The ZM-104
The ZM-104

Figure 2 shows the ZM-104. It’s a classic looking device, with black chassis and analog meter movement. 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 temporarily 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 reading should be before the measurement is made. If 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 prerequisites for any of the following tests. For all tests, the leads on the ZM-104 can be applied without regard to polarity, although they are shown as red and black in the figures. http://www.jyins.com/