Hamfest inductors

[tggzzz]

A hamfest or two ago I picked up a couple of inductors because... do I need to spell it out?! Anyway, I like Muirhead and Cambridge Instruments kit.

I checked for continuity but unsurprisingly my cheap component tested didn't bother to measure L. Hence I was dreaming up ways to measure the inductance, but never got around to it.

The, while lazing in the most productive environment known to mankind[1], I finally remembered that I had picked up one of those wretched universal bridges which make you appreciate modern TE: a Marconi TF2700.

The TF2700's spec is +-1%, the 1mH measures 0.99mH (see photo) and the 10*10mH are also all good. Job done, they can be stuffed in the cupboard and left to irritate my daughter when she clears things out.

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[1] almost submerged in a hot bath, of course - one of the crowning achievements of civilisation.

[Zenith]

The TF2700 is a bit clunky but they do work. I haven't used mine in 20 years.

I'm sure when they came out the ads claimed they could measure the inductance of a paper clip.

[vk6zgo]

The TF2700 is a bit clunky but they do work. I haven't used mine in 20 years.

I'm sure when they came out the ads claimed they could measure the inductance of a paper clip.

Back in the day, at my old work, they were the "go to" bridge!

[tggzzz]

Played around with it again last night.

Some 1µH inductors disguised as resistors with a silver band measured 1.1µH. Can't argue with that.

Some relatively chunky 0.75µH inductors measured around 1µH. Conceivably there is a frequency dependence of the core, but I'm unconvinced.

[Zenith]

The slippery slopes of LCR nuttery.

[tggzzz]

The slippery slopes of LCR nuttery.

R yes, because you can expect sufficient sig fig to be interesting.

L and C unlikely, because <0.5% seems to be "high accuracy".

[EC8010]

Then, while lazing in the most productive environment known to mankind, almost submerged in a hot bath, of course - one of the crowning achievements of civilisation.

Many of my better ideas for work occurred either in the bath or towards the end of a beer. I requested a bath and a beer fridge in my office, but nothing came of it.

The whole point of twiddling the knobs on a TF2700 is to make you think about whether the answer is plausible; always question the number on an LCD display, where it be on a calculator or test equipment.

[tggzzz]

The whole point of twiddling the knobs on a TF2700 is to make you think about whether the answer is plausible; always question the number on an LCD display, where it be on a calculator or test equipment.

Yes, you need to know the expected answer before you measure it. It is surprisingly difficult to get people to act on that principle[2]

However, the TF2700 ain't nice. Well, to be more accurate, manual RLC bridges aren't that nice. I found that even when measuring reference capacitor and resistors, it was easy to get unacceptable answers[1]. The "solution" was to twiddle the non-orthogonal knobs a bit more. And a bit more. And a bit more. And then to wonder whether a bit more twiddling would give a better result. And then to note it was only 1% anyway. EDIT: 0.1%, doh.

I'm still going to keep it, since "it might come in".

[1] in fairness, I even found that to be an issue with an Aim LCR Databridge 401.

[2] and now LLMs are coming: we're doomed. LLMs are not only statistical parrot bullshitters, but also the revenge of the unskilled.

I'm tempted to set up a company called "LLM-busters", inspired by the GhostBusters theme tune "There's somethin' weird and it don't look good / Who ya gonna call (ghostbusters)"

[EC8010]

Practice makes perfect. I found the TF2700 very nice to use. And it can quite easily be made more accurate. After all, it's only a bridge. The resistors on the large knob are only 2%, so they can easily be improved. Likewise, the range switch can be changed to 0.1%. Everything reactive is compared to its reference capacitor, so that can be tweaked if necessary. It's not difficult to make a TF2700 much more accurate than as received. Try doing that with a tester full of digital demons.

[tggzzz]

Practice makes perfect. I found the TF2700 very nice to use. And it can quite easily be made more accurate. After all, it's only a bridge. The resistors on the large knob are only 2%, so they can easily be improved. Likewise, the range switch can be changed to 0.1%. Everything reactive is compared to its reference capacitor, so that can be tweaked if necessary. It's not difficult to make a TF2700 much more accurate than as received. Try doing that with a tester full of digital demons.

Improving the capacitors is more difficult than improving the resistors. The capacitors are the standard "decent stable type for LF filters" used at the time.

Improving digital stuff? Easy: the solution to all signal processing problems is to integrate for longer.

(The solution to all programming problems is to add an extra level of indirection)

[EC8010]

If memory serves, it's a single 100nF polystyrene. A close tolerance Soviet 100n silvered mica (suitably tweaked) is all that would be required. Seriously. I've measured a few recently and they are very good.

I'll take your word for it on programming.

[tggzzz]

If memory serves, it's a single 100nF polystyrene. A close tolerance Soviet 100n silvered mica (suitably tweaked) is all that would be required. Seriously. I've measured a few recently and they are very good.

I've finally bothered to look at the manual, and you are right: "0.1% plastic", not the 1% I claimed. Mea maxima culpa.

I do have some (very) old small mica capacitors around, but how accurate would they (still) be.

I also have some fag-packet sized micas, which really ought to be accurate - but only up to 40nF. And then there's the 500nF book-sized Sullivan mica.

[Robert]

Many years ago, more than 40, I was very happy to have a TF2700 at work. It's portable and measures small capacitors. Try lugging a 25 pound Wayne Kerr b221 onto a aircraft.
That reminds me. When I was working in Jersey (CI, not USA) we had a pair of Shorts 330 twin turboprop aircraft with slightly different radios to the rest of our fleet. One had reccuring issues with the ADF (150kHz to 2 MHz direction finder used for navigation). Having rouled out the easy stuff I was suspecting the sense antenna (an electrically short E-field antenns used to resolve the 180 degree ambiguity of the H-field loop). The antennas primay charatistic was it's capcitance but this was undocumented. Unfortunatly it was belly mounted and access was by removing the floor. To get the floor up you had to remove 2/3rds of the seats. So when the airframe guys had the floor up on the "good" aircraft I thought I'd take the opportunity to get a reference value with the TF2700. I was just finishing up when I got an urgent call to the line as a departing aircraft full of passengers had an issue. So I went and sorted that out. I then went back and picked up the TF2700 and lead from the bench where I'd left it. Later that evening, as I was eating I suddenly had this nagging doubt: Had I reconnected the BNC to the antenna? I finished dinner and went back to the airport (I was the only avionics engineer on the island at the time). Powered up the aircraft, tuned the ADF to the local beacon and the pointer swung to the correct heading and then flipped 180 So I went to operations and asked them to ring around to see if any of the airframe guys wanted some overtime..... I did help them and bought late night food. Four hours work to access a BNC that took 5 seconds to reconnect. The aircraft was scheduled for the first flight in the morning. The company had what we would now call a good safety culture. Make a genuine mistake and as long as you do the right thing there is no punishment. Try to hide your lazy mistake and it's another matter. I got a don't do it again. I didn't claim an overtime though.
So that's my TF2700 story.

Robert.

[tggzzz]

The company had what we would now call a good safety culture. Make a genuine mistake and as long as you do the right thing there is no punishment. Try to hide your lazy mistake and it's another matter.

True in so many walks of life.

I taught that to my daughter.

In HP someone was caught using the internet "inappropriately". When confronted, he denied it and was sacked. If he had admitted it he wouldn't have been. In another case it was using someone else's phone to call his girlfriend in Singapore; trying to disguise it was the critical mistake.