"Power - Line Monitor"
Radio Corporation of America,
Cambden, New Jersey, USA

RCA Description

    The RCA WV-120B Power-Line Monitor is designed for use by radio-TV repair shops, laboratories, and industries where it is important to know the power-line voltages at all times. This instrument provides continuous, accurate indication of the AC line at a glance, without the inconveniences of handling probes and switches or the necessity of selecting one of several scales to determine the proper meter reading.

    Used in conjunction with the RCA WP-25A, WP-26A or WP-27A isotap isolation transformer on a 50-60 Hertz power like, the WV-120A is a valuable aid in selecting line voltages for such applications as TV servicing, test instrument calibration, and the operation of electronic equipment that requires a known line voltage supply. A convenient power-line socket is located on the side of the unit.

    The WV-120A can be used on 120 volts AC power-line sources with frequencies of 25 Hz, 50Hz, 60Hz, or 400 Hz. The instrument features accuracy of +/- 2% at 120 volts and +/- 3% at 100 volts and 140 volts. The meter has a special expanded scale with large, easy-to-read voltage designations. Also, the top of the meter pointer is made wide enough to be seen clearly at a considerable distance from the instrument. As a convenient means of reference, the nominal AC line-voltage standard of 120 volts is printed in red.

    The meter movement in the WV-120A is a moving-vain type. This type of meter indicates RMS values even when the waveform of the power-supply voltage is distorted. (Peak reading and peak-to-peak reading AC voltage measuring circuits, such as used in conventional VTVM's are calibrated on the basis of pure sine-wave voltages, and such meters do NOT indicate the true RMS value of distorted waveforms.) The meter movement is only slightly damped and therefore has fast response. This feature is highly desirable because it permits the meter to indicate the presence of line-voltage "bounce" or fluctuations.

    Mounting holes are provided on the rear of the WV-120A so that the meter can be hung in a location that offers the best visibility throughout the area.

RCA Operation

    To use the WV-120A, simply plug the line cord into the AC outlet and allow at least 5 minuets for warm-up. The meter pointer will indicate the RMS voltages of the AC power line. The instrument may be left on permanently, providing a continuous indication of the power-line voltage.

    Since the WV-120A utilizes a moving-vain type meter, it is necessary that the instrument be kept away from strong magnetic fields, such as those developed by power transformers. To prevent static charges from forming on the meter case which would affect the operation of the meter, the clear plastic meter case front has been treated with anti-static solution. Care should be taken to avoid shocks which might damage the meter movement.

RCA Calibration

    The WV-120A is carefully calibrated at the factory in comparison with dynamometer-type laboratory standard reference meters. If recalibration is required, the WV=120A should be sent to the nearest authorized RCA Test Equipment Repair Stations. These stations are equipped with the necessary precision reference standards.

Calibration procedure:

Note: As a safety precaution, it is recommended that an isolation transformer such as the RCA WP-25A, WP-26A, or WP-27A isotop be used during this calibration procedure.

1. Remove the two screws on the rear of the instrument, then lift off the back cover by pulling it downward from the case.
2. Connect the WV-120A, together with a secondary standard reference meter with an accuracy of +/- 0,25%, to a variable AC power source. Allow at least five minuets for the WV-120A to warm up.
3. Set the voltage source at exactly 120 volts as indicated on the reference meter. Adjust the variable 500 ohm resistor R1 so that the meter reads 120 volts.
4. Set the voltage source at 100 volts as indicated on the reference meter. Carefully insert a soldering tool or similar pointed device through the small hole in the front of the meter. Engage the slotted adjustment lever, then move the lever slightly sideways as required to bring the WV-120A meter pointer to 100 volts.
5. Again set the voltage source at 120 volts, as in step #3 above. If necessary, reset the adjustable 500 ohm resistor so that the WV-120A meter reads exactly 120 volts.
6. Repeat steps 3, 4 and 5 if necessary and check accuracy at 140 volts. Slight compromises may be required at higher end of the scale, or at the center, to obtain maximum accuracy throughout the range of 100-140 volts. Before reassembling the case, make certain that the terminals of the resistor are not shorting against the metal case of the instrument.

Height...4 inches
Width...5 1/8 inches
Depth...3 1/8 inches


This web page was last updated: November 10, 2013