Driving a Rexophone crystal set! - UK Vintage Radio Repair and Restoration Discussion Forum

high_vacuum_house · 16th Mar 2023, 9:11 pm

Good evening,
I acquired a nice looking twin detector Rexophone crystal set from the BVWS Biggleswade event last Sunday. I am looking for some tips to drive it.

I was using a piezo element crystal type earpiece though I cannot get much sound at all from it yet other than a rather feint 50Hz hum. I have been trying for a few hours both in the day and at night to see if conditions improve. I was thinking do I need a high impedance type headphones of say 4000 ohms in order for enough current to flow through the cats whisker detector in order for it to function.

I have a long wire aerial from the bedroom window to a tree at the bottom of the garden which I would say is 30' long. The instruction shows 100' long. It looks like I would need to find a way to make it longer to harness enough RF power to power the crystal set earphone. Or I could just blame my hearing !!

Many thanks,
Christopher Capener

Simon Gittins · 16th Mar 2023, 9:28 pm

It will probably be better with 4000 ohms headphones. I'm sure you know it needs an earth connection as well. One thing you could try is using it with an amplifier to start with and then connecting headphones when you have tuned in a strong signal.

Herald1360 · 16th Mar 2023, 11:05 pm

Perhaps try with a 4k7 or so resistor across the piezo earpiece?

Mr 1936 · 16th Mar 2023, 11:45 pm

Hi

Are you using the original galena crystal and point contact ('cats whisker') detector ? You might find it easiest to temporarily use a more modern germanium diode until you have sorted out other aspects.

Sensitive high impedance headphones or a crystal earpiece are needed, or as suggested couple up an audio amplifier to get you going.

The longer and especially the higher the aerial the better. The 100 foot limit was once imposed by the Postmaster General, I guess that was to prevent anything too outlandish. The far end of the aerial is a high impedance point, and needs a good insulator between it and any mechanical support (wooden pole, tree etc) or signal will be lost in the shunt resistance. A 6 inch piece of plastic strip should do the job.

A good radio earth (rod driven into the soil etc) is important

These sets were only ever intended for fairly local reception, albeit with transmitters that were feebler than todays. With the decline of medium wave you might struggle if there is no transmitter near you, but with luck there should be one dominant signal. Unless you want reception from hundreds of miles away you will be getting a ground wave, so day and night won't make much difference.

Good Luck !

Station X · 17th Mar 2023, 2:24 am

Crystal sets will not work without an earth connection as they rely on the capacitance between aerial and earth for tuning.

I've found that connecting the earth terminal to the copper pipe feeding a radiator in an upstairs room worked well.

I've also found that crystal earpieces, high impedance headphones and even low impedance headphones work well.

Mr 1936 · 17th Mar 2023, 9:14 am

Hi

I must confess that I'd forgotten that the detector diode needs a DC load to function. Unlike a traditional electromegnetic headphone, the piezo type earpiece "looks" like a capacitor, so a resistor is needed in parallel. Value is open to experiment, but I agree that 4.7 K is a good starting point.

Pete_kaye · 17th Mar 2023, 9:33 am

I have found that a 47k or 56k works best.

Mr 1936 · 17th Mar 2023, 10:08 am

Hi

My favourite book as a (very) young experimenter in 1960 was "The Boy's Book of Crystal Sets"

It covers the background as well as presenting several designs for home construction.

You can read an online version here: https://worldradiohistory.com/UK/Ber...l-Sets-v12.pdf

Dave757 · 17th Mar 2023, 12:19 pm

Hi,

The fairly common DLR5 military headset seemed to be one of the best for crystal sets.

My old friend Howard Turner of Centre Electronics Birmingham (now SK), did a
test on 23 different pairs of headphones, - mainly high impedance, - and the
DLR5s were better than all the others.

Another well thought of headset was the RCA "Big Cans", but we never tested those.

Kind regards
Dave
G0ELJ

Mr 1936 · 17th Mar 2023, 2:34 pm

Hi

I'm pretty sure it was DLR 5 phones I used back in 1960, no doubt purchased from my Dad's favourite army surplus store.

Although medium impedance (about 600 ohms) they were very sensitive. Inside, the diaphragm was corrugated and at the centre was an exceedingly small nut and thin rod coupled to the magnetised part within the coil. Most rival phones seemed to use a plain disc akin to that found in a telephone handset.

