Help -My Crystal Set Won't Work! PDF Print E-mail
Written by Bryce Ringwood   
Tuesday, 05 November 2013 11:37

My first crystal set didn't work either. At least, not at first. That must have been many years ago in the 1950's. Nowadays, there are many more reasons why a set won't operate than there were then – as we shall see. (More Illustrations to follow...please be patient!)

In South Africa, the majority of crystal sets will be made for medium-wave reception. There will be a few made for shortwave maybe a few more for FM bands and, although it is possible, none for “True FM” reception. I'm going to concentrate on medium-wave crystal sets here. Note that some areas of South Africa do not have medium wave transmitters near by. This means that you will only get reception in the evening from stations some considerable distance away.  The aerial and earth have to be well executed. 

Starting Out

Beginning at the very beginning – please be sure to use a circuit that is known to work. The circuit in the projects section of this web-site is a time-honoured working circuit, for example. You will find circuits on the Internet, such as the “Mystery Crystal Set” (shown below) and a reprint of the “Boy's Book of Crystal Sets”. You will also find bad circuits in popular books and magazines (and on this site in the previous chapter). It might be an idea to check out the design of the coils using the calculators on these web-pages.Mystery Crystal Set

The next step is to check out your components. If you are using a 1N34A or 1N60 diode, or a diode from the “Junk Box” - be sure to check that it works using a digital multimeter. Why digital ? - because they have a diode checker. Even the ubiquitous el cheapo DT830B multimeter has a diode checker. It should display approximately 300 in one direction and overload in the other. This corresponds to the "diode drop" in millivolts. If  you are using a crystal and cat's whisker – see later. The tuning capacitor should not have any short-circuits – conversely, the tuning coil should not have any breaks in continuity.

Having made these simple checks – we can proceed to build the set.

Construction

Either you are soldering – or you are not. Either way, the connections have to be sound – perhaps better than a set with some amplification because it is the signal itself that must power the headphones. Check that all connecting wires are scrupulously clean before making the connection. Enamelled wire must have the insulating enamel scraped off. Be sure to use fine emery paper on the coil where the slider is to make contact. Use a solvent cleaner on the diode leads (You can get “proper” solvent cleaners from RS Electronics). If you are using a cat's whisker and galena – dunk them in clean solvent and let it evaporate before using. If using “junk box” components make sure the leads are clean.

Now the set can be wired using suitable woodscrews or connectors.

If you are soldering a crystal set, you need to be careful not to make “dry joints”. Hopefully, none of the parts to be soldered are aluminium. Aluminium requires special solder, but worse is to follow, because it conducts heat away from the joint like crazy. So leave soldering to aluminium until you have mastered soldering to tinned copper, brass, etc. The other thing to avoid at this stage is the use of “RoHS” or lead-free solder. It requires higher temperatures and tends to corrode soldering iron bits quite quickly. If you have a choice, I recommend multicore “savbit” alloy for soldering. This has a small amount of copper in the solder, which makes your bits last longer. It also has 5 cores of resin flux, which is very important. Your soldering iron should have a stand and a wet sponge on which you can wipe the tip. Before use for the first time, the bit should be “tinned” by placing a small piece of solder against the bit. If the bit is the correct temperature, the solder will flow on to it and amalgamate with the surface. You will see the flux from the solder bubble and vapourise, giving off a small amount of smoke. Now the tip can be wiped clean on the wet sponge. The tip should have a silvery plated surface. 

To begin with - you don't have to buy a fancy soldering iron. I would avoid a gas operated one - because the flame can inadvertently melt or damage other components. You probably are best off with an inexpensive 25 - 50 Watt electric iron. Be sure you purchase/make a stand for it because scorching the kitchen table can cause family rows. On the other hand, if you intend working with printed circuit boards, it might be as well to get a temperature controlled iron from the word go. If you decide to buy a very expensive Weller, remember you are buying quality - I still use the one I bought 45 years ago. Set the soldering temperature to 320 - 350 degrees C for normal work.  The normal weller TCP tip should have a '7' on it. '8' is for lead free solder. "Hobby" soldering irons - even if they are Weller will be very hot indeed, and may burnthe soldering iron flux. 

The joints should be well-connected prior to soldering by twisting the wires together. If soldering to tags, the wires need only be pushed through, rather than the old style of wrapping round the tag. This may sound like heresy, but, modern solders are really very good and you are not making a military project. Just ensure the wires in the joint can't move. Next, heat the joint with the soldering iron and apply the solder wire. It should melt and flow into the joint, effectively wetting the joint surfaces. Remove the heat and the resulting joint should be shiny with some solidified flux round the edges. The soldering operation should not take more than 2 seconds at the very most!. If the joint isn't shiny (has a grainy appearance) you probably moved it while the solder was solidifying. If the solder is in a big blob perched on top of the joint – you have mucked it up (probably by not cleaning the joint first). These are dry joints – you will have to practice more, and start again. Sometimes those dry joints are hard to recognise. The connection looks good, but the wire pulls away easily. Practice practice practice...

Also, remember to remove fux residues after soldering. This is very important if you use printed circuits, as many fluxes go horrible and corrode the copper. At best they are sticky and attract dust and dirt. You will also see whther your joint was just held together with flux. R300 thinners will remove these residues, if you don't want to buy the correct chemicals.

"I build it by the book and it still doesn't work"

Crystal set radios require a very good antenna and an equally good ground. A really good ground would be a water pipe – but like everything else the connection has to be good. This means using emery paper to make the pipe shiny and then using a jubilee clip to clamp a piece of copper earth wire firmly to the shiny piece. You could also use a piece of lighting conductor rod (available from the hardware store), or bury a zinc bucket in the ground with the earth wire connected and water it every day. These approaches might lead the neighbours to think you are odd. The mains earth lead is not too good, although you may be lucky. It will be noisy, for sure. 

