Gift to

73 Magazine Staff
Field Strength for Free
- a home-brew conversion
Pro bably many amateurs
have ol d VOM s th at
have out lived their usefulness
or have been replac ed
by better test eq uipment. In
our ca se, a ten-year -ol d
Japan ese YOM was start ing
to develop all sorts of problems-
a switch that was acti
vated w hen t he pin tip
from a test lead was inserted
in the Oh ms jack
made erratic contact; resulting
in fa lse resistance
readings, jacks on various
other functions were worn
out, and two current ranges
we re bu rned out. The YOM
had a goo d meter movement
and case, however, so
rather than sim ply di scard
it, we turned it into a very
handy and sensitive fi eldstrength
meter.
Undou btedly, the same
can be done w it h many
other VOMs at minimum or
no cost using ju nk-b ox
parts. O ne can end up w it h
quite an attracti ve looking
instrum ent, as good as the
one shown in th e pho tograph
of t he front of t he
co nvert ed YOM.
In the case of t his YOM,
there was a series of pin
jacks running vertically on
bo th sides and along the
bo ttom of t he space bel ow
t he meter. Th ese w ere
cov ered up wi th a pie ce of
Lexan (the bl ack, wrinklefinish
material seen in the
photograph) and the pin
jack hol es used w it h 6-32
hardware to hold the material
in pl ace. Of co urse, any
sort of att ractive plastic or
meta l can be used as a
covering plate. The miniature
on/off switch for the
a m p lif ier c irc ui t i ncorporated
in th e fi eld-strength
meter isshown in the center
of the plate. You can also
ju st di scern (on the right
side) the knob for a sensitivity
control.
"The instrument's meter
originally had die usual
YO M scales. However, the
scales we re on a plate
which could be unscrewed
and turned around to reveal
a black surfa ce. This surface
was painted gray and
the numbers shown w ere
placed on it using a rub-on
lettering/numbe ring transfe
r sheet. The method of
dressing up an old YOM
w ill var y w ith t he instru ­
ment invo lved, but with a
bi t of im agination o ne
usually can find a method
that w ill resul t in a newlooking
instrument rat her
than one which is a reminder
of a discarded YOM.
The YO M had a 250-I'A
movement, so it was al-
I "IJ 'I
om
'"
'-l1--4
""'0. on S IMILAR
0 -1-"
Photo A. The front o f the YOM as it originall y appeared. It
st ill lo ok s good, but electrically it was falling apart.
Fig. 1. Broadband vo ltage-do ubler/rectifier and de
am p li fier used in the converted YO M.
82 73 Magazine · October, 1980