Sencore Model FE-20 FET V.O.M.
THE latest addition to Sencore's line of solid-state meters is the Model FE-20 FET v.o.m. Several special features add to the usefulness of this instrument. First, the meter has both high-and low-power resistance ranges. In the low-power function, the resistance-measuring circuits are powered by less than a tenth of a volt. This is below the voltage that would cause most semiconductor diodes and transistors to conduct. Hence resistance of these devices can be measured in-circuit without turning the semiconductors "on" and causing incorrect readings. In the high-power function, the usual 1.5-volt battery is used to power the resistance-measuring circuits.
A second feature is the inclusion of a low-voltage range with a full-scale reading of only 0.1 volt. This makes it possible to measure very low transistor bias voltages.
Still another feature is the high-voltage probe that is included with the instrument. The probe is used on three d.c. ranges of the meter: 3 kV, 10 kV, and 30 kV. Now the technician can use his instrument to measure the high voltage at the h.v. electrode of color picture tubes without having to buy a separate probe or other meter.
The meter is battery-operated using standard "C" cells. Because of the very low current drain on these batteries, which are larger than many used in other solid-state v.o.m.'s, the batteries should last a long time. To further conserve battery power, they are disconnected completely when the meter's case is closed.
There are 9 d.c. voltage ranges, 3 high-voltage ranges, 9 a.c. voltage ranges, 7 resistance ranges, and 9 d.c. current ranges up to 1 A. Input resistance is 15 megohms on d.c. and 10 megohms on a.c. for minimum circuit loading. Accuracy is within 1.5% on d.c. and 3% on a.c.
Price of the Model FE-20 is $130.
Eico Model 379 Sine / Square-Wave Audio Generator
A good audio generator is indispensable for testing hi-fi and audio amplifiers and transducers. The generator should have low distortion and, if it produces square waves as well as sine waves, it's even more useful. The Eico Model 379 is just such an instrument. It produces stable sine waves from 20 Hz all the way up to 2 MHz. Hence, the generator is useful at ultrasonic and low radio frequencies as well as audio.
Accurately shaped square waves are available up to 200 kHz. By using square waves and observing on a scope how much your amplifier alters the squareness, you can do a quick check of frequency response.
The Model 379 is one of the manufacturer's new line of all solid-state test instruments that boasts a 5-year warranty for the factory-wired version.
There is no charge for repair parts and labor that may be required for the first 90 days, and there is only an $8 service fee thereafter up to 5 years. No additional charges are made for repair parts, labor, or handling.
The generator that we checked out had good calibration accuracy and flatness of output. Over the first 4 ranges (20 Hz to 200 kHz), the calibration accuracy averaged +0,-2.7 percent over-all, while on the highest range (200 kHz to 2 MHz) it ranged from +2 to-4 percent. These figures are well within the manufacturer's specs. We also found the amplitude response extremely uniform up to about 1 MHz, above which frequency output rises gradually. Sine-wave output voltage was measured at from zero up to 7.5 volts maximum into a high-impedance load. The square-wave output varied from zero to 13 volts (p-p). The circuitry consists of 8 transistors and 11 diodes. Sine waves are produced by an RC oscillator using a bridge-"T" network between output and input. A FET is included in the positive-feedback loop and a lamp, used as a thermistor, is in the negative-feedback loop for good stability.
The separate square-wave circuitry takes the sine-wave signal and feeds it into a 2-transistor Schmitt trigger. The square waves from the Schmitt trigger are then applied to an emitter-follower output stage for good load isolation.
The instrument's power supply includes a power transformer and uses a three-wire power cord for maximum safety.
The Model 379 measures 8 1/2-in wide by 7 1/2-in high by 8 1/2-in deep. Price is $70 in kit form or $95 factory-wired.
----------
Audio Equipment Co. "Rayguard" TV H.V. Monitor.
ALTHOUGH color-TV set manufacturers are working hard to minimize x-radiation, many consumers are still a little wary. The fact remains that some of the early receivers as well as a few of the more recent ones could be made to emit x-radiation that exceeded the safe limits as set forth in government standards. During tests on such receivers it was found that if a set's high voltage were boosted above the value recommended by the maker, x-radiation would go way up.
To make sure that this doesn't happen with sets in use, the Audio Equipment Co. (a division of Walter Kidde) has introduced what they call "Ray-guard"-a TV warning device. This is a high-voltage monitor that the technician installs on his customer's receiver and which is left permanently attached to the receiver in the home. When the consumer sees the pointer on the monitor's meter in the safe area, he knows that his set's high voltage is at or below its normal value. If the pointer rises to the danger area (with its radiation symbol on the meter face, rather than achy al voltage readings), it's time for him to call the technician to check his set.
The device itself is actually a high-voltage multiplier resistor voltage divider and meter-protective circuit in a special high-voltage housing that is placed inside the receiver. A ground lead is then connected to the set's chassis and a clip leading from the divider is placed under the h.v. electrode of the picture tube by the installing technician. The readout meter, with its calibrating pot, is connected to the divider and hung on the back of the receiver. The calibration is originally set at 25 kV, above which a dangerous condition is indicated. However, the technician can readjust the meter for different normal high voltages as recommended by the set's manufacturer.
"Rayguard" has a single-unit list price of around $40 but its price goes down in higher quantities. The manufacturer feels that some technicians will install the monitor at a nominal fee with the hope that they will be called upon to service the set when it becomes defective.
Also see: A Slot-Loading Cassette Transport