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TRF RADIOS
Part 4
THE MEDIUM WAVE MINI

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TRF RADIOS (Part 4) - 'THE MEDIUM WAVE MINI'

Thanks to MDS975 reader Felix Scerri for sending in this interesting project.

1: THE MEDIUM WAVE MINI
Adapted from an article by A.J. Crighton published in the August 1978 edition of Everyday Electronics Magazine

After the introduction of the Ferranti ZN414  Integrated Circuit in the 1970's there were many articles published that  provide details of simple radio circuits that used the I.C.  The ZN414 is no longer made, but a new and almost identical I.C., the MK484, can substituted in all the ZN414 circuits and has proved to be a least, if not more, effective.

In the 1970's the ZN414 could be quite expensive to buy compared to cheap transistors and so this article was published in Everyday Electronics magazine as an inexpensive alternative.  Today some readers may find it difficult to obtain the MK484 so this circuit still holds great value.

This design uses three cheap transistors, the BC548 as can be seen from the diagram below.

Circuit Diagram

The Medium Wave Mini circuit diagram

The tuned circuit, L1/C2 selects the required station, transistor TR1 providing the r.f amplification.  The signal is then passed to TR2, a small amount of the signal is fed back from the collector to the base of TR1 via the tuned circuit.  A mixture of both radio frequency and audio frequency circulates in this feedback loop, the r.f. reinforcing that coming via the tuned circuit, while the a.f. is passed to the audio stage.

Transistor TR3 provides audio amplification that is enough to drive a crystal earphone which is plugged into the socket SK1

Most of the components are mounted on a small piece of plain matrix board having 13 by 7 holes as shown below:



Wiring details for the matrix board and other components





Modification required to the open frame jack socket
to make an 'on-off' switching arrangement, and
other layout details

Connections to the components are made using single cored wire on the underside of the board, this layout enables a smaller layout than using stripboard, however some experimenters may still wish to alter the layout to suit construction on 'strip board'.

The aerial coil L1 is 80 turns of 32 s.w.g. enamelled copper wire close wound onto a small ferrite rod.  The jack socket serves as both a connection for the earphone and a power switch.  One of the switch contacts must be bent slightly out so that they 'make' contact when the phone plug is inserted rather than the usual 'breaking' action when the plug is inserted.  As can be seen from the circuit diagram the power switching is applied in the negative side of the circuit.

The tuning capacitor C2 is a typical miniature polyvaricon type that is widely available from electronic component suppliers and which can be seen in most pocket type transistor radios.  If this variable capacitor has two gangs then usually only one gang will be required although some types may require that the two gangs are wired in parallel in order to be able to tune to the lower end of the medium wave band.


PARTS REQUIRED
Resistors - all 1/4 Watt carbon +/- 10%
R1
100 k
R2
2.7 k (see text)
R3
100 k
R4
1.5 k
Capacitors
C1
0.01 uF Ceramic
C2
250pF Variable
C3
0.1 uF Polyester
C4
0.47 uF Electrolytic
Semiconductors
TR1, 2, 3
BC548  npn silicon
Other Items
SK1
3.5mm open frame jack socket  (see text)
L1
45mm x 10mm Ferrite Rod and 32 s.w.g. enamelled
copper wire (see text)
B1
1.5 volt battery
0.1 inch Matrix Board 13 x 7 holes; Small plastic Case 60 x 40 mm;
Crystal Earpiece; Tuning Dial or Knob for C2; Thin Hook-Up Wire



Circuit Board

The completed circuit board

USING THE MEDIUM WAVE MINI RADIO

Before switching on for the first time, double check that all the components are wired up properly and that the battery is connected the the correct way round.  When you are happy that there are no mistakes plug in the earphone, this will energise the circuit and some background noise should be heard in the earphone.

Adjusting the tuning control should produce a few local transmitters, the number of stations available will depend on the location and the strength of signals available.  The tuning is quite sharp and therefore some care is needed when using the control.  The ferrite rod is directional and so the radio should be rotated in both planes for maximum signal pick-up for each radio station.

If  a whistle occurs then this can be reduced by increasing the value of the 2.7k resistor, R2, to 3.3k Ohms.  It may be possible to replace this fixed value resistor R2 with a sub-miniature preset resistor to make fine adjustments.


Medium Wave Mini

The Completed MEDIUM WAVE MINI Radio


Mini Radio By Karen Orton

Above is a MINI-RADIO made by Karen Orton.
Visit Karen's excellent technical webpage HERE
http://www.techlib.com/Karen.htm#Matchbox%20Radios

Also see this page by Charle Wenzel:
http://www.techlib.com/electronics/reflex.htm#match


Magazine Cover

LINKS:

Visit the EVERYDAY AND PRACTICAL ELECTRONICS Website


PAGES WITH ARTICLES BY RADIO EXPERT FELIX SCERRI:

High Quality AM Receivers:

http://www.crystalradio.net/scerri/index.shtml

http://www.crystalradio.net/

http://www.xtal-sets.com/


Hi Fi AM Receiver Designs:
 
http://sound.westhost.com/articles/am-radio.htm


OTHER ARTICLES

ESP by Rod Elliot - "Mad as Hell" & lead free solder directive:
 
http://sound.westhost.com/madashell.htm

ESP by Rod Elliot - CFL's (Compact Fluorescent Lamps)
http://sound.westhost.com/articles/incandescent.htm







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:

Spitfire & Metzo Complete, 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



BACK TO T.R.F. RADIOS PART 1

  BACK TO T.R.F. RADIOS PART 2

 
BACK TO T.R.F. RADIOS PART 3

TO TRF RADIOS PART 5

TRF RADIOS - PART 6

T.R.F. RADIOS PART 7


LINKS to Other Great Websites >


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AND CAPACITOR CONVERSION TABLE




Visit my Amateur Radio pages . . . .
Visit my Amateur Radio Pages >


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