TF2700 - usage on inductors?

[bugbear]

A Marconi TF2700 has followed me home from a car boot sale.

It's "working" and very easy to use on its DC resistor mode (needle tells you exactly what to do).

I've tried it against capacitors, and eventually got the hang of it, although in the AC mode the error is always in the same needle direction, no matter which side of the NULL you are.

But I am having no success in measuring inductance - I keep getting situations where I haven't found a null (meter at 0), reading nestled in a plateau of large errors (a null "hole"), but I find a situation where the needle moves from null to high error as I cross a critical setting (a null "cliff").

This is not helped by the fact that I don't have inductors of known value. I don't normally use inductors, so the inductors I'm measuring were desoldered from a dead printers switch mode PSU board.

I have two actual questions:

1) How should the null balance be found for inductors - the manual essentially says "play around with the balance and loss controls until you find it".

2) What are the marked values of my 2 inductors (shown montaged with a rule marked in inches) ?

(I have bought a full set of capacitors to replace the fitted electrolytics should that be needed)

BugBear

[Station X]

Perhaps the problem is with the low inductance ranges. You could try measuring the inductance of a mains transformer's primary winding by measuring across the mains plug of an AC radio. With it switched on of course, but obviously not connected to the mains supply.

[Herald1360]

My bet would be 680 and 330 uH.

[Restoration73]

Should be 680uH and 330uH. They look OK.

[Sideband]

I have one of these...at least I THINK it's a 2700.....I'll probably be in the workshop later and check mine. I rarely check inductors but I'm sure I checked it against a known one when I first got it about 15 years ago and it worked OK. If I remember correctly, I checked a 2.5mH choke.

[bugbear]

Originally Posted by Station X

Perhaps the problem is with the low inductance ranges. You could try measuring the inductance of a mains transformer's primary winding by measuring across the mains plug of an AC radio. With it switched on of course, but obviously not connected to the mains supply.

I'd forgotten that a transformer has inductance!

I "just happen" to have a small 15VA toroidal transformer in my parts box, with twin 0-120V primaries, and twin 0-9V secondaries. Any idea what inductance value I should be expecting on any of these windings?

BugBear

[joebog1]

If its a decent quality transformer I would expect to see a minimum of 50 henries on the primary.
Thats when the primaries are wired in series.

Joe

[Uncle Bulgaria]

Have you got the manual? I have one and have found it hard to use until I worked out a knack of the two controls, adjusting one then the other to home in on the null. I'll try it against an inductor tomorrow and see what happens.

[Diabolical Artificer]

Inductor testing on the TF2700 is tricky, it's rather like cracking a safe, sometimes one goes home empty handed, not sure why that is, I've also found I have good days and bad days with my 2700 and easy/hard components, I think this might be where a components is faulty like a high ESR cap or an inductor where there's a large-ish resistive element as well as an inductance. Or could it be down to the 1khz signal? Might a 10khz or 100khz external sinewave be better?

Like any bit of complex (ish) test gear it takes practice to drive the thing successfully, I'd suggest starting practicing with a known inductor, play with the "power transformer & electrolytics" setting as well as slowly increasing sensitivity: get a null on a low setting first.

I know what you mean about the null meter,it always goes to the right, never to the left.... curious...

Andy.

[Radio Wrangler]

It's a scalar (non-vector) AC millivoltmeter looking at the bridge unbalance. It can only dip to zero volts if you get the main potentiometer and the dissipation factor pot set perfectly.

A centre zero meter makes sense on a DC bridge measuring resistance. For AC measurements, it needs vector detection.

HP made a little bridge that had a phase detector operating a pair of neon lights to show which way to turn the main balance control to find the null.

The larger Wayne-Kerr bridges have full measurement voltmeters with usefully scaled meters running on a pair of synchronous rectifiers fed with quadrature reference signals form the oscillator section. Your first stab at a component produces meter readings telling you what steps to set the first ratio transformer sections to (two transformers, one in-phase for real resistance/conductance, one in quadrature phase for capacitance/inductance/susceptance) once you punch these values on the first rows of the button banks, it helpfully adds x10 gain to both channels and the meters give you the next values to punch in. No guessing or searching around!

All these bridges, running at audio frequencies have problems with the low reactances of typical RF value inductors.

To get around this, the serious makers made RF-excited bridges, Q-meters and you could use RF type network analysers.

David

[bugbear]

It can only dip to zero volts if you get the main potentiometer and the dissipation factor pot set perfectly.

With my bridge, that's not the case. When working on inductors I can readily get into positions where ALL settings of the balance (or sometimes loss) greater than "X" give a zero reading, and ALL settings lower give a positive.

This is what I meant by the null not being a "dip".

I think my bridge may well be faulty, hopefully just an old, dry capacitor in need of replacement.

Or I may just live with it - my need to measure inductors is non-existent.

The only real problem with the bridge is that it can't measure BIG electrolytic caps - the sort routinely used in power supplies.

[Wendymott]

I have ditched my digital electronic inductance measurement instruments, in favour of my TF2700 for all inductors, going down to sub 1 uH. It's all a matter of patience, and dual knob control. Was in the storage cupboard until I realised what a "gold" piece of equipment it is. Yes there may be better..... but not in my shack.

[ms660]

I liked the on/off indicator, ideal for battery life...

Sharp nulls with the one I had.

Lawrence.

[Sideband]

Originally Posted by Wendymott

It's all a matter of patience, and dual knob control. Yes there may be better..... but not in my shack.

I agree and as mine cost me nothing (rescued from the Philips clear-out) it makes it even more useful! Sharp nulls on mine.