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TRF RADIOS
Part 1
The  ZN414 / MK484 I.C.
& The Matchbox Radio
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BUILD YOUR OWN MINIATURE 'MATCHBOX' RADIO


Worried About Soldering?  Don't Be:

For some simple ideas on solderless construction techniques have a look at the Crystal Sets 2  and the Crystal Sets 5 pages.  While the ultimate miniaturisation cannot be achieved with solderless techniques, it is still possible to produce a working project in this way.  When constructing these small electronic projects it will be necessary to determine the exact value of resistors, which are colour coded, and capacitors, which sometimes have confusing numbers on them.  I have included a table for both Resistor Colour Codes and a Capacitor Conversion Table HERE.



TRF RADIOS (Part 1)

INTRODUCTION
In the very early days of wireless a TRF (Tuned Radio Frequency) radio was the next step up from a crystal set.   It offered amplification of both the radio frequency and the audio frequencies so that more stations could be received more strongly and the sounds produced would be amplified sufficiently to power a loudspeaker.

From the 1920' to the 1940's it was the glass thermionic valve, the forerunner to the transistor, that was the only component available for such amplification.  A valve is quite a large device, about the size of an eggcup and looking a little like a small lightbulb.   The valve needs its internal working parts , the anode and cathode, to be heated to operate (hence the term THERMionic) and so a valve contains filaments that glow red hot.  Valve radios would consequently get quite warm and therefore use quite a lot of power requiring several large batteries or mains power to operate.  The valve filaments needing just a few volts to get hot, while the voltage required to obtain a flow of electrons in the anode would need 100 volts or more. 

Later radios dispensed with the TRF technology in favour of the more sensitive and selective 'superhet' (supersonic heterodyne) method.   If the set is in good working order the wireless listener will no doubt be rewarded with a wonderfully rich and warm sound quality.  Many of these original valve sets had beautifully hand-crafted wooden cabinets which would often enclose a large loudspeaker that produced the fine sound quality. 

Old mains valve radios can be very dangerous to dismantle and 'play around' with as they invariable have a live metal chassis.  As this chassis will be at 230 volts mains potential the effect of a mis-positioned digit could be fatal, so I advise against this practice.  Instead have a go at building a modern day TRF radio using the latest solid state (i.e. silicon transistors) technology, which operates at low power and with very low current consumption.  It was not until the introduction of the transistor in the mid 1950's that radio sets could be made smaller and truly portable and consume much less power, making battery operation a practicality.

Building a TRF set today is quite straightforward as small transistors or IC's (Integrated Circuits containing several transistors and other components in a small sealed device) will require only battery power in the order of 1.5 to 6 or 9 volts to operate. 

As mentioned a TRF radio has its limitations and was superceded by the Superhet, which is a principle used in all modern receivers, but the results achieved with modern components can be outstanding. 

The TRF radios described here all use transistorized circuits, though some of the the circuit layouts are fairly similar to some early simple valve TRF radios.


BUILD A RADIO IN A MATCHBOX !


The rather battered magazine cover of  Everyday Electronics
http://www.epemag3.com/
 
THE MATCHBOX RECEIVER

When my dad came home one night in the 1970's with a copy of Everyday Electronics magazine I was fascinated by an article describing how to home-build a radio so small that it would fit into a matchbox.  I was a schoolboy taken with the hobby of radio and had recently built a regenerative radio described in the Ladybird book "Making A Transistor Radio".  This book was given to me as a birthday present from an aunt and the method of construction used was a simple solderless breadboard that trapped the component leads with number 6 brass wood screws and screw cups.

The Matchbox Receiver, on the other hand, required careful soldering of the components into a circuit board to achieve the small size needed to fit into a case as small as a matchbox.  I did not have a soldering iron, I was too young.

Dad did have a soldering iron, it was not electric though and had to be heated over a gas flame!  We spent several hours in the kitchen heating this big iron over the gas hob and soldering the tiny components onto the circuit board.

The matchbox receiver uses just two main components, one that I was familiar with - a transistor (the BC107), and a component new to me at the time, an Integrated Circuit (I.C.) containing no less than ten transistors inside a tiny TO-18 style can.  The I.C. used is the Ferranti ZN414 which contains a high gain RF amplifier stage offering up to 72dB of gain, a detector stage and a.g.c. stage.

