Towards the end of the 1960's, rather unwillingly, I had to end my home constructor
activities due to increasing responsibilities towards my family.
The remainder of my circuits resulting from my experiments were sent to the
Editor of “The Radio Constructor” and after a decision was reached they were
published in the coming issues of the periodical during the 1970's.
The production of the ZN414 IC by Messrs. Ferranti of UK falls into this
period, it was rather popular due to the minimum number of components
required to construct a receiver since the IC consisted of 10 transistors integrated
into a metal case with 3 legs similar to a BC107-BC109 .
By adding a simple AF stage more than sufficient volume could be obtained
for earpiece output.It was possible to add a two transistor AF Stage. This could
give much higher volume but in such a case a volume control was necessary for
a comfortable listening level.Another advantage being the need of low voltage
(1.5V) and the size of the battery (AA or AAA) as far as space is concerned.
By adding a powerful AF stage good speaker volume could be obtained.
I do not really know, if the ZN414 was easily affordable by every home constructor
at that time.
Due to the high input impedance of the IC (4 Megohm) a single tuned circuit
was sufficient and in addition, due to the AGC (Automatic Gain Control),
the constructed receiver could be surprisingly selective and receive a lot
of stations especially after dark.Separating several local stations could sometimes
be difficult due to overloading if they are too close to each other.
The ZN414 can be identified easily as the legs are gold coloured whereas normally
the legs of metal cased semiconductors are usually silver , useful when the case
markings have rubbed off after excessive use.
Later models of ZN414 were produced in a plastic case named as ZN414Z.
In 1980's the ZN415Z and 416Z were produced which was the same IC
with an added AF Stage.
Like the fate of all semiconductors after sometime Ferranti stopped the production of
the IC .However, the IC was copied in the Far East under different names such as
MK484 ,YS414, LMF501 and TA7642 etc.Nowdays the most popular one is TA7642 and
in Japan LMF501T By Mitsumi.
Meeting my friend Geoffrey Brown was the spark to start the fire to construct again.
I think I will not be as active as in the past but enjoy the moral satisfaction of producing something, after so many years,by tuning a selfconstructed receiver of some sort.
The area I live in the city is full with tall buildings and not convenient for a crystal receiver where performance depends on a good aerial and earth connection. A loop was also not very efficient. Reconstrucing identical receivers as in the past was not interesting either, although I had all the parts kept carefully in a large box with a padlock for many years, which my grandson named the “Treasure Box” from what he had learned at school.He was surely right, It was a treasure box for me.
Finally I decided to construct a ZN414 IC receiver which was rather simple and efficient. I had no experience with this component and no idea where to find this IC which was produced 40 years ago.
An internet search led me to many pages full of valuable references, related circuit diagrams, datasheets, sources to buy both the MK484, the successor of ZN414 , YS414 and TA7642 etc.
Luckily I was able to find and buy some actual MK484 IC's( the last of the stock). I say actual as many retailers sell TA7642 as MK484. The picture shows MK484 but they sell TA7642 instead with the commentary that they are identical..
The source was Modern Radio Laboratories run by Paul Nelson ,(www.modernradiolabs.com)
a well known person among crystal radio constructors, who was selling the IC's at a very very reasonable price of 80 cents each .
A rather positive attitiude not trying to make use of the situation. There was no info regarding the whereabouts of ZN414 which was normal after such a long time. Finding an NPN AF transistor which I used for the AF Amplification was not a problem.The BC108, BC548 or equivalents are suitable.
In my hometown while looking for these in a small shop mostly visited by homeconstructors, I was very suprised when, after enquiring about a ZN414, the guy took a rather dusted small box from the shelf containing many ZN414 IC's possibly resting since many years.
The price was very very reasonable so I bought enough for myself. The rest was no problem. Nowdays you can Find the MK484 or YS414 Through retailers of Far Eastern origin (Be careful and negotiate) or in USA and Western Europe for a rather high price per unit. The TA7642 can be purchased for a much reasonable price through a trustworthy eBay shop of Hong Kong origin.(See The Links Page for details)
After so much explanation now some info about the circuit.
In the past I always preferred designing RF circuits having a linear pot in series with a resistor of the minimum value required for efficient control of regeneration
This method always worked efficiently without any problem with AF124 , AF125,AF126 and AF127 even with the OC44 germanium RF transistors I used. I therefore planned to use the same method with ZN414. After checking the pages and datasheets I used as reference (Many of them are linked below) I found some diagrams I was in need of.