Ever bought a set of those cheap Chinese-made multimeter probes? They probably had some little plastic inserts in the banana plugs. For what reason I’m not sure, maybe to keep the safety shroud around the plug from deforming. Don’t toss them in the trash. Instead toss the probes in the trash (they’re not that great) and keep the inserts, because you can fix the broken inner jack shrouds on your $300 Fluke DMM with them!
Tektronix may be known best for their industry-leading oscilloscopes, but they also sell other test equipment, including a line of handheld multimeters in the 1990s.
The model documented here is a DMM912, part of a series of multimeters that included the 912, 914, and 916. All are RMS-responding, 4000/40,000-count models with a number of premium features like min/max/average, auto-hold, peak-hold, AC+DC, lead warning, and memory storage. The display updates 4/sec in 4000-count mode or 1/sec in 40,000-count mode. Accuracy and bandwidth increases with each model; this 912 is rated at 0.2% basic DC accuracy and a bandwidth of 1kHz. The 914 and 916 make use of a secondary numeric display for simultaneous ACV and Hz readings, the 912 does not.
Posted in Teardown
Tagged DMM, Tektronix
The basic Beckman multimeter design that seems to apply to many of their 3½ digit models from the late 1970s onward include an LCD/processor chip assembly held together by various plastic parts that simply clip together. The later Wavetek-branded models in the 1990s changed the design slightly to use self-tapping screws to hold the stack together.
The main processor chip (part #270-100) and the LCD are both connected to the main circuit board via elastomeric connectors (aka “zebra strips”). These parts make up a stack that must be held together very tightly in order for the connectors to function correctly. When it fails, the meter may having missing or faded LCD segments, it may display nonsense or bad readings, or simply fail to power up at all.
The first two generations of the popular 70-series Fluke handheld DMMs included the 21 and 23 models, which were the same as the 75 and 77 respectively, except their cases were safety yellow instead of the usual dark gray. The third generation of the 70-series also included the 21-III and 23-III models as well, but they look exactly the same as their 7x-III counterparts. It is my understanding that the only difference was that the 2x-III models were packaged with premium test leads. Just marketing, I suppose.
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.
The HP 3468A multimeter repaired in the last post uses something called “closed-case” or software calibration. Instead of making physical adjustments to controls inside, calibration involves applying known reference levels and having the meter store calibration constants. These are essentially offset values that will allow the meter to show a correct value even though internally there are small errors due to normal component tolerances. It is obviously important to store this data permanently.