Testing the TMCC/Legacy Track Output Signal

The 455 KHz signal coming out of the “Track” or “U” thumbscrew on a TMCC/Legacy Base is too high a frequency for most handheld digital voltmeters to read.  I just “invented” a very simple test to see if signal is coming out the Track terminal if you have a common digital voltmeter.

1.  Disconnect the Base from the track and the Serial output 9-pin.

2.  Set your digital voltmeter to the 2V (2000 mV) scale.  Turn on the meter!

3.  Connect a resistor of almost any value (I tested 100 ohms to 47,000 ohms) from pin 5 on the 9-pin plug (lower right corner for Legacy Base; upper left for TMCC Base) to the Track thumbscrew.

4.  Connect a diode – 1N400x or 1N914/4148 – from the Track thumbscrew to one lead of your digital voltmeter.

5.  Connect the other lead of your digital voltmeter to pin 5/resistor.

6.  Apply AC power to the Base.

7.  Read 1.5-2.0VDC on the meter.  The polarity of the reading will depend upon which way you install the diode, but both orientations of the diode will work.

For my testing I saw about 2.0VDC with the 1N914/1N4148 fast small-signal diode, and about 1.5VDC with a 1N4004 slower power diode.

The diode halfwave rectifies the 455 KHz track signal so that your low-cost digital meter will read a DC voltage.  This way the meter doesn’t need to have any high-performance AC specs to detect the track signal.

The resistor is absolutely necessary because of the output capacitor in series with the thumbscrew inside the Base!!  Without the resistor, this internal capacitor “absorbs” the DC offset created by the diode before the DC offset reaches the thumbscrew terminal.

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