square wave has enough spectral content to produce
some level of output from any loudspeaker compo-
nent. Horns are easily identified over woofers by the
increased loudness and definition of the tone.
Signal Generator
Sometimes you just want to know if a device passes
a signal or not. In such cases, the ZM-104 can be con-
nected across an input. The voltage should be sufficient
to drive any mixer to “meter zero.” Alternately, a butt
set (Fig. 7) can be used across the output of the device
to detect the signal.
Transformer Tester
The turns ratio of a transformer can be found by
connecting the ZM-104 across the primary and a
known resistor value across the secondary (Figure 8).
If the measured impedance is the same as the known
resistor, then the transformer has a 1:1 turns ratio. If
the impedance is higher than the known resistor, then
the transformer is a “step-up” model. If the impedance
is lower than the known resistor, the transformer is a
“step down” model.
Wire Pair Identification
The individual pairs in multi-pair “snake” cables
can be identified by connecting the ZM-104 to a
twisted pair, and then “sniffing” the other end with an
inductive tracer (Fig. 7). These tracers are available at
electronics and home supply stores.
Pin 1 Tester
Many audio products have the internal audio ground
connected to “pin 1” of their input and/or output con-
nectors (Figures 9 and 10). This is a bad practice (it
causes hum and buzz) and it needs to be identified and
corrected. “Pin 1 problems” can be found by connecting
one lead of the ZM-104 to chassis and the other lead to
pin 1 of each I/O connector. If this connection produces
an audible tone from the unit, then the shield of the
connecting cable should be lifted or routed directly to
the chassis with a jumper. The ZM-104 sources about
20ma on the x1 setting. This is enough current to iden-
tify the problem but not enough to damage any internal
components, etc. See AES Journal Reprint June 1995
Grounds and Shields for more information.
Impedance
Oh yes, the ZM-104 can actually be used for what
it was designed for - measuring impedance. The 1kHz
tone is perfect for checking the impedance of any
input or output of an electronic audio component.
Keep in mind that loudspeakers have complex imped-
ance curves, but the 1kHz impedance measurement
can at least reveal whether the voice coil has continu-
ity, or whether a loudspeaker line is open or shorted.
These are by far the most common fault conditions
for installed and portable loudspeakers. If you need to
know the minimum impedance of a loudspeaker (this is
usually the parameter of interest when loading amplifi-
ers and selecting cable) then use your ohm meter across
the loudspeakers terminals. This will measure the DC
resistance of the transducer, which is typically the low
spot on an impedance curve. Note that transformers
and capacitors will not pass DC, so if you need to know
the impedance of such devices you can simply measure
the current with a know voltage applied and calculate it
using Ohm’s law (Figure 1).
In this age of computers, DSP and wireless commu-
nications, don’t overlook the time-proven fundamental
tools of our trade. The mark of a good audio technician
isn’t having the latest gear, it’s knowing how to get the
answer with the tools that are available. pb
Fig. 5 - Balanced outputs can be tested by alternately injecting a signal between 1-2 and 1-3.
From Syn-Aud-Con Newsletter Vol. 32 No.3 3
