Cable Checker repair

The Voltcraft Cable Checker comes in a set of 9V battery powered transmitter and receiver and allows you to check and identify up to 16 cores in a cable at a maximum resistance of 25kΩ. A possible usage scenario is testing cables with indistinct colour code, preferably on a wide distance across rooms or buildings and with only one person. Just connect the terminals 1~16 and circuit ground of the transmitter on one end and the receiver will indicate the number of each core on the other end.

Of course, the Cable Checker is another episode of these cross marked out-of-the-plastic-box units and again it was at least a little bit faulty when I took it home. Hey, don’t you like faulty toys?

The receiver worked fine and so did the transmitter, except for channel 6 which was dead. After opening the box I saw three CMOS 4049 UBCN driving the terminals without additional protection. So, this is why the manual says that „applied voltages in excess of 7 V DC or AC will damage the transmitter and it is therefore crucial to make sure that the cable under test is dead“.

Even with respect to the price the Cable Checker is a cheapo and it will die instantly if you forget to think before connecting it. As thinking is sometimes a difficult business, I decided to install three IC sockets for the drivers. The PWM signals generated by the transmitter are shown on the scope for channels 1 (top) and 8 (bottom) at 5ms/DIV.

The receiver is based on an 8031 compatible Atmel 89C1051 microcontroller with 1K bytes internal flash memory. It directly drives a standard 3½ digit LCD. This box is the most expensive part of the Cable Checker and its only input is well protected with a resistor and two diodes.

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