Toneohm 850A short finder by Polar Instruments - a look under the hood.

[Fraser]

For those unaware of the Toneohm units, they are units that contain a Milliohm meter for measuring track resistance and in some models a signal injection and non contact tracing probe.

The indicators on the unit are an LCD display and an audio tone that changes with the displayed reading.

The purpose of these units is to trace a short circuit caused by a PCB fault or a failed component, such as a power rail decoupling capacitor. In the hands of an experienced operator, these units quickly identify the cause of a low impedance or short, right down to the component or track location.

Several models of Toneohm have been made. The original unit had an analogue meter and the tone generator. It was just a Milliohm meter. The later range contained units that were just Milliohm meters or a Milliohm meter plus the signal tracer capability. I highly recommend the latter as the inductive probe quickly leads the user to the cause of the problem. These work superbly on the power rails of Racal equipment that commonly suffers failed power supply decoupling capacitors.

Models:

500A  Original analogue meter + Audio tone Milliohm meter
550    Digital display + Audio tone Milliohm meter
550A  Facelifted 550. No significant improvements to design
580    Signal injector and tracer only
700    Highly portable Digital Display Milliohm meter + Audio tone + signal injector & tracer probe
850    Bench mounted Digital Display Milliohm meter + Audio tone + signal injector & tracer probe
850A  Facelifted 850 Bench mounted Digital Display Milliohm meter + Audio tone + signal injector & tracer probe

The later 950 model is a very different vector analysis design that is very different to those above and is not covered here.

I own the 550, 700, 850 and 850A models. All perform well but those with the signal tracing capability are my favourites. You inject a signal from the unit into the faulty PCB track and the inductive probe picks up the ac signal along the trace until the point of the short is found. The signal stops when you move beyond the faulty component.

These units have rocketed in price on ebay in the last year or two. I know not why. I was lucky to buy most of mine prior to the price increase. and all cost me less than £70. You see them listed at over £200 these days.

Thanks to fellow forum member Lukier, I became aware of an ebay seller offering a pair of 850A units sans probes for only £50 the pair. I snapped them up as the probes are the same across the models. I have several cable sets and inductive probes to use with them  Lukier bought the other two units that were on offer. A real bargain if they worked.

The units arrived this week and are in far better condition than I was expecting. There is some discolouration of the cases due to UV but this is very common on such cream/grey plastics and is no major issue for me. Both units work fine and perform as they should.

I opened the units and stripped them to enable me to clean the cases. I took some pictures of one unit for your interest. The design and PCB layout is very 'old school' but is of high quality and well laid out. As the bench case is large, the various elements of the design are separated into their own section of the PCB. Very neat and orderly.
I have the service manual and schematics but sadly the file is too large to include on the forum. Sorry.

These units are easy to maintain and spare parts are still available from POLAR Industries. A pair of very sharp tipped Kelvin type probes costs around £40 but you can make your own easily. The inductive probe is also available for around £40. Prices include VAT and delivery.

Now to the pictures......

[Fraser]

Internal pictures of the 850A......

Note the convenient voltage test points in the middle of the PCB. It couldn't be easier.

Aurora

[Fraser]

More pictures.......

[Fraser]

More pictures.......

Note the quality parts used....Dubilier Capacitors in the PSU. Still working fine.

[Stray Electron]

Aurora,

   Great write up!  I've been wondering about these for a long time, particularly the difference in the various models.  You've answered all most, of my questions.  Can you tell us more about the probes?  How hard is it to make them if we buy a unit without probes?

[lukier]

Thanks Aurora for sharing your knowledge on these and teardown pictures so I don't have to I didn't even have a chance to unpack my 850As yet and I don't have the probes. I will email Polar and ask for a quote.

BTW These things are miliohm meters, but is it really usable? I'm thinking about buying 1-10 mOhm Vishay Precision current sense 4 terminal resistor from Farnell to build DIY alternative to Keysight's 34330A (and use it to calibrate power supplies current ranges - Guildline resistors recommended in Keysight's calibration manuals are pure unobtainium).