The ZN414 has a very high input impedance of around 4meg Ohms which minimizes any loading of the tuned circuit thereby improving selectivity, which is important when being used in a simple TRF radio such as is being described.  The useable tuning range available from the ZN414 is from 150kHz (Long Wave) through Medium Wave and up to 3000kHz.  Current consumption is tiny, making it very suitable for the matchbox radio powered by a silver-oxide button cell.

Once dad and I had soldered everything together and assembled the parts into a matchbox I think that we were both amazed that it worked!  We received BBC Radio Four on 285 meters, Radio Three on 464 meters, Radio One on 247 meters, Radio Birmingham on 206 meters and BRMB Radio on 261 meters. 

Matchbox Radio
A Matchbox Radio
(This particular one uses the ZN415 IC - see the notes further down the page)


In 1975 the idea of being able to build a complete working radio that would fit inside an ordinary matchbox seemed absolutely amazing.  Today it is run of the mill of course, but the article from Everyday Electronics of Sept 1975 is still really interesting. 

As mentioned, it uses the Ferranti ZN414 integrated circuit which is no longer available, but the direct replacement MK484 can be used with confidence and is widely available from many outlets including Bowood Electronics , as are a ferrite rods, fixed capacitors, resistors, preset pots and 500pF trimmers. You may also have some similar and useful components in your junk box.  The circuit could still be made up using a miniature tuning capacitor salvaged from a discarded Chinese pocket radio.  See the alternative circuits too.

Another equivalent to the ZN414 and MK484 is, I am informed, the TA7642 which is available from Rapid Electronics and on ebay from Bob's TV & Radio here:
http://www.ebay.co.uk (TA7642)

I have now built a couple of these radios and another one using the ZN415 I.C. which includes an additional buffer stage of audio amplification.

So have a look at the article a little further down the page, I'm sure that you'll find it fascinating.

DID YOU KNOW?

Electronic component sizes have been effectively reduced by half every 18 months!
The same progress in design and manufacture enables computer processor (CPU) speeds to be doubled every 18 months.

This effect of miniaturization is quite noticeable even when working with the very ordinary components involved in the construction of this matchbox radio project, i.e. components with leads intended for use on an ordinary circuit board. 

When I first built this radio in the late 1970's the resistors and capacitors were two or three times the size of the ones I used when re-building the project with current components.  This helps greatly with construction inside a matchbox.

However this process of miniaturization has progressed beyond the 'ordinary' components that are used in everyday 'home-brew' projects, since there is a limit to how small such a component can be and remain usable on standard circuit boards.  Today's powerful personal computers, laptops, mobile 'phones and digital cameras are all made possible by the use of very large scale integrated circuits and miniaturized 'surface mount' components such as resistors and capacitors that are the size of a pin head!  These are rather more difficult to work with for the home constructor.



Photograph showing how components have been reduced in size as time has progressed
The top two are 0.01µF ceramic disc capacitors, the left one from the 1970's and the right one from 2003
The bottom two are resistors, the left one a 0.5 watt from the 1970's and the right one from 2003



BUILD A MATCHBOX RECEIVER !

THE EVERYDAY ELECTRONICS 'MATCHBOX RECEIVER' ARTICLE
.










Notes About Components On The Above Article:
The ZN414 I.C. can be replaced with the current MK484 chip or the TA7642.
The BC107 transistor can be replaced with a BC547, a BC182 or a ZTX300 (and other general purpose devices may work equally well).
I found that the 680K biasing resistor could be replaced with a 470K or even a 100K without detriment.
I used a 1.55 Volt Silver Oxide battery and found that the 1K preset potentiometer must be replaced with 10k preset pot to give an adequate range of control over gain.
When using a Crystal Earphone a resistor of between 4.7k and 10k needs to be included from the collector of the BC107 to the positive rail (after the switched jack socket) i.e. across the earphone terminals.  This will not effect the use of 1000 ohm earphones / earpieces.

Also, have a look at the further information below for some more tips.

SO GO AHEAD... GET BUILDING!



The Pin-Out Arrangement of the ZN414 and
MK484 Integrated Circuits. The TA7642 is
apparently the same as the MK484.

Try to avoid overheating the IC when soldering and keep the wiring of the whole circuit as neat as is possible to avoid unwanted oscillations that could occur with untidy wiring.

