Polar Toneohm 700- how to make some homemade probes?

[auato71]

I found this interesting instrument at a local flea market: a Polar Toneohm 700. It's a faults and shorts locator (an evolution of a continuity tester).

Unfortunately 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). The connector has 4 pin connected and I suppose that each test lead maybe connected by two wire.

Thank you everyone for any tip.
Ciro

[FIXITNOW]

I have one of these, but sadly its probes are also missing.

Let's hope someone has a set.

[KeithsTV]

We have one of these units at work and a 550 both with probes. When I get the chance I'll reverse engineer the probes and post the results here.

We may also have a manual but I suspect that may have been lost during various moves.

Keith

[Radio Wrangler]

It's a 4-wire Ohmmeter. The leftmost two pins on the DIN in the diagram go to two wires to one special clip (called a "Kelvin Clamp") Imagine a crocodile clip with the two halves insulted from each other. One wire goes to each half. The two wires only connect together via the item the clip is clipped to. The two rightmost DIN pins do the same to a second Kelvin Clamp.

One half of each clip passes current through whatever you're measuring, while the other halves make separate connections to sense the voltage. This way the resistances of your test leads and the quality of the grip of the clips doesn't affect your measurement.

Look up 4-wire Ohms and Kelvin Clamps and also the old Kelvin bridge.

David

[KeithsTV]

As mentioned it is a 4 wire kelvin connection. I've had a look at the probes that came with our unit.

One is a 4 wire connection using 4 pins of the Din connector used for resistance measurements. The other probe is a pickup coil used in conjunction with the drive source sockets on the front panel.

I believe it works like this.

The drive source provides an AC source into the board between say ground and power. The pickup coil is then used to trace the current path across the board. The current flow will be greatest next to any short circuit. A amplifier and speaker are used to monitor the signal from the probe. Where it's loudest is the location of the short. Well that's the theory.

The photos show the two test probes plus the connections on the DIN plug to one of the probes. Plus the wiring details.

It looks as if the 550 is the same but only has the ohm meter function.

I did try to find a manual but couldn't find it.

Hope this is useful.

Keith

[FIXITNOW]

thanks Keith i will make up test probe set 1 this will allow me to make low ohms measurement
cheers
vince

[FIXITNOW]

question on the circuit the middle pin has no connection
on you lead drawing you have called this pin 3
but as its a 180 deg din the pins go 1/4/2/5/3
the circuit shows pin 1 as earth
R7 across 4 and 5
pin 2 is n/c
pin 3 goes to c1/d1 junction
is your drawing /pinout correct ?
vince

[KeithsTV]

Well spotted. Must have been half asleep when I did it.

For the ohm meter pin 2 is unused.
Pins 1 & 4 are one connection
Pins 3 & 5 are the other connection.

Pin 2 is only used for the pick up coil

Corrected wiring details attached.

Keith

[FIXITNOW]

sorted lead made which has alowed be to prove the meter works as its been sitting here for years
an expensive bit of kit
not sure if i will ever use it
but nice to get it sorted cheers for your help keith
all the best vince

[auato71]

Hi all.

I would like to bring to your attention the assembly of my homemade probes and to know if in your opinion the behaviour of Thoneohm 700 is right (video). I used that diagram below.

I do not used fine wires but some cheap twin leads for sound installations in the cars. Each cable is long about 1 meter and its diameter is 0.75 mm. Moreover, my suspicion is those tips (retrived from other test leads) introduce too much resistance

Thanks a lot
Ciro

[KeithsTV]

I'll check the operation of the Toneohm we have at work as to how it works.

However the wire used for your probes should be OK. The 4 wires act as a Kelvin connection which compensates for any resistance in the leads. The leads connected to pins 1 & 3 are connected to a constant current generator which is passed through the circuit under test and the voltage across the circuit is sensed by the leads connected to pins 4 & 5. To get the best accuracy the leads connected to pins 1 & 4 and pins 3 &4 should be connected together as close to the end of the probe as possible.

Keith

[Alistair D]

I have a toneohm 550 here with original probes. The wire for each the probes is a thin grade of audio co-ax.

Al

[auato71]

....and with two probes shorted which is the display readout in the range 200 mOhms? (from video above you can see I have about 8/9 with a definite short circuit).

I have to look for two tips similar to those of Keith's

Ciro

[Alistair D]

Originally Posted by FIXITNOW

question on the circuit the middle pin has no connection
on you lead drawing you have called this pin 3
but as its a 180 deg din the pins go 1/4/2/5/3
the circuit shows pin 1 as earth
R7 across 4 and 5
pin 2 is n/c
pin 3 goes to c1/d1 junction
is your drawing /pinout correct ?
vince

Vince,
just to clarify, the 550 does not have a connection to pin2 of the DIN socket. The confusion arises because the circuit does not match the model. I was confused myself by probe 2. None of the 550s at work ever came with anything other than the steel tipped Kelvin probes. I finally realized that probe 2 only relates to the 700.

If anyone needs the operating instructions or service manual for the 550 PM me your email address.

Al

[auato71]

My doubt, now, is on my 700 model. Does anyone have a 700 schematic diagram? I'm courious where that pin 2 of DIN connector (central pin) is connected.

[KeithsTV]

As far as I can see pin 2 is used for the pick up coil. I can't be sure exactly how this works but I think there is an AC output which is put across the circuit to be tested. The pick up coil is then used, possibly in conjunction with the audio amplifier, to trace the path of the current across the PCB.

If there is a short circuit between, say, the 5v supply and ground the coil would be used to follow the short circuit current. (if there was no short the current in the circuit would be zero or very small and the resulting tone would be very low in volume. If there is a short the current in the circuit would be high resulting in a much louder tone from the speaker. By monitoring the volume the path of the current can be followed which hopefully should allow the short to be found.

This technique can be useful on multi layer boards where there is a short between the power planes, usually on an inner layer.

Keith

[Pamphonica]

A small postscript.

I just made up a replacement lead for a recently-acquired Toneohm 550 and the wiring is as stated above but the socket is designed for screened leads to each tip and the screen and centre need to be the right way round, even though they are soldered together at the tip, as the centre cores should carry the return signal.

The 550 service manual is quite clear on where these pins go in the internal circuitry, so the correct way round becomes clear.

Pin 1 (left-hand pin) is earth and is connected to the screen of the cable to the black probe

Pin 4 (next pin round) is -ve input and is connected to the centre of the cable to the black probe

Pin 3 (next pin round) no connection

Pin 5 (next pin round) is +ve input and is connected to the centre of the cable to the red probe

Pin 2 (right-hand pin) is signal drive to the probes and is connected to the screen of the cable to the red probe.

I am still looking for more details of the Toneohm 700, which I also have (again without cables). I can use the 700 with this probe set but cannot make use of its very clever drive source and current trace facility.

I do have a Genrad 2200 BugHound which does much the same but it's larger on the bench.

All these devices cost me £10-20 and are absolutely excellent for quick location of the failed-short tantalums that beset older (70s) test equipment

Jeremy

[Pamphonica]

Having read the thread properly again I realise that Keith has already posted a nice pdf with these details, so my post is rather superfluous. Never mind, at least I got my Toneohm working!

(mods: the thread title is mis-spelled so may make searching hard, it should read Toneohm, not Tonehom)