A good idea or not? IF's in a crystal set! - UK Vintage Radio Repair and Restoration Discussion Forum

Pete_kaye · 8th Mar 2010, 3:27 pm

Would it work if I tried to use one or two matching IFT's (probably 470Kcs) to construct a simple crystal set set up with external aerial and earth .I have a pair of normal size IFs. and thought of fitting a 500pf or 1000 pf cap across the IF to tune away from If to LW band,in particular 198 KCS.I have IN34 diode and high imp. phones .Any comments welcome.i am happy to experiment.

paulsherwin · 8th Mar 2010, 3:57 pm

Any coil should work after a fashion, but I wouldn't choose an IFT myself. A MW or LW coil from a scrap radio (valve or transistor) is likely to give better results, or you could wind your own. It will do no harm to experiment with an IFT of course.

I'm not a crystal set expert though

Paul

wireful3 · 8th Mar 2010, 4:54 pm

Hi Pete, If you just want to make a simple crystal set then a hand wound coil on a piece of plastic drainpipe could be more satisfactory than an IF coil. I agree with Paul's suggestion of using tuning coils from a scrap radio as being better. If you want to experiment with IFTs It might be easier to go for MW by removing turns, going for LW by adding turns would be more difficult. A much larger capacitor as you are thinking of would affect Q and that might be a problem.

It is possible that connecting the primary and secondary in series and removing turns from one of them might work. I have never tried this myself so it may actually be a silly suggestion.

Be warned - once you start experimenting with crystal sets it is very easy to get hooked.

Have Fun

dominicbeesley · 8th Mar 2010, 6:47 pm

Hi Pete, I'd try one first: I'm not sure if you'd get two to work better than one! Anyway I'd be interested in what you come up with! I've never managed to get a crystal longwave set to work up in Yorkshire but I'm sure it must be possible...

Dom

G8HQP Dave · 8th Mar 2010, 10:32 pm

An IFT usually has two tuned circuits, which are approximately critically coupled. Using this for a crystal set could bring two problems:
1) changing the tuning could change the coupling,
2) the incoming energy is split between the two tuned circuits so you immediately have a 3dB loss.
Putting the two coils in series would avoid these issues, so I would try that. You would need to remove the internal capacitors.

Robert-UK · 11th Mar 2010, 10:24 am

There is a design for a crystal set using a scrap IF transformer in WJ May's book "The Boy's Book of Crystal Sets" published in 1954.

I've extracted the relevant fragment.

Robert

Variometer · 12th Mar 2010, 9:41 am

The easiest ready-made coil for a crystal set (or any other receiver) is simply a couple of 100uH RF chokes in series, aerial going to the central junction. They work fine. I have even made superhet frequency changer stages using RF chokes. People usually tell me about Q and technical things like that and how they aren't very efficient, but I have always had excellent results with chokes and rarely use anythg else these days. I put them inside burned-out 1 1/4 inch fuses so I can change them easily, using PCB fuseholders to hold them.
Bob

Variometer · 12th Mar 2010, 6:31 pm

Here is a small crystal set using 100uH chokes. They are the yellow items on the side of the variable capacitor. The germanium diode is inside the fueholder on this set. The small ceramic capacitor is 100pF.
Bob

Pete_kaye · 15th Mar 2010, 9:39 pm

Well it works and R4 isquite good. I linked the two coils and connected the diode to the centre tap effectively and aerial to the top of the primary .I did not remove the 100pf caps .I tuned a350 pf across the aerial and earth and got a little interference from MW.I also just heard RTE1 on1200m when connected to a computer speaker.

G8HQP Dave · 15th Mar 2010, 11:25 pm

Leaving the internal capacitors in place means that you may have a multi-resonant circuit, so you can listen to two frequencies at once.

Pete_kaye · 26th Mar 2010, 5:44 pm

Not sure what 'listen to two frequencies at once ' means .I tried to build something using one IFT with a primary and unconnected secondary ,result Zilch. I also tried joining the two bottom ends of coils to earth with a tuning caps (x350pf) across each one .Again zilch.LW set up still works though

From what I can read ,the two coils should be movable and /or at right angles ,then linked by a small cap across the top ends.

G8HQP Dave · 26th Mar 2010, 10:52 pm

Each winding plus its own capacitor has a resonance, which happen to be on similar frequencies. In addition, the total setup has a resonance at a lower frequency due to your extra capacitor (plus the aerial) resonating with the inductive reactance of the windings below their own resonance.

Leaving the secondary unconnected may fail because it is still coupled to the primary and so will suck energy out of it. You would need to disconnect the secondary capacitor too, so the winding is left open circuit.

Top capacitor coupling is an alternative to inductive coupling.

