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......