Germanium Diode Radio

[Nuvistor]

Originally Posted by Tim

I have tried about half a dozen times to build a crystal set & have never got one to work. .

It’s 60 odd years since I built one, lots more stations around then so that may have helped. Built with a coil former of a loo roll, scrap tuning condenser, hi impeadance headphones and a second hand point contact diode of some form. Worked a treat with a good aerial and earth.

Before lockdown in March I was in a high street shop that had Crystal radio kits for sale among the many other books, craft etc.
The components looked ok, decent size coil etc then I looked at the diode, it was silicon, probably be a disappointment to the budding young builder.

[chompy1]

thanks for your replies guys ........ a very interesting read! I do have a few Mullard OC79's , so I will scrap the full wave bridge arrangement and try just one Mullard OC79.

[Nuvistor]

I don’t know if it’s been mentioned but you need to use a high impedance earpiece not headphones that are used with MP3 players.

[Kala_12]

Did you mean OA79. An OC79 is a transistor isn't it.
Stuart.

[chompy1]

ah! Yes, I meant OA79. Paul

[AC/HL]

You can use just one junction of a transistor if that's all you have handy.

[Philips210]

Hi.

As Chris mentioned, the OA47 is a good germanium diode to use.

The 1SS86 Schottky diode is reputed to be very good as a detector when used in crystal sets. I purchased some of these https://www.ebay.co.uk/itm/1SS86-Dio...-/113712249857 and intend to do some experiments with various crystal set designs.

Regards,
Symon

[Tim]

I don't want to hijack Chompy's thread, but here goes.

Actually I think I did get one to work once, using an electronics set my mum bought me for Christmas, aged 9 .
She actually built it for me, and then wrapped it up so it would work straight away out of the box, as it were. I still have the electronics set somewhere(Anyone remember the Denshi block?), but last time I re-built the same circuit it didn't work.

I have tried a variety of ferrite aerials from scrap radios, along with their tuning caps , and usually use one of those light brown crystal earpieces( the ones with the white twisty wires), all sorts of diodes, both ways round and even tried winding my own coils.
No luck. I tried building one with my daughter a few years ago, following instructions from a book, and that didn't work either, much to my embarrassment.
I guess I just don't have the knack.

[Nuvistor]

My experience was a large coil with a high Q, long wire outside aerial and good earth.
Even the valve sets of the 1930 would have a long wire aerial to a pole in the back yard/garden to work well.

[Radio Wrangler]

A good Earth also makes a useful difference.

David

[FireballX]

Originally Posted by Tim

I don't want to hijack Chompy's thread, but here goes.

Actually I think I did get one to work once, using an electronics set my mum bought me for Christmas, aged 9 .
She actually built it for me, and then wrapped it up so it would work straight away out of the box, as it were. I still have the electronics set somewhere(Anyone remember the Denshi block?), but last time I re-built the same circuit it didn't work.

Again with apologies if this starts a little OT, but to answer Tim - I built the same Denshi Board crystal radio circuit in the early 70s with the kit purchased from Exchange & Mart. It was so much smaller than I expected when it arrived - the base was no bigger than about 6”x4”. The crystal set circuit worked quite well as I recall, picking up several strong shortwave stations.

The diode (along with every other component) couldn’t be “got at” for fiddling with as the whole point of the set seemed to be that it was modular - each component was encased in an individual transparent plastic block with metal prongs emerging from the bottom and a circuit symbol printed on the top. The whole ferrite and tuning capacitor arrangement was the exception as they were pre-wired into the same larger block. I suspected a better diode and a big air-spaced tuning capacitor would have helped.

My Phillips EE1003 kit included a physically bigger OA85 germanium diode but didn’t have a crystal radio circuit amongst its experiments, perhaps because no earphone was included - the set used 9v power and a loudspeaker in its radio projects.

[Philips210]

Hi.

If a crystal earpiece is used as opposed to high Z phones (2 to 4k) then resistor of about 47k will be needed across the earpiece in order that it will work. It will provide a discharge path for the detector diode.

Regards,
Symon

[chompy1]

would 2 germanium diodes connected in parallel work?

[Ed_Dinning]

Hi Chompy, no real point as the rating of one would be sufficient for the currents seen.
However, there are circuits that use FW rectification of the signal to get a higher output. Circuit looks a bit like a FW HT rectifier

Ed

[chompy1]

okay, thanks Ed

[G0HZU_JMR]

The first crystal set I built was from the classic little Babani book called "Solid State Shortwave Receivers for Beginners" by R A Penfold. I still have my copy of this book somewhere.

There's a crude scan of it here on Worldradiohistory.com and I hope it's OK to link to it.

https://worldradiohistory.com/UK/Ber...-Beginners.pdf

In my youth I built quite a few of the receivers in that book. Sadly, the Denco coils used in all the circuits are long since obsolete so I'm not sure how useful the circuits are now to a beginner. However, I'd still recommend the book as a good introduction for a beginner as there is lots of useful background info in it about the LW, MW and SW bands and about antennas and propagation .

The crystal set circuit in the book is very basic and it gave reasonable performance with a good antenna and earth. However, I had better results with a handmade coil that I matched to the antenna and the diode detector. I managed to get enough output to drive a tiny bedside hi Z speaker so I could listen to the largest MW stations with the speaker near my ear. It was a bit tinny sounding though and I had much more fun with the other receiver circuits in the book as these ran from a battery and had much better performance.

[Ian - G4JQT]

For a crystal radio to work you need a good antenna (or a nearby radio transmitter!) a good ground and some high sensitivity high impedance headphones.

Germanium diodes can vary quite a lot so hopefully you have a few that you can select from. Modern Schottky diodes will also work as they have a similar forward voltage drop to germanium diodes.

The link to the Boys Book of Crystal Sets has as good selection as any. I made circuit no 7 in 1972 and it was better than any I have made since!

[Sideband]

The first crystal set I built at 11 years old was basically built around a 500pF tuning gang. I remember stripping down an old mains transformer and using some of the primary wire, I think about 100 turns wound around a cardboard tube from a toilet roll. I remember some taps along the coil as well but the whole thing was just hung around the tuning capacitor. I used a crystal earpiece and amazingly it worked. The aerial went right down the garden and the earth was a water pipe. I could only get two stations, the Light Program and the Home Service (as they were then) but to me it was the best thing since sliced bread!

A few months later I built a two transistor reflex receiver with a ferrite rod aerial and then I was hooked since with that I could receive Luxembourg and two pirate stations, Caroline and Radio London......