The HP 970A is a handheld multimeter introduced in late 1973.

[This article appeared in the October 1973 issue of CQ magazine.]

Hewlett-Packard Digital Multi-Meter Fits In the Palm of Your Hand

Seems that we just get accustomed to a new concept when someone comes along and introduces still a newer concept. At least that's the way it seems in the area of digital multi-meters which amateurs have begun to accept and use. Just recently, however, Hewlett-Packard broke the multi-meter market wide open by introducing their Model 970A DMM, a palm-sized probe/meter tagged at a relatively modest $275.

The Model 970A is an auto-ranging-type instrument measuring a.c. or d.c. volts, and ohms over five ranges. All electronics including the display and rechargeable battery pack are contained in a small, rugged hand-held package. Only one function control need be set, to select the parameter to be measured, and only two terminals are used for all functions.

HP uses a five-digit LED (Light Emitting Diode) cluster to provide a 3-1/2-digit DMM, so that all probe voltage readings are in volts, and resistance readings in kilohms. There are no scales to misinterpret. Decimal placement is automatic.

From an operational standpoint, automatic decimal placement and automatic polarity indication save time and increase accuracy. After setting the function selector (ac V, dc V or k&ohms;), the user simply touches the probe tip to the test point, presses a Push-to-Read bar, and the solid-state LED readout automatically displays the correct reading and polarity. When measuring ohms or d,c, volts it takes typically less than 2 seconds to range and settle to a proper reading.

Since the display is close to the point of measurement, a user working in closely-packed circuits can hold the probe in one hand without having to look away from the circuit to read the meter. The display can even be electronically inverted, so the operator needn't worry about reading 6's for 9's, or misplacing the decimal point if the probe is used upside down.

D.c. voltage from 0.1000 v. full scale to 500 v. is read to an accuracy of +/- (0.7% of reading + 0.2% of range). Full scale ranges are 0.1, 1, 10, 100, 1000 v. (500 v. maximum input).

A.c. voltages from 1 volt through the highest range, (500 volts r.m.s. maximum) from 45 Hz to 1 kHz, are read to +/- (2% of reading + 0.5% of range). Accuracy from 1 kHz to 3.5 kHz is +/- (3% of reading + 0.5% of range). On the 0.1 volt range and below, accuracy from 45 Hz to 1 kHz is +/- (2% of reading + 0.5% of range).

Resistance measurement accuracy is +/- (1.5% of reading +0.2% of range). Ohms rnages are 1 kilohm full scale (1 ohm resolution) through 10 megaohms. Maximum test current does not exceed 10 ma.

Nickel-cadmium rechargeable batteries and a battery charger come with the Model 970A. More than 2000 measurements can be made using the 'Press-to-Read' switch with a full charge. Batteries can be recharged in about 14 hours. The probe case is made of high-impact plastic with built-in stress reliefs to improve its resistance to an accidental drop. The probe is 6-1/2 inches long by 1-5/8 inches wide. The instrument weights 7 ounces.

For a full color brochure and more detailed information, circle A on the Reader Service Coupon on page 94.

[This article appeared in the January 1974 issue of Radio-Electronics magazine.]

RE Cover

I thought I'd lost the capacity for being amazed, but I really haven't. They handed me a beautiful little soft leather pouch, about the size and shape of a big hot dog bun. "What's that?" "Oh, that's just a self-contained, automatic-ranging digital multimeter. It'll read ac or dc up to 500 volts. and resistance up to 10 megohms." "Oh, ís that all? Hmm." Opening the zipper pouch, I found a device not as big as an electric toothbrush, with a coiled pigtail lead! (See this month's cover.)

It was a DVM, with all of the features mentioned above. This is the Hewlett-Packard HP-970A Probe M1lultimeter. It is, too -- a "probe". A folding test prod on the business end can be set to several angles. To take a reading. just clip the pigtail lead to the low side or ground of any circuit, and use the instrument itself as the "test prod"! The readout is a 5-digit LED unit, on the end of the case.

