Cable Repair Part 2
with Kevin Micka

The Mysterious “Electrostatic Shield”

A week or so after finishing up last month’s column on repairing instrument cables, I was spending some quality alone time repairing and making some cables of my own. I came across a predicament that I must share as it may be crucial to completing a quality repair on your cables.

A cable I had just repaired now sounded horrible. It sounded fine the day before and now (after re-soldering) it sounded thin and the overall volume and quality had degraded. I finally found, after much head-scratching and a call to a wiser tech than I, that there is a second shield within the cable surrounding the center insulator and conductor! This second shield has a high resistance! Often referred to as the “electrostatic shield” (or “triboelectric shield,” depending on your source) and usually black, it comes in contact with the braided shield and runs along the entire length of the cable. In my case, it was touching the center conductor wire (this being your signal), which was greatly attenuating the signal by shunting it to ground.

Imagine adding a resistor (10k or so) between your ground and signal, or adding a volume control and having it around “1” or so. The relationship between the two shields also acts like a capacitor in this situation, like a tone control in where only high frequencies are allowed to pass. The frequency at which this is done is dependent upon the amount of resistance and capacitance (otherwise known as an RC circuit). More on this later.

So the solution to this was to cut back the thin second shield at both ends about a quarter of an inch (leaving the rubber insulator underneath it) so that it does not touch the center conductor or terminal of the plug. I have had some luck with a razorblade and needle nose pliers. Once you make a little cut it tears quite easily.
Now, this second shield actually does help create a much quieter cable. It is less porous than the braided shield (especially when flexed) and blocks interference better. Also, you may not find this type of shield on all instrument cables. I have only had this problem twice in the last 10 years. Most of the time I believe I was just lucky that the shield and conductor never touched. Anyway, it is very much worth looking out for as it can easily create a useless cable.
Keep sending your questions, comments and suggestions to

workshop@performermag.com