Toa ZM-104 Manual

One of my favorite field tools is a basic analog
impedance meter. It can be used to troubleshoot a host
of sound system problems, either stand-alone or with
some additional gadgets. The TOA ZM-104 has been
around for many years and many experienced audio
people have one. Mine has served me well and has the
scars to prove it. It will be the example meter used in
this article, although in principle the tests can be con-
ducted using any similar device. Here is an overview
of the meter characteristics and some of the basic tests
that it can run.
Impedance
Electrical power sources produce voltage and cur-
rent. Electrical current between a source and load may
flow in one direction only (direct current or DC) or
in both directions (alternating current or AC). Imped-
ance is the opposition to the flow of electrical current.
It has two ingredients - resistance and reactance. Only
resistance need be considered in a DC circuit. Both
resistance and reactance must be considered in an AC
circuit. An ohm meter measures the opposition to DC,
which is why it is sometimes call a resistance meter.
The meter itself can serve as the current source. In
alternating current circuits, another form of opposition
is present. Reactance is the frequency-dependent oppo-
sition to the flow of current. It cannot be measured with
an ohm meter, because DC flows in only one direction.
An impedance meter generates an alternating current
which can be fed to the device-under-test (DUT from
here on).
Measuring Impedance
Several methods can be used to determine the imped-
ance of a load. The most straightforward way is to use
Ohm’s Law to determine impedance from the volt-
age across the load and the current through it. Figure
1 shows how to make the measurement with a volt-
ohmmeter (VOM). Impedance can also be measured
using a constant current source. This is the method that
the ZM-104 uses. The meter has a high output imped-
ance and acts like a constant current source (the current
through the load is independent of the load impedance).
The small voltage drop across the load is proportional
to the load impedance. This voltage produces the meter
deflection on the calibrated scale.
The ZM-104
Figure 2 shows the ZM-104. It’s a classic looking
device, with black chassis and analog meter move-
ment. There is an “off” button along with three range
switches, a zeroing button and a zeroing potentiometer.
The range switches are used to change the range of
the analog display by factors of ten. As with all analog
meters, the range setting that produces a mid-scale
reading should be used. The zeroing button temporar-
ily shorts the output terminals of the meter together,
allowing the zeroing pot to be used to “zero” the meter.
The meter probes can be shorted as an alternative to
using the zeroing switch. Note that the meter must be
re-zeroed if the range switch is changed. As with all
electrical or acoustical measurements, the measurer
should make an educated guess about what the read-
ing should be before the measurement is made. If
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Trade
Tools
of the
The Analog Impedance Meter
Fig. 1 - The “manual” method of determining impedance.
Z = V/A
From Syn-Aud-Con Newsletter Vol. 32 No.3 1

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

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
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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

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Fig. 6 - Identifying loud-
speaker wiring.
Fig. 7 - A pas-
sive “butt” set
increases the use-
fulness of the ZM-
104 (www.jwdavis.
com). An inductive
tester is equally
useful.
Fig. 8 - Identifying transformer parameters.
Fig. 9 - Performing the “Pin 1 Test” on an input.
Fig. 10 - Performing the “Pin 1 Test” on an output.
From Syn-Aud-Con Newsletter Vol. 32 No.3 4