the extra-sturdy safety yellow enclosure.
This unit arrived with a small problem in the two least-significant digits. Some segments were very dim or flickering. This is the classic “faded segments” problem that often plagues older instruments with liquid-crystal displays. It is usually caused by contamination on the contact surfaces of the elastomeric connector (a.k.a. zebra strip) that couples the very low-current drive signals from the printed-circuit board (PCB) to the glass LCD. This problem is often affected by mechanical pressure on the LCD. In other words, you can squeeze it and make it look better or worse.
Mere Alcohol Doesn’t Thrill Me At All
A quick cleaning of the PCB pads, connector strip, and the glass LCD contact surface with isopropyl alcohol (IPA) usually clears up the faded segment problem, just as it did here. The simple cleaning procedure doesn’t warrant a lengthy explanation, so let’s just have a look at the restoration process from start to finish, and then take a good look at how it’s made.
Missed It By THAT Much
I’m not very impressed with the extra digit of resolution in this unit. It tends to “bobble” quite a bit, even for a very stable input. It is normal for a DMM to flicker between two sequential values on occasion, but this one tends to make excursions both up and down by 1 count. Not a huge problem, but it does not instill confidence in the device’s precision. Without documentation, I can’t say for sure this unit meets its accuracy specifications. The extra digit of resolution makes it hard to appear “spot-on”, but with an error of 0.06% at 5VDC, I would hazard a guess that it is in good shape.
Many DMM models have a simple calibration procedure that involves the use of a single low-voltage DC reference level (usually around 90% of full-scale) and a single trim potentiometer to adjust the unit’s basic DC measuring ability. The accuracy of the rest of its functions will then simply depend on the tolerances of components like the precision resistors used in the voltage divider network. Others have a more complicated set-up. This particular model falls into the latter category.
The HD140 has 3 variable resistors and 3 variable capacitors. None of them are labeled. The trim caps are there to compensate the voltage divider network at AC frequencies higher than just 50 or 60Hz mains. Proper adjustment requires the manufacturer’s recommended calibration procedure, and a means to generate the stable reference signals required. Lacking both the procedure documentation and the necessary equipment, no effort was made to adjust this unit’s calibration.
DMMCheck calibration check results for this Beckman HD140:
|Ohms||100Ω||0.1006K||0.0006K with leads shorted|
Current consumption was measured at 1.42 mA on both AC and DC function settings. The low-battery annunciator comes on at a battery voltage of 7.15V. Accurate measurements were still possible down to 5.00V. Open-circuit voltage on the 2KΩ (diode-check) range was 3.88V, enough to illuminate and measure the Vf of a red LED.