A simple but handy cable tester...

As we have many hundreths of MIDI-cables, XLR microphone cables, 1/4" jack patch cords and very often run into a cable that seems to be suspect, we were in need of a simple tool to thoroughly test these cables.

The circuit for such a tester is barely worth to be called a circuit, but for sure it saves lots of time both in-house as on the road. The principle is simple: the test goes in two phases:

1.- Conductivity test on all conductors in the cable. The LED's on the right part of the circuit board (numbered 3, 5, 2, 4, 1 corresponding to the somewhat odd numbering of pins on a 5-pole DIN connector) should all light up with a good cable. In case one or more of the LED's do not light-up, the corresponding conductor is either broken or unconnected. For this test the cable should be connected to both connectors on the long sides of the board. So both ends need to be connected. To give an example: a working MIDI cable should result in LED's 2,4,5 lighting up. If we have to do with a 5-wire cable, all LED's will be ON. Cross-connected cables however cannot be detected, but these will not pass the second test.

2.- Short circuit test: For this test only one side of the cable should be connected to the mating side connector on the test PCB. Push the button to perform the test. For a cable with no shorts, all LED's should be lit. If a short exists between any pair of wires, the corresponding LED will stay dark. The LED(s) staying dark will tell you which conductors are shorted or cross-connected.

The 9V battery required can stay in its holder as the circuit, without cables connected, does not consume any power.

Here is the utmost simple circuit:


Here is the PCB for this circuit:

If you want to etch it, save the gif image on your computer and laser-print it on transparent foil reduced to 50% in size. Assembled testers can be bought from the Logos Foundation for approximately 50 Euro's. Here is a picture of a finished circuit:


Godfried-Willem Raes

Ghent, february 15th 2016

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