The Circuit :

The circuit consists of an IC the ZN414 or MK484 and possibly with some

modifications a TA7642. The AF output  transistor is not at all critical,

a BC108 , BC547, BC548 etc. can be used with excellent results.

In the past I always used transistor sockets to avoid overheating damage during soldering.These are also useful  while experimenting but always remember to

disconnect the battery before removing or inserting the IC to prevent damage.

Wire wrap IC sockets are ideal as the longer leads are very easy to solder to the construction board, also a linear pot is used to adjust the regeneration and AGC for maximum efficiency.Both of these principles were used with this circuit before

reaching the final version.Keep all connections as short as possible and the ferrite

aerial well clear of the battery etc.

As my interest was not a matchbox type or a mini receiver a long ferrite slab is preferable  with a coil wound with Litz Wire  and an air spaced variable capacitor.

Component values quoted in the parts list are suitable for the ZN414 or the MK484 but the

TA7642 will require a higher value for R * .

The TA7642 has an input impedance of 3M whereas  both ZN414 and MK484 have 4M.

Using a TA7642 may require either a tapped coil or a coil with a coupling coil of a few turns.Replacing the ZN414 with a MK484 was not a problem but the value for the above resistor should be increased to maybe 680 Ohms -1K for TA7642.

During experiments both with former one's I started with a value of 560  Ohms later I reduced this to 470 Ohms as per the datasheet. Do not go lower than this value as you will destroy the IC .Please bear in mind that IC's of the same type may still require individual selection of the resistor due to manufacturing tolerances.

The battery consumption with the use of a crystal earpiece was 0.25mA which is extremely low. Some constructors use 32 Ohm earphones connected in place of the 10 K ( R3 resistor)   The use of 32 Ohm Earphones will increase battery consumption to around  3mA.

  I tried something different ,which I explain below.

Some Hints for Guidance :

Check all connections against the circuit diagram before switching on paying carefull attention to the orientation of IC's and transistors and battery polarity.

Initially set the pot to minimum and tune in a local station, if reception is weak turn the pot towards maximum until reception becomes clear, if you go too far the signal will become distorted and you will have to back off the pot a little to reduce it.

As MW transmissions are effected by daylight  you will be able to tune in many more distant stations after dark.

With the prototype I am able to tune at least 15 foreign transmissions such as 3  Greek

stations Kol Israel, Russian, Bulgarian , Romanian, Serbian , Arabic, One German station in Potsdam an English Transmission possibly a relay of BBC and RadioChina after dark

which is really something  positive for such a simple receiver.

Due to the signal strength it is necessary to adjust the AGC each time you are tuned to a

different station for efficient reception.

You will also experience some fading, that is some stations coming and going which is normal.

Some transmissons were so strong that a volume control is recommended but you can cure this at the cost of sensitivity by adjusting the AGC level  towards minimum.

Due to  increasing electronic devices used in our homes you have to have in mind that

that some EMI  (Electromagnetic Interference) may be experienced depending upon the location the receiver is being used specially if  nearby such a switched on device. In areas far from such devices the selectivity and outoput superseds  all two transistor and some three transistor receivers I built in the past.    

If available, a Vernier or Slow Motion Dial will ease tuning at the high frequency end of the Band.

After the motto “Never play with a working circuit” I tried something without altering the original receiver. 

I experimented with various types of headphones, 2000 Ohm, Crystal, Low Impedance  and Magnetic. Some of the older  headphones were souvenirs from olden times. Finally a pair of  16 Ohm and a pair of 32Ohm Mp3 Type earphones were also tried.The result was in favour of crystal earpiece .Without neglecting the above motto, when I plugged in both the latter one's, as the plugs were stereo and the socket was mono the result was not promising.

Then It came to my mind to use a stereo input and mono output adapter and plug in the earphones via the adapter.The 16 Ohm Pair was not satisfactory but the 32 Ohm pair immedately gave good results and high volume. The sound was more audible and clear but the battery consumption was 2.5mA. This has the advantage of better hearing in a crowded or noisy area as the crystal earpiece is a single phone for one ear only.

This gives you two alternative outputs without altering the original receiver.

I want to add a few words before I end the article.

All the  IC's referred to  have a a frequency range of 150kHz to 3mHz which is both

LW and MW plus tropical bands. 

On the pages you will find links to, or you will reach via the search engine Google

similar or alternative circuit diagrams for Short Wave coverage, possibly the 49 Meter

band which sounds really interesting.

I have not tried receiving Short Wave transmissions as the IC cut off is 3 mhZ.

But a 10-15 turn coil would be a good starting point for any experiments along these lines.

            

 

Creative EP50

Earphones

Volume Control

Mini Switch

Linear Pot

Ferrite Antenna

Old Type

Jack Socket

Crystal Earpiece

ADDENDUM  (1) JUNE2011 :

It is of human nature to sit down and relax after a certain goal is reached.However after sometime this nature starts to push him and one is after new ideas to correct or modify a certain project.

I was used to this from my active times, therefore this feeling did not disturb me at all. First of all I tried several other earphone pairs to find an alternative for the Creative EP50. Everytime I was able to test a different type, I used this opportunity, either borrowing from a friend or remarked someone using a pair of earphones.The idea behind this comparison is to avoid unnecessary costs and disappointment for those of you interested with the circuit.

a)The first type was a pair of unknown Impedance given during my recent flight. I was not very hopeful  as the system used on an aircraft was not comparable with my simple receiver, also not recommendable since getting such a pair of earphones was rarely possible for the others.The performance of this was something similar to my 16 Ohms earphones.The remaining three were all having 32 Ohms Impedance.Herewith some specifications of the manufacturers:

b)The second one was a pair of Piranha PRN-302 earphones . Mini type that fits into the ear.Sensitivity 102db, 20Hz-20KHz Rated Power 100 mW.This also did not perform better.Neither with nor without the stereo to mono adapter plug.

c)The third pair of these earphones was a Maxell  Model  EC150 with an earpad and clips for the ear.The model details as follows: Sensitivity 100dB +/- 3dB,20Hz-22KHz,Anisotropic Ferrite Magnet. 

d)The last one was a neck type Creative HG65, sensitivity 117 dB.20Hz-20KHz with binaural transducer.The performance was not better than the others, although I had much hope on this.

The EP50 has similar specifications except the sensitivity is 111db/mW and has a neodymicron rare earth magnet and is to be used definitely, if you do keep the component values and do not make any changes.In addition it is cheaper than the others.   

e)The other point in this addendum is the use of YS414 IC, which I luckily received a few from a friend of mine.The use of YS414 did not create any problem and there was no need of any component adjustment or change of values.

It worked perfectly as did  the ZN414 and MK484. If the YS414 is easier to obtain  than a ZN414 or MK484 or a TA 7642 ,it can be used without hesitation.

One point I have to draw to your attention is the pin leadouts of the YS414  which are not identical with ZN414 IC and the two outer leadouts must be transposed.Please have in mind that  the YS414 leadouts may differ depending upon the manufacturer which I remarked when comparing with a MK484.Rather complicated I must say. As I am using sockets, finding this out was rather easy since no soldering or desoldering was necessary.Do not forget to switch off, while checking, if you are also using sockets . In future I may be trying some other variation regarding the circuit.