faults & shorts locator: Polar Toneohm 700. How to create new homemade probes?

[auato]

I found this interesting instrument at a local flea market:



It's a Polar Toneohm 700. It's a faults and shorts locator (an evolution of a continuity tester).


but it isn't equipped by its original test leads. I tried to look in an old caisson of that flea market but it was filled of various and every kind of cables but I only found a long lead with the right DIN connector. I also found a similar schematic diagrams of the toneohm 550 (I belive the wires connection of front DIN-connector is the same) and my question is how to connect the two probes (I mean at which pin I could connect the two probes). Here the circuit of frontal din connector to connect probes:



Thank you everyone
Ciro

 

[betwixt]

Re: faults & shorts locator: Polar Toneohm 700. How to create new homemade probes?

You need two screened cables. The screen of one goes to ground with it's inner core wired to the pin next to it. The other cable screen goes to D1/C1 with it's inner core wired to the pin next to it.

You can use any kind of testmeter probe but you must wire the inner to the core on each cable together AT THE PROBE. These machines use four wire measurement to eliminate the cable resitance from the reading so where the inner and core are joined is the place of least measuement error and should be as close to the probe ends as possible.

Brian.

 

[auato]

Re: faults & shorts locator: Polar Toneohm 700. How to create new homemade probes?

Thank you Brian.
Sorry, but I don't understand the second part of your reply when you wrote:

You can use any kind of testmeter probe but you must wire the inner to the core on each cable together AT THE PROBE.

What do you mean with the word "inner" and the word "core"?

Has each cable a single central wire (the inner) and a single external shield (the screen)?

 

[betwixt]

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.