Re: faults & shorts locator: Polar Toneohm 700. How to create new homemade probes?
Yes, that's exactly right. The inner and screen are joined where they meet the probe. The original Toneohm cables were audio grade screened cables, the kind of cable that might connect units together in a stereo system.
Although it might seem sensible to short the pins together at the DIN plug and just use a single wire to each probe, it won't work properly. The method of measurement relies on there being one connection to feed signal to the probe and a different connection to read it back, in this case, the inner and screen of the cable are used as the two connections.
It should be possible to measure resistances down to one or two milliOhms (0.001 Ohms) although these were not intended to be used to measure resistance directly. The idea is that you can localize short circuits on circuit boards byapplying the two probes across shorted points. If you move the probes closer to the actual shorted point, the tone increases in pitch. For example if you had a short across the ground and supply rails on a large board, it could be anywhere where the tracks were close together or it could be inside a component. By moving the two probes between each gound connection and supply connection it could detect the extra resistance of the copper tracks and help localize the fault. The fault would be where the resistance is lowest and therefore the tone highest frequency.
Brian.