Test Equipment Product Report

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(source: Electronics World, Aug. 1963)

RCA Model WR-51A FM Stereo Signal Generator

A VERY specialized signal generator is required for servicing and maintenance of FM stereo adapters, tuners, and receivers. The new RCA WR -51A is such a generator; it produces all the signals needed for proper adjustment of stereo circuits. This unit must be used in conjunction with a scope to observe response and output waveforms of the equipment being adjusted.

The generator produces a composite stereo output signal with either left -only or right -only modulation. A special left-plus-right signal is also available for accurate phase adjustment of subcarrier transformers. Various audio frequencies (400 cps, 1 kc., and 5 kc.) are utilized for modulation. A 19-kc. crystal -controlled subcarrier signal is provided for checking the lock-in range of stereo receivers. Sine-wave outputs at 28 kc., 38 kc., and 67 kc. are generated for band pass and SCA filter network adjustment.

Built into this unit is a 100-mc. r.f. oscillator that may be frequency modulated with the above signals. This permits the stereo information to be applied to the front ends of FM tuners and receivers for overall r.f. - i.f. alignment checks. In the event that there is an FM station right at 100 mc., it is possible to shift this frequency slightly by means of a front -panel control.

Another crystal oscillator in the generator (at 5.35 mc.) is used to produce a 10.7 -mc. i.f. marker for the tuner's i.f. response curve. In addition, harmonics of this oscillator may be picked up at 90.95 mc., 96.30 mc., 101.65 mc., and 107 mc. for a check of a tuner's r.f. and oscillator alignment.

The instrument also includes a zero center meter that measures 1.4 v. r.m.s. either side of zero. This meter is used to check the electrical balance of a stereo amplifier audio output. Two cables at the rear of the instrument are connected to the voicecoil output terminals of the amplifier. With a mono or an L +R signal applied to the amplifier, there should be equal outputs from both channels and the meter will read in the exact center. With a left -only signal, the meter will deflect to the left; with a right -only signal, it will deflect to the right.

A block diagram of the instrument is included. Considering all the functions provided, the unit is remarkably simple.

It uses only 6 dual -purpose tubes, 4 crystal diodes for the balanced 38-kc. modulator and meter rectifiers, 2 crystals for frequency control, and a silicon rectifier.

Accompanying the WR-51A is an excellent manual: This not only goes into complete details on the operation and use of the generator, but also includes a fine section on maintenance and adjustment of the unit itself. Some of our test equipment manufacturers would do well to study this manual to see how it should be done.

The generator is fairly portable, weighing 14 pounds and measuring 13',_ "x 10 "x 8 ". The price of the unit is $ 249.50.


Associated Research 2850 Megohmmeter

A NEW test instrument designed for field and bench testing of insulation on electrical components and equipment has been introduced by Associated Research, Inc. as the "Vibrotest" Model 2850. This is a megohmmeter that is able o measure resistances as great as ten million megohms at 500 volts d.c., or one million megohms at 50 volts d.c. Accuracy of ±2 percent is achieved over most of the range at 500 volts, and ± 5 percent over most of the range at 50 volts.

The meter can be used to measure resistance between conductors on a printed circuit, as well as insulation resistance of transistors, miniaturized parts: cables, and motors. It also measures leakage resistance of capacitors and can be used for measuring grounded and ungrounded sections of three-terminal resistors.

Many megohmmeters use a hand cranked generator or batteries to produce the test voltages. In this instrument, a.c. line power is rectified by silicon diodes to produce the d.c. required.

To prevent line-voltage variations from affecting the readings obtained, a cathode-follower voltage-regulator circuit is used (see block diagram). A fused three-wire a.c. line cord grounds the case for shock protection.

The instrument is protected from overload damage, even in cases of direct shorting, by an electronic overload circuit. The metering circuit uses miniature vacuum tubes, and the meter itself is a 200-µamp. unit.

Guard circuits are employed to control stray and leakage currents to assure accuracy at maximum sensitivity. A terminal for external guard use is provided, and a selective-return grounding circuit offers maximum guarding flexibility when the instrument is utilized to test production components or permanently installed equipment.

The instrument weighs about 10 pounds and is available for $250.


Sencore CR125 Cathode-Ray Tube Tester

THE most expensive component in a TV set is the picture tube. Fortunately, modern picture tubes are quite ruggedly built and they ordinarily last a long time. Although the tube's heater does burn out or open up completely, it is far more common for the tube to operate month after month but at reduced emission (and consequently, a darkened picture). Another common defect is the depositing of cathode or other foreign material between the tube's electrodes, resulting in inter-element leakage. This has many effects, ranging from lack of brightness control, poor contrast, dark picture, bright picture, and others, depending on the location of the leakage and the circuitry.

The new Sencore CR125 not only is a complete tester for picture tubes, but it also is frequently able to remove inter element shorts, restore emission by rejuvenation, and weld together open cathodes. The instrument can check tubes ranging from an 8-incher all the way up to the 30-inch tube, and this includes color-TV picture tubes as well.

The tests performed include those for inter-element shorts, cathode emission, and a check on the operation of the control grid. For these tests, d.c. is used on all tube elements (except for the heater) and emission of over 300 pa. is indicated as "good" on the meter scale.

New tubes normally measure 900-1500 pa., but since the high end of the 500-µa. meter is suppressed by a crystal diode, the meter does not go off scale. The grid bias can be adjusted from 0 to -100 v., and the effect can be noted directly on the emission meter.

Suppose the tube you are testing shows leakage or a short between control grid and cathode. The tester can then be used to remove this short. It does this by applying momentarily to these electrodes a high voltage (450 v.) from a large electrolytic capacitor (C5-B in the diagram). The length of time that this voltage is applied depends on the amount of leakage; with high leakage (low resistance) a large current flows for perhaps 1/6 second or so; with low leakage, less current flows for a second or more. If the material causing the leakage is burned away during this process, you may see a momentary flash in the neck of the tube. This procedure can be tried several times until the short is burned out.

If the tube being tested shows low emission, then the CR125 is set up for rejuvenation. During this procedure, we attempt to bring new active emitting material to the surface of the cathode and to enlarge the aperture in the control grid. There are three steps available for the rejuvenation procedure. In the first step, a high voltage is momentarily applied to the control grid. If this does not do the job, then the second step is tried. Here, the heater voltage is raised slightly while the high voltage is applied. If the tube still does not respond, then the third step is tried. The heater voltage is raised still more and the length of time that the high voltage is applied is lengthened.

When color-TV picture tubes are tested or rejuvenated, each of the three guns is checked and connected to the tester separately, as determined by the position of the "Color Gun" switch.

Like all Sencore's instruments for the service technician, the cathode-ray tube tester is designed for convenience and portability. It is available at the company's distributors for $69.95.

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Updated: Friday, 2017-10-06 20:34 PST