I also have DE-5000 and robrenz did interesting experiments showing that it could be used as a miliohm meter. However, in my case it will be sub 10 mOhm so a meter even with with 1 mOhm resolution (not to mention accuracy) is not going to cut the mustard.

I don't have 34420A that would be good for the job, but I have Stanford Research SR850 and I thought I might measure such low resistance by pushing AC sine excitation (that has output resistor/impedance to limit the current) through the current shunt and connecting signal input to sense terminals. SR850 can resolve down to 2nV and has sub 8nV/root Hz noise floor if I remember correctly.

Does the route with lock-in amplifier makes sense?

[Fraser]

@Stray Electron,

Making your own probes.....

1. The Kelvin probes are easily DIY built. You just need the correct 5 pin DIN plug, some thin coax cable and a pair of decent sharp tipped probes. Each coax is connected to 2 pins in the DIN plug and the coax braid and inner conductor are connected together at the metal probe tip. Not true Kelvin probes but close enough for these units and this is how they are made by POLAR.

2. The Signal Injector cable is very simple to DIY. It is just a pair of test clip leads connected to the required pins in the appropriate DIN plug

3. Signal tracer probe is not a DIY option until its internal parts are identified and understood. The probe is likely a coil on a ferrite core and it is very directional. I may be able to X Ray my probes to see what can be revealed. I bought new probes from POLAR industries for £40 as I wanted good performance and the time to make the probes would have exceeded the new parts purchase price. If I can reveal more about the probes contents, I shall post it in this thread shortly.

The DIN plugs are standard 'off the shelf' items and they do not contain any components.

Aurora

[Fraser]

There is a 550A for sale on ebay UK at the moment. It may go for a bargain price?

http://www.ebay.co.uk/itm/181852968902?_trksid=p2060353.m1431.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Aurora

[Fraser]

@Lukier,

My two units cleaned up really well and work perfectly. We have two different versions of the 850A. One has the Grille on the front and was made on 1996. The other has the speaker grille underneath and a plain front and was made in 1999.

The Toneohm 850A uses a common ICL 7106 3.5 digit LCD voltmeter chip. IIRC this is not a super accurate device.

I would not trust it as a Milliohm meter. It can measure very low resistances and provide a reading useful for finding shorts, but it is not designed to be a Milliohm meter so will not have the resolution or accuracy that your are looking for.

I regret I am not able to help on your question further as measuring low resistances or impedances is not something I have been involved with over the years.

Aurora

[lukier]

My two units cleaned up really well and work perfectly. We have two different versions of the 850A. One has the Grille on the front and was made on 1996. The other has the speaker grille underneath and a plain front and was made in 1999.

I've noticed the grille difference on eBay photos and got intrigued. Thanks for solving this mystery

The Toneohm 850A uses a common ICL 7106 3.5 digit LCD voltmeter chip. IIRC this is not a super accurate device.

I would not trust it as a Milliohm meter. It can measure very low resistances and provide a reading useful for finding shorts, but it is not designed to be a Milliohm meter so will not have the resolution or accuracy that your are looking for.

I suspected so.

I regret I am not able to help on your question further as measuring low resistances or impedances is not something I have been involved with over the years.

No worries. Maybe someone else on the forum will enlighten me if a lock in amplifier makes sense for microohm measurements

[kony]

No worries. Maybe someone else on the forum will enlighten me if a lock in amplifier makes sense for microohm measurements

Sure it does.

[lukier]

Thanks for confirmation!

I found this article by Keithley on AC vs DC for sensitive measurements: http://www.keithley.com/data?asset=50671 and they (of course) suggest their nanovoltmeters, but now I think lock-in amp will do fine for my purpose.

[Fraser]

I was feeling in a helpful mood so have taken some pictures of my complete 850 and probes set.

I got it wrong on the inductive probe in that it has a 1 Ohm resistor in its DIN plug. I attach a quick sketch and images.

The inductor in the inductive probe measures 9.6 Ohms but I will have to measure its inductance tomorrow as I am not in the lab.

I have also X-Rayed the probes and attach them in later posts for your interest and information. The component at the tip of the inductive signal tracer probe appears to be a simple tiny ferrite bobbin inductor. This should be simple to copy once I have the inductance value. The Injected signal is not a very high frequency so COTS bobbin inductors should be available.
 

