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Old 30th Nov 2009, 6:00 pm   #1
Join Date: Sep 2009
Posts: 487
Thumbs up Mixer LO/Tuning coil progress.

I`v had what is for Me a bit of a breakthrough with my super-het project, an experiment that I`d been putting off and worrying about it`s feasability for the last few days has finally paid off!
namely the Coils and tracking for the mixer.
despite trying (and failing) to get any 1/4 inch Dia formers for the coils, I discovered some Cans in half a TV board that I`v had doing nothing for over a decade and stripped them down.
I also redesigned the entire front end of a Murphy 198 that I had for spares, fixed up a few problems it had, and got it working as just MW with bare bones basic circuitry (so no wave changer etc...).

I got some Coil data thanks to Harry Lythall and had a go at making my own LO and Tuning coils to replace the MW coils in the 198.
when I powered it up there was silence until I hooked up the antenna (the ferrite rod antenna was now redundant), and I was on SW!

oddly there was very little in the way of Tuning needed to get some stations and all this worry I had about half a turn the wrong way and it won`t work etc... was ill founded.
in fact I find it to be Very forgiving indeed, reinforced by the fact that after re-reading Mr Lythalls data, instead of 27 turns tapped at 4, I`d done 4 turns, tapped and then 27, making 31 turns total!
(yeah, so I hadn`t had my morning coffee yet when I wound them :P )
I`v put a couple of pictures of these coils and my new SW radio below, the 1`st one is the Tuning coil (white wire is the antenna), the second is the LO and the complete absense of all the LW/MW stuff.

I`ll have a hunt around in the morning to try and find some suitable board, and then start work with the ECH81 using these Tried and Tested homebrew coils to make my Mixer, and then ultimately marry that to the other half of the super-het.
for that few that Are following my progress, take heart as the end is now in Sight!

edited to add: the above winding data takes you to SW, and the top end is roughly 10 MHz as I was listening to Radio Japan today on 9.8MHz and still had some cap left, and should go down to 4MHz.
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Old 1st Dec 2009, 11:32 am   #2
dominicbeesley's Avatar
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Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,696
Default Re: Mixer LO/Tuning coil progress.

Well done!

Winding coils used to always scare the stuffing out of me too until I had a go. So far like you I've (nearly) always been pleasantly surprised - and its not a big deal to start again!

The real difficuly on a superhet is getting the tracking right i.e. the oscillator circuit to stay precisely the IF (455kHz) away from the RF circuit.

This can be checked roughly by getting it working at one end of the scale then tuning to the other end and adjusting the padder to line it up then check it works well all the way through the range....or you could do what I did and have a seperate tuning condenser for RF and oscillator but then you need extra arms!

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Old 1st Dec 2009, 11:57 am   #3
Join Date: Sep 2009
Posts: 487
Default Re: Mixer LO/Tuning coil progress.


I think what concerned me as well was the fact that although the coil data was good, it asked for 32SWG wire, I`m not sure I had any, so I looked that up as well and it`s about 0.2mm dia, so I looked through my copper wire collection and found the one closest to fitting 5 side by side into a 1mm space.
and the Original windings in the 198 where ridiculously sensitive that half a turn and most of the stations would go quiet or lost, I expected the same result that even if I DID make them correctly it would be near impossible to get anything out of it.

I found an Easy(ish) way to get all the stations to work across the band though, since it has no slow tuning aparatus, turning the variable cap from one side to the other in a single move is easy.
as you do this you get a "chipping" sound as you pass a station, you can Tune the coils as your listening and watching the caps position so that it gets Louder and spreads nicely across the range.
it`s not exactly High-Tech, but it works
Now it makes sense why the outer blades on a variable cap are often bent outwards slightly in some positions!
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Old 1st Dec 2009, 1:37 pm   #4
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Join Date: Jun 2006
Location: Chard, south Somerset, UK.
Posts: 6,300
Arrow Re: Mixer LO/Tuning coil progress.

Originally Posted by YT2095UK View Post
Now it makes sense why the outer blades on a variable cap are often bent outwards slightly in some positions!
Just a side-comment on that remark.
The usual practice of s/het tracking is to get the difference in the resonant freqs. of the RF stages and the oscillator to be the intermediate freq. Unfortunately, this difference cannot be maintained at all freqs. across the RF tuning range, so it is usual to get this required difference to be exact at 3 freq. points - a.k.a. "3-point tracking". At its optimum, there will still be small errors between these tracking points, so the idea is to adjust the cut outer blades (inwards or outwards) of the tuning capacitor to minimise these errors.

Al. / Skywave.
"Your idea is crazy! But the question is this: is it crazy enough . . . . to be true?" (Niels Bohr)
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