PARTS LIST
1 :  MK484 (or TA7642 or ZN414) Integrated Circuit
1 : BC107 or BC547 or ZTX300 Transistor
1 :  500pF Miniature Postage Stamp Trimmer Capacitor

1 :  Crystal (Ceramic) Earphone
1 : 0.01 µF Ceramic (or similar) Capacitor (103)
1 : 0.1 µF Ceramic (or similar) Capacitor (104)
1 : 0.047uF (0.05µF) Ceramic (or similar) Capacitor (473)
  2: 100k Ohm ¼ watt Resistor
1 : 470 Ohm ¼ watt Resistor
1 : 4.7k Ohm ¼ watt Resistor (optional)
1 : 10k Miniature Preset Potentiometer
1 : 10mm Dia Ferrite Rod 100 or 150 mm long
1 : Reel of 0.5mm (approx) Enamelled Copper Wire
1 : 3.5 mm Jack Socket (for earphone)
1 : AA Battery Holder

1 : 1.5 Volt AA Battery
1 : On/Off Switch (optional)
1 : Tagboard or Verostrip board

BOWOOD ELECTRONICS
is a useful source for many of these components

You May Also Try:
J Birkett, The Strait, Lincoln for surplus items such as capacitors


Here are links to more component suppliers >>


AERIAL COIL DETAILS

MEDIUM WAVE: 60 to 65 Turns of 0.5 mm dia enamelled copper wire on a 10mm dia Ferrite Rod of about 35 to 40 mm long.

LONG WAVE: As above but with 250 turns of wire.



MY REBUILT MATCHBOX RECEIVER

My original matchbox receiver had got rather battered and worn, so being as I needed to solder a new battery into place I took the opportunity to refurbish the radio.  I used the original Ferranti ZN414 i.c. , BC107 transistor and 500pF trimmer capacitor used for tuning.  The resistors and capacitors I replaced with new smaller ones and I also replaced the circuit board and the 3.5mm jack socket with a new one as the original looked rather corroded.   A 4.7k resistor was also included across the earphone output as I use these radios with a crystal earpiece, as mentioned above.  I also replaced the medium wave coil with a Long Wave coil (purchased from Maplin Electronics) so that the radio could tune into 198 kHz for BBC Radio Four and also 252 kHz for RTÉ Radio One. 

(I built another matchbox radio, using the newer MK484 i.c, and this covers Medium Wave. See further down this page. A TA7642 could also be used).



The photo above is a little trip through England's Glory matchbox history.  The box top left is from 2004, and the box top right is probably from the 1990's while the box at the bottom is probably from the early 1980's.  Incidentally England's Glory matches are now made in Sweden - so shouldn't they be called Sweden's Glory?

SCRAPE OFF THE VERO-STRIPS UNDERNEATH THE FERRITE-ROD AERIAL!



The photograph above shows the underside of the VeroStrip board that I used to build the matchbox radio.  Note how three rows of copper strips have been removed using a sharp blade.  The ferrite rod aerial sits above this area and leaving the copper in place reduces the "Q" (the effectiveness) of the tuned circuit very markedly. 

Leaving the strips in place would result in poorer selectivity and sensitivity of the radio and thereby rather poor reception.  I strongly recommend removing any strips from immediately below the ferrite rod aerial when constructing such a radio.  Also keep the aerial away from any other metal objects that will mar the reception.



Above photo is a close up look at the re-built ZN414 matchbox radio
with the new Long Wave tuning coil.


DON'T DESPAIR

Don't despair if you cannot find some of the components required for the Matchbox Receiver.  The resistors will be no problem, of course, and can be 1/4 Watt, 1/2 Watt or 0.6 Watt at any tolerance.  The capacitors can be Ceramic Disc, Resin Dipped Ceramic or Polyester and the values stated should not be varied for best performance.

As mentioned elsewhere the ZN414 is no longer available but the MK484 i.c. is widely and there should be no problem obtaining one of these. The TA7642 is another equivalent to the ZN414.

There will be no difficulty in finding a small piece of 10mm diameter ferrite rod and some 0.5mm dia, enamelled copper wire for the aerial.

There may be a little difficulty in finding the 500pF postage-stamp trimmer, and the 6BA brass screw and nut to extend the tuning shaft, but some careful searching should be rewarded with the necessary items.

