May 1959 Popular Electronics
[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from
RF Cafe visitor Terry W. recently wrote to me about a topic
that ended up mentioning RCA's vacuum tube voltmeter (VTVM)
known as the VoltOhmyst. VTVMs were the era's high-input-impedance
multimeter, before field effect transistors (FETs) came on the
scene. The higher a meter's input impedance, the less it loads
the circuit under test. Any meter connected across a circuit
appears as a parallel load to the source, so the closer it is
to an open circuit, the better. Terry mentioned how the VoltOhmyst
was a key component on test benches of many professionals. You
can still buy various models of the
VoltOhmyst on eBay.
Popular Electronics Builds a Vacuum-Tube Voltmeter
Printed-circuit board makes the RCA VoltOhmyst
an easy-to-assemble kit
The first piece of quality electric test equipment you should
have for your test bench is a vacuum-tube voltmeter. A VTVM
found in service shops and industrial plants throughout the
country, the RCA "VoltOhmyst," Type WV-77EK, is now being offered
in kit form.*
What It Can Do. The VoltOhmyst WV-77EK measures
a.c. (r.m.s.) and d.c. voltages up to 1500 volts, peak-to-peak
voltages to 4000 volts, and resistance up to 1000 megohms. There
is a high input impedance on all d.c. and a.c. voltage ranges,
allowing the use of this VTVM in circuits where VOM's with a
lower input impedance would result in loading of the circuit
under test - and a resultant error in voltage reading.
The VoltOhmyst utilizes a push-pull balanced d.c. bridge
with the meter in the plate circuit, which affords excellent
linearity of response, good stability, and very high input impedance.
Additional features include: provision for zero-center indication,
useful in discriminator and bias measurements; separate scales
for low a.c. voltage measurements to assure accurate readings;
a circuit design which allows measurement of a.c. in the presence
of d.c. and vice versa; a separate d.c. probe with a 1-megohm
resistor which minimizes capacitance-loading effects; and electronic
protection against meter burn-out. Also, the resistors in the
ohmmeter ranges are protected by a separate fuse.
* For complete information on the VoltOhmyst, write to Radio
Corporation of America, Commercial Engineering Dept., Section
PE-10, Harrison, N. J.
Putting It Together. The WV-77EK utilizes
a printed-circuit board to facilitate assembly. This board provides
a rugged, pre-wired mounting for the components and, if the
parts are properly inserted and soldered, makes for a neat and
Large, clear meter face permits quick, accurate
The symbol number of the part to be mounted is printed on
one side of the board, and the copper wiring is etched on the
other side. When the assembly instructions call for mounting
a part, make sure the leads of the component form a right angle
to the body of the part. The leads, when properly bent, form
the two long portions of a "U" shaped unit, the bottom of the
"U" matching the dimension between the two holes. Some parts,
such as the disc capacitors, tube sockets and the selenium rectifier,
do not require bending.
Completely wired unit. Note the three internal
adjustment potentiometers. Diagram below is from the construction
Place the leads in the holes provided and pull the part snug
to the board, so that the leads protrude on the etched side
of the board. Spread the leads slightly to prevent the part
from falling out. After soldering the leads to the copper foil,
cut them to 1/8" from the board.
Well planned illustrations reduce complex
wiring to a simple task.
Recommendations. The instructions for assembling
and wiring the VoltOhmyst have been carefully thought out and
presented in seven major construction steps. The POPULAR ELECTRONICS
editor who assembled the VoltOhmyst makes the following recommendations.
- Insert R14 as the first substep in Step 2. This section
of the printed circuit is crowded. Installing R14 first
will permit you to mount the adjoining components with ease.
- After mounting the two snap-in sockets on the printed-circuit
board as instructed in Step 2, be sure to solder each connection
point as instructed. Do not be fooled into thinking that
these connection points make a good electrical connection.
- When connecting R10 to the printed-circuit board in
Step 4, do not cut the leads. Otherwise, wire must be added
to reach a connection point later on.
- In Step 4, the instructions call for tinning the negative
and positive terminals of the dry cell. When doing this,
be careful not to apply too much heat with the soldering
iron. Excessive heat will damage the cell.
The instructions state exactly
how to calibrate the VoltOhmyst. After calibration, our model
was checked against laboratory standards. All scales except
the a.c. ranges were found to be accurate to within 3%. The
a.c. ranges were off because the home 117-volt a.c. power line
was used as an a.c. calibration voltage. Unfortunately, home
a.c. power line fluctuates throughout the day. Night voltage
could be as high as 122 volts, but when the power demand is
high, the line voltage often drops to 105 volts.
In general, the calibration procedure permits satisfactory
calibration for most servicing purposes.
Comment. The VoltOhmyst assembles without
difficulty, works nicely, is fairly rugged, and has ample accuracy
for most practical work. The meter face is large, calibrations
are fairly easy to read, and the knife-edge pointer permits
readings with good accuracy. This reliable measuring device
will prove extremely useful in television and hi-fi repair as
well as in many industrial applications.
The VoltOhmyst uses a time-proven RCA circuit
for optimum results.
Posted February 20, 2013