8th Mar 2010, 3:27 pm	#1
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	A good idea or not? IF's in a crystal set! Would it work if I tried to use one or two matching IFT's (probably 470Kcs) to construct a simple crystal set set up with external aerial and earth .I have a pair of normal size IFs. and thought of fitting a 500pf or 1000 pf cap across the IF to tune away from If to LW band,in particular 198 KCS.I have IN34 diode and high imp. phones .Any comments welcome.i am happy to experiment.

8th Mar 2010, 3:57 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: A good idea or not? IF's in a crystal set! Any coil should work after a fashion, but I wouldn't choose an IFT myself. A MW or LW coil from a scrap radio (valve or transistor) is likely to give better results, or you could wind your own. It will do no harm to experiment with an IFT of course. I'm not a crystal set expert though Paul

8th Mar 2010, 4:54 pm	#3
wireful3 Retired Dormant Member Join Date: Sep 2003 Location: Bristol, UK. Posts: 808	Re: A good idea or not? IF's in a crystal set! Hi Pete, If you just want to make a simple crystal set then a hand wound coil on a piece of plastic drainpipe could be more satisfactory than an IF coil. I agree with Paul's suggestion of using tuning coils from a scrap radio as being better. If you want to experiment with IFTs It might be easier to go for MW by removing turns, going for LW by adding turns would be more difficult. A much larger capacitor as you are thinking of would affect Q and that might be a problem. It is possible that connecting the primary and secondary in series and removing turns from one of them might work. I have never tried this myself so it may actually be a silly suggestion. Be warned - once you start experimenting with crystal sets it is very easy to get hooked. Have Fun

8th Mar 2010, 6:47 pm	#4
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: A good idea or not? IF's in a crystal set! Hi Pete, I'd try one first: I'm not sure if you'd get two to work better than one! Anyway I'd be interested in what you come up with! I've never managed to get a crystal longwave set to work up in Yorkshire but I'm sure it must be possible... Dom

8th Mar 2010, 10:32 pm	#5
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: A good idea or not? IF's in a crystal set! An IFT usually has two tuned circuits, which are approximately critically coupled. Using this for a crystal set could bring two problems: 1) changing the tuning could change the coupling, 2) the incoming energy is split between the two tuned circuits so you immediately have a 3dB loss. Putting the two coils in series would avoid these issues, so I would try that. You would need to remove the internal capacitors.

12th Mar 2010, 9:41 am	#7
Variometer Retired Dormant Member Join Date: May 2008 Location: Preston, Lancashire, UK. Posts: 428	Re: A good idea or not? IF's in a crystal set! The easiest ready-made coil for a crystal set (or any other receiver) is simply a couple of 100uH RF chokes in series, aerial going to the central junction. They work fine. I have even made superhet frequency changer stages using RF chokes. People usually tell me about Q and technical things like that and how they aren't very efficient, but I have always had excellent results with chokes and rarely use anythg else these days. I put them inside burned-out 1 1/4 inch fuses so I can change them easily, using PCB fuseholders to hold them. Bob

12th Mar 2010, 6:31 pm	#8
Variometer Retired Dormant Member Join Date: May 2008 Location: Preston, Lancashire, UK. Posts: 428	Re: A good idea or not? IF's in a crystal set! Here is a small crystal set using 100uH chokes. They are the yellow items on the side of the variable capacitor. The germanium diode is inside the fueholder on this set. The small ceramic capacitor is 100pF. Bob Attached Thumbnails

15th Mar 2010, 9:39 pm	#9
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: A good idea or not? IF's in a crystal set! Well it works and R4 isquite good. I linked the two coils and connected the diode to the centre tap effectively and aerial to the top of the primary .I did not remove the 100pf caps .I tuned a350 pf across the aerial and earth and got a little interference from MW.I also just heard RTE1 on1200m when connected to a computer speaker.

15th Mar 2010, 11:25 pm	#10
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: A good idea or not? IF's in a crystal set! Leaving the internal capacitors in place means that you may have a multi-resonant circuit, so you can listen to two frequencies at once.

26th Mar 2010, 5:44 pm	#11
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: A good idea or not? IF's in a crystal set! Not sure what 'listen to two frequencies at once ' means .I tried to build something using one IFT with a primary and unconnected secondary ,result Zilch. I also tried joining the two bottom ends of coils to earth with a tuning caps (x350pf) across each one .Again zilch.LW set up still works though From what I can read ,the two coils should be movable and /or at right angles ,then linked by a small cap across the top ends.

26th Mar 2010, 10:52 pm	#12
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: A good idea or not? IF's in a crystal set! Each winding plus its own capacitor has a resonance, which happen to be on similar frequencies. In addition, the total setup has a resonance at a lower frequency due to your extra capacitor (plus the aerial) resonating with the inductive reactance of the windings below their own resonance. Leaving the secondary unconnected may fail because it is still coupled to the primary and so will suck energy out of it. You would need to disconnect the secondary capacitor too, so the winding is left open circuit. Top capacitor coupling is an alternative to inductive coupling.