A flexible plastic band around the case is the function switch. It can he set for ac or do volts, or resistance (in kilohms). From then on, all you have to do is touch the prod to the circuit: the HP-970A does the rest by itself! Voltages read in five ranges: 0.1. 1.0. 10, 100 and 1000 (500 V maximum input, ac or dc).

The HP-970A is an "auto-ranging" type meter. Special circuitry adjusts the readout to whatever scale will display the reading best. Oddly enough, I used it for quite a while, taking several different voltage readings, before it dawned on me what this little thing was doing! It was setting itself to display the voltage, anywhere from about 0.5V up to about 250V. The decimal point moves, so that you always know what the reading is.

For resistance, the HP-970A reads in kilohms: thousands of ohms. For example, a 470-ohm resistor reads out as 00.470. A 2200-ohm resistor as 02.200, etc. For those times when we inadvertently do something that we know better than to do, a fuse resistor is included in the ohmmeter circuit! It will withstand 115V rms for up to one min ute, and 250V rms for 10 seconds. It is recommended that we take HP's word on this: I know I will!

Accuracy: the manual says 2% of reading plus 0.5% of the range in use. Accuracy of these tolerances is guaranteed. with measurements traceable to National Bureau Of Standards calibration. The input impedance is a very high 10 megohms on all ranges.

A separate Current-Shunt/Bench Cradle unit can he used with the 970A for reading alternating or direct current. Current ranges are from 100μA up to 1.0 A. The insertion voltage drop is only 0.2V or less.

The HP-970A is powered by a self-contained NiCad battery, which is somewhere between 9 and 11 V (depends on state of charge). It slips into the end of the probe case. A special charger is included: the battery unit slips out and plugs into the charger housing. Both probe and charger are completely idiot-proof. A rib along one side of the battery case must fit into a slot in both housings before the battery will go in.

The battery will operate the 970A for about three hours: when it drops below about 9.5 volts, the display dims and it's time for a recharge. The on-off switch has a "push-to-read" position for the first click. This can extend battery life considerably. The second click is normal on position.

The LED display is bright enough to read even in pretty bright sunlight. A Display Invert slide switch can he used to turn the figures over so that the readout can he right side up no matter what position you or the instrument are in. (They have a cute little thing for this! Tiny man-figures are on the case: one is always covered by the invert switch slider. Set this so that the little man you see is not standing on his head, and there you are!)

The actual test-prod is a short one, mounted on the underside of the case. It can he folded out, with several detents so that it can he set at any angle you want. The center part telescopes, and is completely insulated. It has a very sharp tip, to make perfect contact to PC hoards. etc. Two extension test prods are provided: one is a 5-inch with a sharp tip, and the other a special 2-inch, with a hollow, cupped tip. This one is very handy for hitting test-points, wire-wrap terminals, ends of wire leads on a PC hoard, and similar things. These extra tips tit in the socket on the end of the prod: the regular one just pulls out. Special pockets in the carrying case keep the extra rods safe.

HP-970A Internals

The electronics in the 970A are a miracle of compactness. Out of the 6 inches of the case, three inches is battery. and the rest is the works. Out of this small number, most of the space is taken up by the switches. The heart of the unit is a monstrous (electroni- cally) device roughly 0.5 by one inch. All that this contains is an analogue to digital converter, the attenuators, the auto-ranging circuitry. the auto-polarity circuitry and the logic! Outside of this, only 19 components and three switches are needed to make the whole thing!

The whole instrument is designed for "one-hand operation". This makes it potentially very useful in today's crowded instruments. The zippered carrying case even has a clip for carrying on your belt. With this, you can be the "Quick-Draw McGraw" of the electronics maintenance set!

I know too well the hazards inherent in firm statements: I have the lumps to prove it. So, I'll confine myself to saying that us of right now, this instruments looks to like the ultimate in "portability" when its versatility is taken into account. It should certainly he very useful to field engineers and anyone working with solid-state equipment. It would be most appreciated where the equipment to he checked is located in tight places!

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Updated May 21, 2020