Syn-Aud-Con Newsletter
completely unsure, then start with the x1 button and
work your way up until you get a mid-scale reading.
The meter’s output signal is a 2V peak square wave at
20mA (x1 range).
The analog meter movement is great for finding
intermittent problems. There is no “boot up” required
and the 4AA batteries last for a very long time between
charges. It is simple, durable, low in cost and easy to
use. A great deal of sound system problems can be
found by a trained user armed with a ZM-104 and a
few other gadgets.
Note that impedance meters can be used to test
components that are switched “on.” It is important that
the component NOT be passing a signal! So, “powered
on” and “no signal present” are important perquisites
for any of the following tests. For all tests, the leads
on the ZM-104 can be applied without regard to polar-
ity, although they are shown as red and black in the
figures.
Differential Input Test
Differential inputs have inverting (-) and non-invert-
ing (+) inputs (pins 2 and 3 on XL connectors). An
active differential input can be identified by connect-
ing one lead of the ZM-104 to pin 1 and the other to the
plus and minus alternately. Both connections should
produce an output signal from the DUT (Fig. 3).
Common-mode Rejection
This one is just like the differential input test, except
that instead of driving the + and - terminals alternately,
lay the test probe across both (Fig. 4). The
signal will be cancelled by the common-
mode rejection ratio (CMRR) of the input if
it is differential.
Balanced Output Tester
A balanced output will have the same
output impedance from each output leg. Use
pin 1 as common and check the impedance
into the plus and minus terminals respec-
tively. The value should be low (typically
less than 600 ohms) and within 1% of each
other (Figure 5).
Continuity Tester
The ZM-104 will tolerate a dead short
between its output terminals, so it makes
a great continuity tester. The analog meter
movement makes spotting intermittent con-
nections quite easy. I will often use this test
to verify that all metal panels on an electronic
product are at the same (ground) potential.
Don’t be afraid to scrape a little paint when
doing this test.
Loudspeaker Wiring Identification
The ZM-104 will produce a distinguished
tone from a loudspeaker, without the use of
an amplifier. I will often use this to identify
pairs in multi-way loudspeakers. The 1 kHz
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Fig. 2 - The TOA ZM-104 Impedance Meter
Fig. 3 - Differential input and input
impedance test.
Fig. 4 - Common-mode rejection test.
From Syn-Aud-Con Newsletter Vol. 32 No.3
2