Alternatives To The 500pF Trimmer For Tuning
Since components are so small these days, there will be an opportunity to use a miniature 200pF (approx) poly-dialectric (polyvaricon) tuning capacitor of the type that may be salvaged from a pocket transistor radio and that is generally more widely available from component suppliers.  This would take up more space in the matchbox and the tuning knob would protrude from the back face of the box rather from one end.  With careful re-arrangement of the components and some neat soldering there is a very good chance that everything could be still be accommodated in a matchbox, though I have not tried this yet.

Another alternative would be to have the matchbox receiver permanently tuned to a favourite station. The tuning circuit could then consist of a fixed capacitor of perhaps 50pF or 100pF or even a miniature trimming capacitor of similar value.  The required favourite station could then be set by experimenting with the number of turns on the coil, and 'fine-tuned' with the small value trimmer, if that is what is used.  This is just an idea but then the receiver would easily fit inside a matchbox.

The 3.5 jack socket has to be the open type, i.e. not enclosed in a plastic case, so that the switching operation can be changed from opening when the earphone plug is inserted, to closing when the plug is inserted.  A pair of pointed nose pliers is all that is required to bend the switching contacts into the required position.


ANOTHER OF MY MATCHBOX RADIOS
This One Using The MK484 IC



I have recently built another matchbox receiver, which is shown above and uses the MK484, a replacement for the ZN414.  This radio uses a ZTX300 transistor as the audio output device which, in this case, has a 100K biasing resistor, rather than the 680K resistor specified in the Everyday Electronics magazine article above.   In fact a BC107, a ZTX300 or a BC547 transistor can be used in this circuit, with a 100k biasing resistor all with equally good results.

I strongly recommend using a crystal earphone rather than the high impedance magnetic earphone referred to in the article since such earphones are almost impossible to come by.  My own Matchbox radio will drive a pair of 32 Ohm 'Walkman' type headphones to reasonable volume when the two earpieces are wired in SERIES to provide a 64 Ohm load, but a crystal earphone is much MUCH louder which means that more of the weaker stations can be heard.


I therefore recommend the use a crystal earphone, however a 4.7K Ohm resistor (up to about 10K works) needs to be soldered across the earphone output; i.e. between the collector of TR1 (the BC107, BC547 or ZTX300) and the positive (+ve) rail - before the preset potentiometer and after the on/off switch (or switched jack socket if that method of switching is employed, as it is in my radio).

I would also recommend the use of a 10k preset potentiometer to set the gain, as my radio was far too loud and rather distorted when using a new 1.55 volt silver oxide button cell and even a 4.7K preset potentiometer could not introduce enough resistance into the circuit to sufficiently reduce the gain.  The 10K pot works very well in my set.

As with all these designs don't forget to scrape off the copper strips from the Veroboard below the area where the ferrite rod aerial will be mounted.  Failure to remove these strips will not prevent the radio from working, but the performance of the tuned circuit will be marred.
 


Photograph showing a close-up of the inside of my Matchbox Radio
The tiny silver oxide battery is covered in black insulating tape.
The skeletal jack socket has been modified so that the contacts make a circuit when inserting
 the earphone jack plug rather than breaking the circuit as is usually the case.
 This then automatically switches the radio on when the earphone is plugged in.


SOLAR POWERED MK484 MATCHBOX RADIO !
Solar Power !
Solar Power !
Hi Mike,

I was very pleased to read about the ZN414 Matchbox radio and its variants (MK484 & TDA7642) as I have experimented with it before. Readers might be interested in the fact that this little radio can even be driven using an old solar panel salvaged from a pocket calculator! These panels usually provide up to 2 Volts being exposed to bright daylight and a current of 1mA or more.

If you are worried about voltage overload simply add a 1.5 V zener diode or a rechargeable button cell in parallel which will cut down Voltage, thus protecting your precious ZN414 and avoiding self oscillating.

With best regards from Germany, Dietmar DH7AMQ   [April 2013]

Thanks Dietmar - What an excellent idea!


SOME ALTERNATIVE MK484 (ZN414) (TA7642) CIRCUITS
(Which Also Work Really Well!)

Even simpler than the 'Matchbox Radio' described in the above 1975 Everyday Electronics article would be to use the circuit below which omits the amplification (BC107 transistor) stage.  The ZN414, TA7642 or MK484 is still able to drive a simple Crystal earpiece to reasonable volume, though obviously not as loud as with the additional transistor included.  This approach would make it even easier to assemble the receiver into a matchbox.  I have also constructed this set (though not inside a matchbox) and it works brilliantly with a simple crystal earpiece.  I decided to use a 4 inch (10cm) long ferrite rod, which produces better signal pick-up so that stations will be more clearly heard. 

