Test Equipment 1980s Toneohm 500A

Looks like something you chase shorts down with, looking at the resistance ranges?

Spot on Mike 👍 

Indeed, it is a precision milliohm meter which also employs a variable frequency tone to trace shorts. The frequency of the audio tone is dependent on the resistance between the two Kelvin probes, and can locate a short with an accuracy of 2mm without even looking at the meter, just using the tone to guide you. The meter reads ohms according to scale, and the audio frequency generated is lower the further away you are from the short, and higher the closer you get.

This version I believe was the first device manufactured by Polar instruments, the Toneohm 500A. They are still in business and the latest offering is the Toneohm 950, which now does multi-layer, and it now includes arrow indicators showing the direction you should go to probe the fault.

Here is a demo of that model

Impressive!

These were in more or less everyday use at a place where I worked in '89 to '91. We had a lot of trouble with shorts between layers of prototype multi-layer PCBs and the Toneohm would usually find the offending spot.

One particular board proved resistant to the Toneohm, the shorted area was too large - the electronics engineer who designed the circuit in question ended up wring the shorted tracks across the mains, that found and removed the short involving a loud bang and some smoke. The engineer, Gordon, was known as "Flash" after this.

John

@jjl A good few years ago now one of my colleagues had a short on a multi-layer PCB in a Thomson camera (a 1647, I think). As it was dead as it stood, we had nothing to lose by charging up a 22,000uF electrolytic to about 50V and discharging it across the fault.

Said camera went back into service until replaced with Sony E30s...

Hi guys, here crusty goes diving the deep end on something he knows nothing about, namely the Toneohm and Op Amps. Just for clarity, I do not have a circuit diagram or description for the Toneohm.

Toneohm 500A Components

• 12 diodes
• 5 op amps (4 x MC1458CP, 1 x MC1458JG)
• 2 Transistors (BC327)
• 40 resistors
• 10 caps
• 2 electrolytics
• 1 Bridge Rectifier
• 1 TX
• 1 multi-switch
• 1 on/off/volume control
• 1 Meter
• 1 telephone speaker
• 1 LED indicator
• 1 200mA Fuse

Toneohm Problem or Not?

I've noticed two interesting behaviours the Toneohm exhibits, both of which my gut thinks are questionable, perhaps incorrect.

First, when the unit is powered on, the meter pegs fully over to the right-hand side. Then, when you start probing the circuit under test, the meter drops back, displaying the ohms it reads. My initial thought was Whoa, that needle should not go over that harshly, but when I started testing circuits it would sedately drop back and read the ohms.

For example, if I set the range to 30R f.s.d. and then use the cal test points, which reads a 10R 1% resistor, the meter correctly reads the 10R resistor on the meter and the audio frequency can be heard. However, and this is the second behaviour, you will note from the video below, the meter intermittently bounces, and the tone follows suit. This again feels wrong.

Video of Toneohm Meter action

Toneohm 500A demo

My initial thoughts turned to this appears to be a fault. Monkey brain said could this be a cap exhibiting leakage, so I replaced the only two electrolytic caps, though I suspect they are just the smoothers as they sit each side of the bridge. I then wondered if the probe cables, needles and 5-pin DIN, might have an intermittent. I replaced all of these and still had the intermittent reading.

Attention then turned to where the probes enter the circuit, and it so happens both kelvin pairs route into U1, the MC1458JG. This being the only ceramic dual-op-amp. Obviously I've no op amps in stock, no idea how to test them either, in fact I'd only ever heard the term op amp, never understood what they do. I found some MC1458CP online also some MC1458CU which are allegedly compatible ceramics for the JG, so ordered a load of both types, they should be here later in the week.

Impatient to make progress, yesterday I looked into testing op amps and found an op amp tester that can do mono and dual op amps for just over £10 delivered on Amazon, as I'm a Prime member (next day delivery) one was ordered and it arrived today. All that arrived is the PCB so I mounted into the bottom shell of a Raspberyy Pi case and added a push button for momentary test.

Again in uncharted territory here, but here is the best it describes about its function, first a pic, circuit diagram, then a circuit description.

Application:

Single op amp and dual op amp screening circuit board, the circuit board can judge whether the IC can be used for amplification, and can roughly judge the conversion rate of the chip at the same time. Can find out those low-speed chips that pretend to be high-speed chips.

Working Voltage: DC 12V

Working Current: Above 100mA

Introduction:

IC1a in the circuit is connected to a multi-vibrator with a frequency of about 1Hz, and the OP Amp to be tested is connected to a unity gain inverting amplifier working state.

