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Test Equipment TMK Model TEM 3000; Milliohm Meter

Test Equipment

Last Post by Sundog 3 years ago

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crustytv

(@crustytv)

Posts: 12221

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Product: TMK Model TEM 3000 Milliohm meter

Year: Unknown, late 70s, early 80s?

Resolution: 2 ohms, 20 ohms, 200 ohms

Accuracy: Within +/- 0.3% of range at unity factor

Test Current: 100mA on 2 ohm range

Factor Control: Compensation in accordance with IEE wiring regulations for temperatures from 5C to 25C in 5C steps. Setting for unity factor (true resistance reading)

Connectors: Coaxial, Four-terminal network, fully compensated

Background

As I'm sure you're all aware, I also enjoy finding and using various pieces of period test equipment. I've been on the lookout for a milliohm meter for quite a while, you might well ask what on earth for. The simple answer is after seeing a YouTube video a few years ago demonstrating how easy it was to locate component shorts. On a number of occasions, I've faced a shorted component somewhere, and it's often resulted in me unsoldering a fair few components before finally discovering the culprit, a milliohm meter takes a lot of that pain away. For locating the short, the process is to measure, with the milliohm meter, many places across the shorted rail. Rather than me waffling, here's a little demo.

Scenario; one of these components is short-circuit, but which one?

We have five components, resistor, capacitor, diode, transistor strapped as a diode, and finally a choke, as shown in the photo below. One of these components is shorted, but which? The second photo shows the short detected with a standard multimeter. The trouble with a multimeter is it only shows that there is a short across the rail, it cannot tell you where or which one. This next photo shows the short detected across the rails.

This is where the milliohm meter comes into its own, as it has a much lower resolution. The following test demonstrates how you locate the exact component with the short. By taking a reading across each component, the milliohm meter provides you with the low resolution reading, from this, it is possible to accurately pinpoint which component is the one with the short.

I'm sure you've all worked it out from the display which is the shorted component, but if not, click below to reveal the answer.

Spoiler

Reveal Which Component Was Shorted

Yes, it was component number 3, the diode. When tested, this gave the lowest reading of all the tests, thus could be pinpointed as the short. Four-Wire Kelvin measurement makes it possible to accurately measure resistance values less than 0.1 Ω while eliminating the inherent resistance of the lead wires connecting the measurement instrument to the component being measured. The screen of the leads is used for the current supply (current supply lead), and the core for sensing (sense lead)

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Posted : 27/05/2022 6:39 pm

Red_to_Black, Marconi_MPT4, Lloyd and 6 people reacted

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Milliohm meter TMK TEM 3000

Sundog

(@sundog)

Posts: 173

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@crustytv I wish I'd had one of those when I once needed to trace a short circuit. Since I didn't, I decided to supply a current source and 'sniff" where the voltage was closest to zero. With 100mA there was no voltage to sense, then with 1A I could see some voltage but not enough resolution to be sure. Up to 5A and I was tracing down to the lowest voltage when suddenly there was a bang, and some smoke came from backplane of the kit. I can only suppose that a whisker of conductive material had got caught between 2 pins of an edge connector to cause the short, because once the boards were replaced, it worked fine.

Posted : 30/05/2022 8:49 pm

crustytv, crustytv and crustytv reacted

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