The aerial coil consists of 60 turns of 0.56mm diameter enamelled copper wire which gives good coverage of the Medium Wave band even when using a small 220 pF tuning capacitor.  Battery power is again only 1.5 volts, this time I used a penlight AA cell which will last for a very very long time indeed.  The 10k Ohm preset pot sets the internal gain of the MK484 integrated circuit and while not critical in many cases careful adjustment is needed in strong signal areas to help prevent overloading.

Keep construction of the circuit very neat with component leads as short as is practicable.  Ideally the radio should be built on a small piece of VeroStrip, but if you wish to experiment with components and values then an ordinary piece of tag-board will be quite suitable.  A 6 x 6 tag board will be more than adequate for this circuit and I have built these sets in this way very successfully, but bear in mind to keep the output components and battery away from the coil and tuning capacitor.

Other component values should be adhered to to obtain best results, the 0.1uF capacitor at Pin 1 (output) of the MK484, the 100k Ohm resistor and the 470 Ohm resistor should not really be changed.  The 0.01µF capacitor can be experimented with and could be between 0.01 and 0.0068 µF.  The 0.05µF output de-coupling capacitor is not too critical and the resistor across the crystal earpiece could be 2.7K Ohm 4.7K,  6.8K perhaps up to 10K depending on the particular earphone used.  I found that a 2.7K resistor allows my crystal earphone to work very effectively indeed.  The battery voltage should not be changed and should be between 1.4 and 1.6 volts.  Either an original Ferranti ZN414 IC or the later MK484 IC can be used in these circuits,  the newer MK484 may work even better than the excellent, but now obsolete, ZN414.



The Pin-Out Arrangement of the ZN414 and
MK484 (TA7642) Integrated Circuits.

Try to avoid overheating the IC when soldering and keep the wiring of the whole circuit as neat as is possible to avoid unwanted oscillations that could occur with untidy wiring.

AERIAL COIL DETAILS

MEDIUM WAVE: 60 Turns of 0.5 mm dia enamelled copper wire on a 10mm dia Ferrite Rod of between 100 and 150 mm long

LONG WAVE: As above but with 250 turns of wire with the coil ideally being shunted with a 220k Ohm resistor

Maplin Electronics have been selling a 10mm diameter ferrite rod with a pre-wound medium wave and long wave coil included and this may provide another option. Ferrite rods are also available from Bowood Electronics.

Miniature MK484 radio

+ See note below

PARTS LIST

1 :  MK484 (or TA7642 or ZN414) Integrated Circuit
1 :  220pF or 500pF Tuning Capacitor
1 :  Crystal (Ceramic) Earphone
1 : 0.01 µF Ceramic (or similar) Capacitor (103)
1 : 0.1 µF Ceramic (or similar) Capacitor (104)
1 : 0.047uF (0.05µF) Ceramic (or similar) Capacitor (473)
1 : 100k Ohm ¼ watt Resistor
1 : 470 Ohm ¼ watt Resitor
1 : 2.7k (or 4.7k) Ohm ¼ watt Resistor
1 : 10k Preset Potentiometer
1 : 10mm Dia Ferrite Rod 100 or 150 mm long
1 : Reel of 0.5mm (approx) Enamelled Copper Wire
1 : 3.5 mm Jack Socket (for earphone)
1 : AA Battery Holder

1 : 1.5 Volt AA Battery
1 : On/Off Switch (optional)
1 : Tagboard or Verostrip board

BOWOOD ELECTRONICS
is a useful source for many of these components

You May Also Try:
J Birkett, The Strait, Lincoln for surplus items such as capacitors


Here are links to more component suppliers >>

+ Our correspondent Chas Castagana, in the USA, had some trouble with this circuit he writes: I constructed this circuit as close to your spec's as I could, ...perhaps it may be better to feed the output into an external transistor amplifier stage. 
I think Chas may have been using headphones which possibly may not work as well as a Crystal Earphone that I had intended.
This design can certainly be fed into a discrete transistor amplifier stage and will give excellent results in this way, but even connecting the crystal earphone between the output and ground (as shown) I find that there is a more than usable audio output on all local stations.  It is worth including the circuit here due to its simplicity and good performance.  it may also be worth experimenting with connecting the crystal earphone between the output, via 0.05 uF capacitor, and the positive rail (i.e. to the positive side of the battery) and connecting the 47k Ohm resistor across the earphone output.  Either way I don't think constructors will be disappointed.  Thanks Chas for the input!