The antenna has to be a reasonable long wire. (You don't have to have a few hectares and erect a rhombic). I use 20 metres of wire between my VHF antenna mast and my chimney. This is suspended between two perspex insulators. The down-lead goes through my window to the crystal set antenna connector.

This, of course is all well and good if you live on your own property not subject to “rules” of a management committee. Worse still, you may be living in a high-rise apartment penthouse, with no opportunity to erect an antenna. Medium-wave signals do not penetrate reinforced concrete – so what now? There are a number of designs using “frame aerials”, ferrite rod and so on. They will not work inside a concrete-and-steel building, but should work in the open in reasonable proximity to a transmitter. I struggle to get a superhet to work in such circumstances. I think the only way is to get away from the building and make some sort of portable antenna. In Victorian homes, the plan was to wrap an antenna round the picture-rail...but they weren't made of concrete-and-steel.

Detector Problems

First and foremost, use a Germanium diode detector. You can use a 1N34A, a 1N60, OA79 or a modern-ish NTE109 from Mantech. Try one of these time-honoured diodes before anything else! If this works, you can substitute your own cat's whisker detector, silicon diode, LED, power diode or whatever (possibly) foolish (certainly time-consuming) experiment you are conducting. If the set still doesn't work – you need to examine your headphones.

Solid state diodes are made from two semiconductor materials in close proximity. Semiconductors, such as silicon or germanium can be manufactured as p-type (excess of “holes”) or n-type (excess of electrons). A “hole” is not so much a physical particle as a place-holder for an electron. In a Schottky diode, metal serves the purpose of the p-type material. In a way, this forms a “mini-battery” with a potential of about 0.6 volts for silicon and 0.3 volts for germanium. Schottky diodes can have less than this. Assuming you connect a battery and resistor across the diode with the positive terminal connected to the anode and the negative to the cathode via a resistor, then current will flow in profusion provided this small voltage is exceeded. If you do the experiment with a LED, the LED will suddenly illuminate once this built in voltage or "diode drop"  is exceeded. With LEDs, it is quite high, varying from 1.5 volts or so for red LEDs to well over two volts for blue.

The radio signals at the crystal set antenna are quite small- not enough to overcome the diode drop of a silicon diode and probably not enough for a germanium diode either. If you plot a graph of forward voltage against current for a germanium diode, you will see they begin to conduct almost immediately – which is fortunate, because it means we can use a germanium diode without any additional batteries to place it in a conducting situation.

Silicon diodes and LEDs can be made to work-but then you will have to bias them with a battery nearly equal to the diode drop of .6 for silicon, or 1.5 volts for an LED. Be careful not to send that voltage through your headphones or earpiece – especially if your ears are attached!!! If you are close to a station and have biased your LED, you can have the added entertainment of the LED lighting up when you are on the station, or so I'm told.

Cat's whisker detectors are a metal cat's whisker and a semiconductor spot on a piece of galena or other crystal. Some materials, such as carborundum have semiconducting spots, but need a bit of bias to make them work. Cat's whisker detectors are examples of Schottky diodes. .Ivalek Cat's Whisker Kit - 1950s

I have attempted to use commercial Schottky diodes in a crystal set, but so far have only got very distorted sound. The exception being a VHF crystal set using special Schottky diodes HSMS-285B from Avago technologies. They are surface mount devices.

If you are making your own cat's whisker detector, you will need a suitable crystal about the size of a pea. This can be clamped inside a bronze cup and held in place with three grub screws, as in the picture below. The cat's whisker is traditionally a phosphor-bronze spring mounted in such a way that it can form a contact with the crystal surface. It should have a sharp point. The crystal and cat's whisker need to be scrupulously clean. There is a bit of fiddling about involved before you hit a sensitive spot on the crystal. It probably best to cheat and tune the set in first using a germanium diode. You should find a good spot after a few minutes.

Finally, there's nothing to stop you using a valve, such as half of an EB91 as the detector. Of course, its no longer a crystal set, and you might as well go the whole hog and use the control grid as well. Whether its still a crystal set if you use a FET or transistor is moot.

In my youth, you got germanium diodes – and that was it. Transistors were yet to be commercialised.

Summing up this section – use a 1N34A diode.

 

Headphones

By decree, you are not allowed to use an amplifier,otherwise you will be banished from crystal set societies the world over.

Headphones need to be high-resistance to avoid loading the tuning coil and they need to be sensitive. The “Ivalek” headphones sold to youngsters in the '50s were high-resistance, but were not sensitive. I got them to work, but the results were disappointing. Headphones for crystal sets should have a 2000 Ohms resistance or better.

Stereo headphones are low resistance and low sensitivity. They will not work with crystal sets.

High resistance headphones are difficult to find. The nearest thing you will find are the innards of an old telephone. You need to dismantle the handset and remove the earpiece. Wire it to the crystal set and you will be surprised at how good the reception is. I think the microphone is the same device.

High-resistance earpieces may be available from some hobby suppliers. The SG Brown headphones illustrated were purchased from a UK antique shop.

I was surprised that some WW2 military headphones (DLR No 5) with a 600 Ohm resistance worked really well. Working them through a small output transformer gave the best results of all.

If all else really does fail, then use an amplifier. You can use the microphone input to your PC sound card.

Finally

The reason my first attempt failed was simply bad solder joints. The first thing I heard was a soprano on BBC third program. Let me know the memories of the first thing you heard on your first home-made radio - its always better to share.


 

Last Updated on Thursday, 19 December 2013 15:56
 
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