IC2 is an EX-OR gate, and its truth table is that the output is 1 when the two inputs are inverted.

Therefore:

If the OP Amp is normal, points A and B of IC2 must be reversed, so C is 1, and it is still 1 after IC1b and 1d, so the yellow LED D3 lights up;

If the OP Amp output is open, the point C of IC2 must always be 0, and it becomes 1 after IC1c, so that the red LED D4 lights up.

If there is an internal short circuit in the OP Amp, it makes the output voltage fixed to positive or negative, the output of IC1c is also a 1Hz low frequency square wave, and the red and yellow LEDs will flash alternately.

For chips with a low conversion rate, the status indicated by the LED is always yellow, and red will flicker faintly. This can also be used to roughly judge the conversion rate of the chip. Because most of the second-hand chips on the market use low conversion rate instead of high rate, it can also judge whether the chip is counterfeit or not.

In the actual test of low conversion rate chips such as: OP07, LM741, the yellow light is always on, the red light is slightly bright.

In the actual test of high conversion rate chips such as: TL071, NE5534, MC34071 and other chips, the yellow light is always on, the red light is off.

Single op amp model: LM741, LF356, NE5534, TL071, TL081, OP07 and other standard package single op amp.

Dual op amp model: LM358, NE5532, TL072, TL082, MC1458, RC4558, OP275, AD827 and other standard package dual op amps.

Testing My MC1458JG

Inserting the op amp into the dual channel, results in both yellow LED lights illuminating, which suggests it's OK. The red LEDs only mildly flicker, which means it looks to have a low conversion rate.

Conclusion

I've no idea how to go about debugging this fault, no diagram and no idea make for unhappy bedfellows.

Could I have, like the recent teletext fault on my Salora TV, a chip socket problem? Maybe I should solder the three pluggable op amps, one of which is the probe op amp, to the board to eliminate the sockets. However, it does not explain the meter needle pegging to the right, unless John @jjl can confirm or not the action is normal or not.

I really do pick some fruity projects.

Once again, if you've made it this far, thank you.

@crustytv - I seem to recall the meter would show full scale+ when the Toneohm was presented with an open circuit or over range resistance.

Elektrotanya has a service manual for the Toneohm 550 and 580 models that feature a digital panel meter but may have similar analogue circuitry to your unit.

If you want to learn about opamps then Rod Elliott has some excellent pages on them starting here - they really are marvellous devices and not too hard to master.

John

@jjl

Posted by: @jjl

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I seem to recall the meter would show full scale+ when the Toneohm was presented with an open circuit or over range resistance.

Hi John, thanks, that at least puts that one to bed, and makes perfect sense. I think what threw me was how violently (imo) it flies to the right, it seemed so brutish.

Posted by: @jjl

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Elektrotanya has a service manual for the Toneohm 550 and 580 models that feature a digital panel meter but may have similar analogue circuitry to your unit.

Wonderful, that will give me some idea, I see that device also has current probe abilities that are not availble on my 500A.

Posted by: @jjl

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If you want to learn about opamps then Rod Elliott has some excellent pages on them starting here - they really are marvellous devices and not too hard to master.

That'll certainly put some flesh on the bones, thank you.

That just leaves the odd behaviour of the meter and tone when probing, I would expect the reading and tone to be perfectly static, especially when calibrating. I will remove the sockets and solder the op amps directly to the board, and if that fails to resolve, I'll replace the lot with new when they arrive next week.  All that's left after that is to check the two transistors. If they're OK, I'm down to the tropicals, resistors and diodes. I accept my approach here is non-scientific.

As a side note, I hunted through my stores and unbeknown to me, I did have some Op amps, well five to be precise, three dual and a couple of mono's. I was pleased to find the mono's as it allowed me to test the other side of the op amp tester.

Today I removed IC sockets U1, U2 & U3 and soldered all but U1 to the PCB. I decided to keep U1 socketed (this is the one the Kelvin probes input) but replaced the socket with a better quality stand-off.  IC's U2, U3, U4 & U5 were all replaced with exactly the same but new MC1458CP opamp. U1 was replaced with an MC1458CU (ceramic), the previous was a JG. I also replaced the 5-pin DIN socket.

The result? Now the wildly erratic behaviour of the meter and tone has been tamed, much easier to calibrate, and I suspect the tiny pulse you do see & hear, is normal expected operation. Look forward to test-driving it on a Tandberg TV that has a short, somewhere.