Play Meter

TECHNICAL TOPICS
BY Randy Fromm
Building a Field Strength Meter
For the field mechanic that works
on wall games, it is important to have
some method of testing and tuning
the R.F. (Radio Frequency) section
of wall game transmitters.
A weak transmitter will cause an
erratic game operation that can
lower the play appeal of an otherwise
reliable money-maker. A transmitter
that seems to work from one part of
the room but not in another, or one
that has insufficient range , is
probably not working to the peak of
its designed capabilities .
Since we already have a method of
determi'ling the presence of the R. F.
carrier and a procedure for testing
and tuning the audio section (see
Update for April and May 1977 and
Figure 3 below) , all that is needed for
a complete field testing service
procedure is a way to " peak " the
R.F . section. Peaking refers to the
process of maximizing the signal
strength in a circuit . In this case we
w ant to peak th e transmitter' s
output signal to give it maximum
rang e and reliability.
BwJding a Cheap
Field Strength Meter
Th e sch ematic in Figure 1 shows
an inexpensive and easily con-
stru ct ed R. F. detector circuit . This
devic e will sample the transmitter's
R.F . output and convert it to a D .C.
(Direct Current) voltage. When you
use this in conjunction with your
V.O.M. (volt-ohmeter) , you will have
a fi eld stiength meter that allows you
to read a relative output signal for the
transmitter and obtain a peak signal
indication while tuning the R.F .
circuits .
Just about any construction
method can be used to build the
detector . I built the prototype by
soldenng the component leads
together and wrapping the whole
thing in a wad of duct tape . The
ant enna is just an eight inch piece of
insulat ed wire . The device has been
ki cking around in my tool box for a
whil e now and has proven to be a
vauable aid in solving my wall game
problems .
An alternative and clearly superior
construction method would be to
use a small piece of vector board
(perf board) to mount the com -
ponents and to attach clip leads for
connecting the meter . (See Figure
2. )
Testing the Detector
Set your V.O.M. to a low D.C .
scale (10 volts or lower) and connect
the meter as indicated by the
schematic in Figure 1. The detector's
antenna may be placed near, or
wrapped around the transmitter's
antenna. When energized, a working
transmitter should cause a meter
deflection. If you find that your
meter is not sensitive enough,
ground the transmitter and the
detector together by connecting the
negative meter lead to the metal base
which serves the " ground plane" of
the transmitter. This will increase the
output voltage
circuit .
of
the
Tuning the R. F. Section
Tuning the R.F. section of a wall
game transmitter is an easy two-step
operation with the use of your new
field strength meter.
Variable tuned coil L-2 is used to
peak the R.F. input to the final R.F.
amplifier. To adjust L-2, melt the wax
on the coil by touching it with a hot
soldering iron . Insert a hex alignment
tool, energize the transmitter, and
adust the slug for a peak reading on
thea meter. Remelt the wax to lock
the slug in place .
Tuning capacitor C-9 peaks the
R. F. output ofthe final amplifier. Use
a plastic screwdriver to adjust C-9
and obtain a peak reading on the
meter .
That's all there is to it!! The
process is so simple that a trans-
mitter can be tuned for maximum
output in the same amount of time
that it takes to change batteries.
+
A N TE NNA ca '' l
FIG .2
R.f. DETECTOR CIRCUIT
, IMH
METER-
METER+
detector
D
D
FH3.3