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Old 22nd Jan 2018, 1:36 am   #1
MrBungle
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Default Rewinding slug tuned IFTs

I am in the process of designing something and I've come across the requirement for a certain slug tuned inductor which appears to be unobtainable. Need 3.5uH. Doesn't seem to exist. Spectrum sell 5.3uH and 2.6uH cans so I've decided to try and rewind one with only one winding. I've got one 2u6L left to experiment with.

Some questions:

1. Is this a terrible idea?
2. How best to go about this? Do I start low and measure it with no core or do I stick the core in it and measure it?
3. Can I reuse the wire that is in them or should I buy some and just empty it and start again.
4. How can I crack one open without destroying it?

Alternatively does anyone know if I can tune the 5.3 or 2.6 coils into that range?
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Old 22nd Jan 2018, 9:19 am   #2
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Default Re: Rewinding slug tuned IFTs

Unwind a few turns from the 5u3?
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Old 22nd Jan 2018, 9:48 am   #3
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Default Re: Rewinding slug tuned IFTs

Will try that thanks. I’ve got to get hold of a 5u3L first hence checking viability. Haven’t rewound or dismantled any new ones before, only the big ones you used to get on very old transistor radio chassis’.
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Old 22nd Jan 2018, 10:08 am   #4
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Default Re: Rewinding slug tuned IFTs

I can't remember if they have a couple of small crimps on the bottom that need to be straightened first, but basically you very carefully push them out of the can from the top (ie push down from the top so they slide out the bottom).
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Old 22nd Jan 2018, 10:33 am   #5
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Default Re: Rewinding slug tuned IFTs

Thanks - I just tried that and cut my finger open (doh!) so doing a more scientific method first.

I stuffed the 2u6L I have across a 100pF mica 1% capacitor and put a 47k resistor in series with the tuned circuit and stuffed it on my scope and signal generator. Solved resonant circuit equation for the two limits of the coil and it gives:

2u6L tuning range of 1.72uH to 4.17uH.

That's well within the tuning range of what I need so I'll probably grab some 2u6L's and just tune them appropriately.

I was half asleep last night at half past midnight when I was thinking this through. This morning I woke up with a "there must be an easier way here".

I'll grab some 5u3L's as well just in case they don't all tune into that range and I need some headroom.
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Old 22nd Jan 2018, 10:50 am   #6
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Default Re: Rewinding slug tuned IFTs

One other thing to consider - which I've done in the past - is wiring an additional coil in series [externally] to your variable one.

OK the tuning-range will be restricted so your choice of the additional coil's value needs to be made with intelligence - and it'll be unscreened - but the little axial-leaded coils that look like resistors are ideal for this sort of thing.
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Old 22nd Jan 2018, 11:05 am   #7
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Default Re: Rewinding slug tuned IFTs

That's an interesting idea actually. Will bear that in mind in future as well.
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Old 22nd Jan 2018, 11:15 am   #8
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Default Re: Rewinding slug tuned IFTs

It worked just fine as a technique for me when I wanted to retune a 10,7MHz FM IF strip down to 5.5MHz for TV sound.
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Old 22nd Jan 2018, 1:21 pm   #9
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Default Re: Rewinding slug tuned IFTs

You can effectively shift the inductance up a bit by adding a parallel capacitance. Or down by adding series capacitance. It all depends on context. What was the 3.5uH needed for?
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Old 22nd Jan 2018, 2:35 pm   #10
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Default Re: Rewinding slug tuned IFTs

It’s mainly a cost saving. It’s part of a narrow filter with extremely low passband ripple. If I don’t use 3.5uH then I have to combine a few capacitors as a substitute and they are quite expensive.
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Old 24th Jan 2018, 1:46 pm   #11
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Default Re: Rewinding slug tuned IFTs

Ok got my 5u3L's this morning and have tested a couple and taken averages across the board:

2u6L range from 1.7uH -> 4.2uH (sample size 6)
5u3L range from 3.7uH -> 8.1uH (sample size 5)

Looks like 2u6L will do the job here. If it doesn't meet the pass requirements, I'll swap the transformer out for a 5u3L and try again.

These were tested with a calibrated DDS and 1% 100pF mica capacitor in parallel. DDS was swept until peak was found on a scope. DDS was connected to the tank circuit with a 47k resistor so as not to completely ruin the Q of the tank and get a good peak.
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