STAR*TECH
Journal
November 1995
CD
RUNNING THE SHoP: EssENTIAL Tool5
USING THE
V ARIAC
AND ISOLATION TRANSFORMER ON THE BENCH
Douglas "Enzo" McCallum
Shiawassee Technical Services
Lansing, Michigan
To replace a bunch of parts, flip on the power and watch
something blow right back up is wasteful and frustrating.
THE VARIAC
When a piece of gear blows fuses,
there are two ways to proceed:
keep throwing fuses in it while
you try to troubleshoot or use a
Variac. A Variac is simply an ad-
justable autotransformer that al-
lows you to vary the AC line volt-
age from O to 140 volts. (Yes,
Variac is a brand name, but any
brand will do.) The Variac is used
with a current meter, allowing
you to monitor the current draw
of a unit under test while slowly
bringing up the power line volt-
age.
Some Variacs have an ammeter
built right into them. A Variac
rated at 10 amps with meter is
available from several suppliers
for under $100. If you are
wealthy, Sencore sells their
Powerite for about $500. It is a
Variac and isolation transformer
in one with a big meter and even
a probe to measure chassis leak-
age. I'd rather roll my own.
should have appropriate fuses
and an off switch in there, too.
Plug the unit under repair into
the Variac and slowly tum the
voltage up from zero. Watch the
current draw measured on the
ammeter.
As you raise the line voltage, a
normal unit will start to draw a
little current as the filter caps
charge up in the power supply
section. A unit that blows fuse
will start to draw a large amount
of current even at very low line
voltages. In this case, turn it right
back down and continue the re-
pair. If the unit can pull five amps
out of the line when it is only up
to ten volts, imagine what it could
have drawn if you had turned it
all the way up. Make sure the
excess current draw, even at very
low line voltages, doesn't exceed
the fuse in the unit or it will blow.
The Variac saves blown fuses and
further damage to circuitry.
PROCEDURE
CAREFUL, CAREFUL
How do we use it? Our basic
setup is an isolation transformer
plugged into the wall, then a
Variac plugged into the iso. You
A word about safety. (Yes, even
after 40 years of soldering, I am
still careful.) Always use an iso-
lation transformer. The metal
chassis of most video monitors is
connected to the AC line, it is hot.
A larger potential or voltage will
exist between such chassis and
ground unless you isolate it. Even
just grounding your scope to it
can destroy the circuitry or you
might even hurt your little self.
Use an iso ... please.
SOURCE
Where can I get an isolation
transformer? Every video game
you own includes one. It will ei-
ther be a separate transformer for
the purpose or it will just be an
extra winding on the main power
transformer, but it will be there.
A scrapped vid-game cabinet
should provide an isolation trans-
former, a power cord, a handy
push-pull power switch, a suitable
power connector for at least one
brand of monitor, maybe even a
loose quarter or two.
T'SHOOTIN'
Variacs are especially helpful in
repair of monitors, power sup-
plies, and amplifiers. Let's
troubleshoot a monitor.
0 First, inspect the unit for burnt
components and blown fuses.