The Working Single Chip MK484 Radio Constructed On A Piece Of VeroStrip



At our location I can receive the three national stations, BBC Radio Five Live, Talk Sport and Absolute (Virgin) Radio from a main transmitter about 25-30 miles away plus BBC Five Live from a main transmitter about 80 miles away on 909 kHz.  Two low power local stations (990 kHz 0.1 kW and 828 kHz 0.2 kW) are also received at good strength from their transmitters located 6 miles away. Before it closed, a community station on 1350 kHz at only 0.001kW  located about 4 miles away could also be received. Additionally three other local radio transmitters (3 to 6 kW) about 15-20 miles away were also receivable.  At night a number of other broadcasts can be heard easily, e.g. 1440 from Luxembourg and 1512 kHz from Belgium and some others such as 567 kHz from Eire and 675 kHz from The Netherlands.

ADDITIONAL AUDIO AMPLIFICATION USING THE BC548 TRANSISTOR

The MK484 receiver shown below uses a BC548 transistor as the audio output stage.  It also uses a larger ferrite rod aerial for better signal pick-up, a standard "AA" battery cell and a widely available miniature polyvaricon tuning capacitor of approximately 200pF for ease of construction.  This really is a superb radio!










Above the completed "Cook's Matchbox Radio"
Housed in a larger matchbox to accommodate a longer ferrite rod aerial for improved pick-up
and a standard "AA" battery cell together with the more orthodox polyvaricon type
tuning capacitor.  The larger housing also makes construction a little easier.
See the circuit diagram for this radio below:


MK484 circuit diagram

Above: Circuit Diagram For The Excellent MK484 (or TA7642 or ZN414) with the BC548 transistor stage of amplification.


The tuning capacitor can be any standard type of between about 200pF and 500pF and the polyvaricon

type commonly found in pocket radios and should be available new from many component suppliers.
A crystal earpiece should be used, although excellent results may be obtained with a pair of good quality and
sensitive 32 Ohm 'Walkman' type headphones.  If these earphones are to be used, the 32 Ohm earpieces must
be wired in SERIES so that the total load is 64 Ohms.  This can be arranged by using a stereo jack socket and
connecting the output across the first two (small) rings of the headphone plug and making no connection to the
upper (longer) part of the plug which is the common/ground connection of the 'phones.



THE ZN415E and ZN416E

Have a look at the circuit below which uses the ZN415 integrated circuit, and is certainly worth using if you happen to have one in your 'junk box'.  The ZN415 includes an additional buffer stage which increases the output from the 30 to 60 millivolts produced by the ZN414E up to about 100 to 120 millivolts, enough to directly drive a pair of Walkman type headphones.  The two 32 ohm earpieces must be arranged so that they are wired in series to give the necessary 64 ohm load. The ZN415 makes assembly even easier.

The ZN416E is similar to the ZN415E except that the output is raised still further to about 300 to 330 millivolts.

ZN415 circuit

Circuit digram of the Ferranti ZN415 single chip radio.  This circuit can also be used for the later higher output ZN416 and ZN416E integrated circuits - if you can find one.
ZN415 Matchbox radio
The ZN415 / ZN416 / ZN416E is, like the ZN414, also discontinued by Ferranti, but you may be able to find one from somewhere, there may even be a replacement IC, but I have not come across one.


READER'S MK484 RADIOS:

I've read your site on and off for a few years now and finally got around to building a matchbox radio. I'd made one as a child during the '70s from the PW article, with the original Ferranti ZN414 and I'd always wanted to make another one to have, particularly so that I always had a mini radio to listen to the cricket on LW.

I did cheat slightly this time, as I ordered a kit (cheapest and not many people sell all the parts these days) and it came with a PCB board, my original was down on Vero board. I will have to get some Vero board so that I feel that I have made it 'properly'. I did wind my own aerial coil though.

What I find surprising, is that all the kits come with a AA battery holder, why do you want a huge battery on a mini radio when it will happily work off a soldered on button battery. The other thing is they all come with a mini on/off switch, what happened to have the earphone socket in line with the battery, so you switched it on by plugging the phones in.