16th Mar 2023, 9:11 pm	#1
high_vacuum_house Octode Join Date: May 2003 Location: Belper Derbyshire Posts: 1,936	Driving a Rexophone crystal set! Good evening, I acquired a nice looking twin detector Rexophone crystal set from the BVWS Biggleswade event last Sunday. I am looking for some tips to drive it. I was using a piezo element crystal type earpiece though I cannot get much sound at all from it yet other than a rather feint 50Hz hum. I have been trying for a few hours both in the day and at night to see if conditions improve. I was thinking do I need a high impedance type headphones of say 4000 ohms in order for enough current to flow through the cats whisker detector in order for it to function. I have a long wire aerial from the bedroom window to a tree at the bottom of the garden which I would say is 30' long. The instruction shows 100' long. It looks like I would need to find a way to make it longer to harness enough RF power to power the crystal set earphone. Or I could just blame my hearing !! Many thanks, Christopher Capener __________________ Interests in the collection and restoration of Tefifon players and 405 line television

16th Mar 2023, 9:28 pm	#2
Simon Gittins Heptode Join Date: Jan 2006 Location: Cornwall, UK. Posts: 506	Re: Driving a Rexophone crystal set! It will probably be better with 4000 ohms headphones. I'm sure you know it needs an earth connection as well. One thing you could try is using it with an amplifier to start with and then connecting headphones when you have tuned in a strong signal.

16th Mar 2023, 11:05 pm	#3
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Driving a Rexophone crystal set! Perhaps try with a 4k7 or so resistor across the piezo earpiece? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

16th Mar 2023, 11:45 pm	#4
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 523	Re: Driving a Rexophone crystal set! Hi Are you using the original galena crystal and point contact ('cats whisker') detector ? You might find it easiest to temporarily use a more modern germanium diode until you have sorted out other aspects. Sensitive high impedance headphones or a crystal earpiece are needed, or as suggested couple up an audio amplifier to get you going. The longer and especially the higher the aerial the better. The 100 foot limit was once imposed by the Postmaster General, I guess that was to prevent anything too outlandish. The far end of the aerial is a high impedance point, and needs a good insulator between it and any mechanical support (wooden pole, tree etc) or signal will be lost in the shunt resistance. A 6 inch piece of plastic strip should do the job. A good radio earth (rod driven into the soil etc) is important These sets were only ever intended for fairly local reception, albeit with transmitters that were feebler than todays. With the decline of medium wave you might struggle if there is no transmitter near you, but with luck there should be one dominant signal. Unless you want reception from hundreds of miles away you will be getting a ground wave, so day and night won't make much difference. Good Luck !

17th Mar 2023, 2:24 am	#5
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Driving a Rexophone crystal set! Crystal sets will not work without an earth connection as they rely on the capacitance between aerial and earth for tuning. I've found that connecting the earth terminal to the copper pipe feeding a radiator in an upstairs room worked well. I've also found that crystal earpieces, high impedance headphones and even low impedance headphones work well. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

17th Mar 2023, 9:14 am	#6
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 523	Re: Driving a Rexophone crystal set! Hi I must confess that I'd forgotten that the detector diode needs a DC load to function. Unlike a traditional electromegnetic headphone, the piezo type earpiece "looks" like a capacitor, so a resistor is needed in parallel. Value is open to experiment, but I agree that 4.7 K is a good starting point.

17th Mar 2023, 9:33 am	#7
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: Driving a Rexophone crystal set! I have found that a 47k or 56k works best.

17th Mar 2023, 10:08 am	#8
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 523	Re: Driving a Rexophone crystal set! Hi My favourite book as a (very) young experimenter in 1960 was "The Boy's Book of Crystal Sets" It covers the background as well as presenting several designs for home construction. You can read an online version here: https://worldradiohistory.com/UK/Ber...l-Sets-v12.pdf

17th Mar 2023, 12:19 pm	#9
Dave757 Heptode Join Date: Apr 2009 Location: Scratby, Norfolk, UK. Posts: 651	Re: Driving a Rexophone crystal set! Hi, The fairly common DLR5 military headset seemed to be one of the best for crystal sets. My old friend Howard Turner of Centre Electronics Birmingham (now SK), did a test on 23 different pairs of headphones, - mainly high impedance, - and the DLR5s were better than all the others. Another well thought of headset was the RCA "Big Cans", but we never tested those. Kind regards Dave G0ELJ

17th Mar 2023, 2:34 pm	#10
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 523	Re: Driving a Rexophone crystal set! Hi I'm pretty sure it was DLR 5 phones I used back in 1960, no doubt purchased from my Dad's favourite army surplus store. Although medium impedance (about 600 ohms) they were very sensitive. Inside, the diaphragm was corrugated and at the centre was an exceedingly small nut and thin rod coupled to the magnetised part within the coil. Most rival phones seemed to use a plain disc akin to that found in a telephone handset.