85 Tek scope to trouble-shoot my AC-DC set, got sparks and burned out the wire
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
85 Tek scope to trouble-shoot my AC-DC set, got sparks and burned out the wire
I tried to use a Tek scope to trouble-shoot my AC-DC set, but when I
connected the probe ground, I got sparks and burned out the wire.
What's wrong?
US AC-DC sets typically have one side of the line connected directly
to chassis ground. Some European sets may also have a direct connection
between one side of the supply mains and the chassis. Virtually all US
test equipment built over the last 40 years uses a three-prong plug with
a direct connection between the ground prong and the test equipment
chassis. What happened here is that the radio was plugged in with the
high side of the line connected to its ground, and you connected the
ground strap across the line voltage. While in US power distribution
systems, the "neutral" wire is connected to earth ground at the
distribution panel, grounding the line neutral at the radio may cause
currents to circulate in the neutral-ground circuits (ground loop).
The best way to avoid a shock hazard with an AC-DC set is to use an
isolation transformer. It is possible, but not recommended, to "float"
the test equipment ground by using a two-prong "cheater," but this may
cause other problems. Plugging the set in so that the grounded side is
neutral may also work, particularly if you use a .01 mfd or larger cap
in the ground circuit to the scope. However, with any method other than
an isolation transformer, the scope and the radio may have some voltage
between them, posing a shock hazard as well as problems making
measurements.
AC-only sets were often connected with a .02 mfd cap from each side of
the AC line to the chassis to provide an AC reference ground between the
chassis and the AC line. If either of these capacitors is shorted, the
chassis is directly connected to one side of the line. Find these caps
and check them before doing any trouble-shooting.
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