My reason for emailing you is the following. Rapid Electronics appear to be selling off their AM radio kits.They're selling them at £8.00 plus VAT (£9.60) for 5 kits. The kits don't include the PCB, which is about £3.00 (again, that's for 5 PCBs) and the tuning knob isn't included either (they say they've run out of them), but they have suitable knobs for sale that are about 25p each, it's a 6mm spindle on the tuning capacitor

I thought that it might be of interest to you or maybe the readers of your site, if you spend £20 plus VAT it's free delivery. It could be cheap radios for those that might want to put it together on a vero board and have tuning knobs lying about or rescued from old dead radios.

I've just ordered two packs (10 radio kits) with the PCBs and knobs and I'm going to start experimenting with them. I want a good working LW/MW/SW (individual radios), I'll go down the matchbox route and then afterwards see if I can buy a tidy project box that's about the same size. Then I want to see if I can have a combined MW/LW radio in a matchbox. I purchased a Maplin ready wound coil / ferrite rod for MW/LW and without the rod in the coil it picks up some MW stations, and then LW when the coil is inserted.

I have a feeling that my wife will probably disown me over the next few weeks, when I bore he rigid with how the projects are coming on.

Thanks for your informative website.

All the best, Al.
(April 2013)

 

Click HERE to see some MDS975 Reader's Radios >>

.


No AM radio stations or transmitters in your locality or country?

http://www.vcomp.co.uk AM Medium Wave Transmitter from Vintage Components

Has your local medium wave broadcast station closed or been moved to VHF/FM or Digital? Don't worry. You can still build and experiment with crystal sets and TRF radios by also buying or even building a simple low power AM transmitter. So, not only can you use your crystal sets but you can also run your own radio station that can be heard in and around your home - playing the music or programmes that you want to hear!


SSTRAN AMT3000 Superb high fidelity medium wave AM transmitter kits from SSTRAN. Versions available for 10kHz spacing in the Americas (AMT3000 or AMT3000-SM) and 9kHz spacing in Europe and other areas (AMT3000-9 and AMT3000-9SM). Superb audio quality and a great and well designed little kit to build: http://www.sstran.com/pages/products.html

SSTRAN AMT3000 low power AM medium wave transmitter
http://www.sstran.com/

Other AM transmitters available:

SpitfireComplete, high quality ready built medium wave AM Transmitters from Vintage Components:
http://www.vcomp.co.uk/index.htm  Vintage Components offer a choice of the high quality Spitfire and Metzo transmitters:

SPITFIRE AM Medium Wave Transmitter with 100 milliwatt RF output power:
Spitfire AM transmitter from Vintage Components

METZO AM Medium Wave Transmitter with built in compressor:
Metzo AM Transmitter fromVintage Components




AM88 LP  A basic AM transmitter kit from North County Radio.
http://www.northcountryradio.com/Kitpages/am88.htm



RESISTOR COLOUR CODES
AND CAPACITOR CONVERSION TABLE >>>


Having difficulty in finding components?  I have added some ideas for component sources here.
Sources For Older Components >>>


From Al:

Hi Mike,

I've read your site on and off for a few years now and finally got around to building a matchbox radio. I'd made one as a child during the '70s from the PW article, with the original Ferranti ZN414 and I'd always wanted to make another one to have, particularly so that I always had a mini radio to listen to the cricket on LW.

I did cheat slightly this time, as I ordered a kit (cheapest and not many people sell all the parts these days) and it came with a PCB board, my original was down on Vero-board. I will have to get some Vero-board so that I feel that I have made it 'properly'. I did wind my own aerial coil though.

What I find surprising, is that all the kits come with a AA battery holder, why do you want a huge battery on a mini radio when it will happily work off a soldered on button battery. The other thing is they all come with a mini on/off switch, what happened to have the earphone socket in line with the battery, so you switched it on by plugging the phones in.

Anyway, my reason for emailing you is the following. Rapid Electronics appear to be selling off their AM radio kits.They're selling them at £8.00 plus VAT (£9.60) for 5 kits. The kits don't include the PCB, which is about £3.00 (again, that's for 5 PCBs) and the tuning knob isn't included either (they say they've run out of them), but they have suitable knobs for sale that are about 25p each, it's a 6mm spindle on the tuning capacitor

I thought that it might be of interest to you or maybe the readers of your site, if you spend £20 plus VAT it's free delivery. It could be cheap radios for those that might want to put it together on a vero board and have tuning knobs lying about or rescued from old dead radios.