Aurora

[Fraser]

X-Ray images of the Inductive probe

They are quick and dirty so apologies about the quality !

[Fraser]

X-Ray of the connections to the contact probe tips. As you can see the inner conductor and coaxial braid are connected together at the metal tip solder bucket.

[lukier]

These Faxitron x-ray images are a thing of beauty.

It really looks like a COTS SMD inductor.

[Fraser]

Lukier

I agree. I imaged it at two angles and it certainly looks like an SMD bobbin inductor with a base to which the wires are attached.

The images that I produced this evening are very rough and far from what is achievable with the MX-20. I started it from cold, did a quick calibration and did not even give the sensor array time to warm up properly. Good enough for our present needs though. The MX-20 has a lot of image manipulation and enhancement menu options that can really tidy up an image and improve clarity where needed. Sadly it is limited in terms of penetration though as it tops out at 35KVp. This is why I cannot penetrate through the inductor core. At least not at 30KVp with a relatively short 5 second exposure. I could try a longer exposure and 35KVp but time was short this evening.

The MX-20 can also measure the bobbin for us if we need dimensions as it is able to calculate dimensions even when using the upper (magnification) shelves in the cabinet. I will supply the inductance value as soon as I can get in the lab. Other tasks to do first though. I can also take some close up pictures of the probe head if needed by others. It isn't very sophisticated though. Fortunately I found no other components in the handle of the probe so it is just a simple inductive pick-up probe.

Aurora

[lukier]

I just spotted this on eBay: http://www.ebay.co.uk/itm/291555781429

This is I believe the most advanced model with some sort of triangulation to find power/ground plane shorts.

I'm fine with 850A but I thought this might be of interest to other forum members. With 950 model (still manufactured and with probes) it is likely that bidding will go pretty high.

[Fraser]

Yes the 950 is the current model and is designed to cope with shorts on power planes. Using triangulation. They always sell for a lot of money and in my opinion, are not that much useful to the hobby user as they really are for production line fault tracing where a PCB power plane has suffered a supply to 0v short due to PCB production fault. Triangulation does not work on conventional PCB's that do not contain a power plain as the stimulation probes need to be positioned at the four corners of the PCB.

The 950 is very clever, but very much a production tool rather than general repair. Computer repairs may benefit from the triangulation mode though as the motherboards have a power plain.

Aurora

[nowlan]

I saw a repair video once, where the kid had some flash 6.5 digit multimeter, and just dragged the probes along power rail, until he found the shorted capacitor watching the track's ohm decrease.

Ive only seen tone generators for tracing wires, like in car stereo, to see where they run behind panels.

[Fraser]

A tone ohm just uses basic physics in its operation. It's function can be mimicked to a degree by a Milliohm meter. The Toneohm has the added benefit of a varying frequency audio tone and a signal tracer probe. The venerable HP Current probe is also capable of finding shorts.... But it can be an expensive purchase these days.

The Toneohm is a classic case of an instrument that contains several elements that make it a very versatile and useful tool on the bench. As can be seen in the pictures though..... There is no rocket science inside the case. It's an oldy but goody 

Aurora

[Fraser]

OK I have just measured the inductance of the probe inductor......

1KHz ... 220uH Q =0.145

10 KHz ... 220uH Q=1.442

100KHz ... 225uH Q= 12.68

Aurora

[lukier]

Thanks!

With the inductance value and those beautiful Faxitron images I'm really tempted to DIY something (both current and pointy kelvin probes) instead of contacting Polar. 40 quid here, 40 quid there and it adds up. Instead I can buy a bunch of DIN connectors and various SMD inductors from Farnell, sharp probes from eBay/Aliexpress, I have  some RG158 coax , current probe handle I can probably 3D print out of ABS and I will still have money left for some other useful gear

[Shock]

If your interested in current tracers check out the HP 547A. The HP Journal from December 1976 has some real good reading in it as well.

[Fraser]

@Shock,

That is the HP current tracer probe that I have 

An ex MoD new set of HP current probe, logic pulser, logic probe and logic clip cost me £60, but they sell for around £100 these days.

Aurora