I've just ordered two packs (10 radio kits) with the PCBs and knobs and I'm going to start experimenting with them. I want a good working LW/MW/SW (individual radios), I'll go down the matchbox route and then afterwards see if I can buy a tidy project box that's about the same size. Then I want to see if I can have a combined MW/LW radio in a matchbox. I purchased a Maplin ready wound coil/ferrite rod for MW/LW and without the rod in the coil it picks up some MW stations, and then LW when the coil is inserted.

I have a feeling that my wife will probably disown me over the next few weeks, when I bore he rigid with how the projects are coming on.

Anyway, thanks for your informative website and all the best, Al.
April 2013



From Sean O'Connor:


Hi Mike, You might be interested in a Power MOSFET TRF circuit I have designed and built:  http://litetec.hubpages.com/hub/A-Power-MOSFET-TRF-Radio-Circuit

MOSFET TRF Receiver by Sean O'Connor
Best Regards, Sean O'Connor



From Liz Costa:


Hi, MDS975 folk. I bought a MK484 from Maplin and didn't have a circuit for it so I entered the chip number into Google and I was taken directly to your site. You've given better and more detailed info on this than Maplin have and I certainly will be building the TRF soon. By the way I also LOVE pussycats. Yours are really gorgeous!  Thanks for a great site!
Liz Costa 2E1FQN



From Dave Summer:

Hello Mike

An interesting site. I have made valve TRF radios since a boy in the 50’s. I heard about 60 amateur countries on a two valve 1.4 volt set using AM. I know many hints and kinks about making these sets work. Another good circuit is to use a valve followed by a transistor. If you use a 6.3 volt mains valve, such as the 6AK5 or 6AM6, it can work perfectly OK with 6.3V HT as well, and is very sensitive. Of course, if you use an RF stage the set is isolated from the aerial, which is more steady in frequency. I find the 1.4 valves to be poor performers and prone to microphony.

If you want the set for short wave reception, you can use it in oscillating mode for CW, SSB and AM. In the case of AM, if you use a high anode load resistor, the circuit pulls-in slightly to the carrier. But if you do not like this, use a low resistor, then it does not. In practice, these two conditions are best obtained using a pentode with its screen acting as the oscillator anode. Use either a high screen dropper resistor, or a low resistance 5k potential divider for the screen depending if you want pull-in or not.

Modern valves are very high gain, and will work down to a few volts of HT. It is necessary to avoid too much regen'. A good way is to use a trimmer as the grid condenser, as a small capacitance of say 5 pF may be all that is needed. If you use a small grid condenser, you are in fact tapping the grid down the coil – it is an impedance matching action.

Always locate the grid condenser right at the grid, with a short lead; this prevents hum pick-up.

As for aerial coupling, the best way is via a small trimmer. In this way, the loading can be adjusted. A coupling coil is not so satisfactory and may introduce unwanted resonances and dead spots.

A resonant aerial is not desirable as it causes rapid changes in loading across the band. Choose a length such as a third of a wavelength.

If you want bandspread, a good method is to tap the bandspread capacitor down the coil, until it just gives the swing you want.

As far as hand-capacitance goes, this is a big problem and makes the use of traditional baseboard sets more or less useless. To avoid hand capacitance you must have the set in an enclosed metal box, and the phones lead must be decoupled.

RF chokes are a bit undesirable, but the Hartley oscillator (cathode tapped into the coil) avoids their use. Regen can usually be obtained using a resistor instead of a choke.

If you use an iron cored choke in the detector anode circuit, I believe the circuit will suffer from threshold howl.

If you use a battery valve, or you need more gain from a valve with very low HT, you can take the grid leak to a positive supply, either filament positive or HT. This increases gain but reduces overload capacity.

Finally, you cannot use a long wave ferrite rod in a set using a power audio IC, as the chip ”radiates” noise in the long wave band which will be picked up by the rod, and the circuit will howl.

Finally, finally, although not TRF, a superb simple superhet can be made using a crystal controlled mixer ECH81 etc followed by an ordinary low frequency TRF. 

I hope these things are of some interest!!

Dave





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