How it Works
The circuit uses five transistors and is a broad
band amplifier with a bandwidth of 5 MHz. The signal is applied
at points 1 and 2
(ground) and is taken through C1 to the
first stage which is a preamplifier and is built around Q1. In
the output of Q1 are DC coupled Q2,3 which amplify the signal
more, and as they are DC coupled to the preamplifier there is
virtually no distortion and the
amplification is quite high.
Finally the signal from the out put Q3 is fed to the output
transistors which are Q4 & Q5. These two transistors are
complementary and the signal from their common emitters is taken
to the signal distribution R-C network from where it is sent to
the various devices which are driven by the circuit. The circuit
needs a 12 VDC power supply and it is much better if it is a
stabilised one like the circuit printed elsewhere in the
instructions.
Construction
First of all let us consider a few basics in
building electronic circuits on a printed circuit board. The
board is made of a thin insulating
material clad with a thin
layer of conductive copper that is shaped in such a way as to
form the necessary conductors between the
various components
of the circuit. The use of a properly designed printed circuit
board is very desirable as it speeds construction
up
considerably and reduces the possibility of making errors.
Smart Kit boards also come pre-drilled and with the outline of
the components and their identification printed on the component
side to make construction easier. To protect the board during
storage from oxidation and assure it gets to you in perfect
condition the copper is tinned during manufacturing and covered
with a special varnish that protects it from getting oxidised
and also makes soldering easier.
Soldering the components to
the board is the only way to build your circuit and from the way
you do it depends greatly your success
or failure. This work
is not very difficult and if you stick to a few rules you should
have no problems. The soldering iron that you use
must be
light and its power should not exceed the 25 Watts. The tip
should be fine and must be kept clean at all times. For
this
purpose come very handy specially made sponges that are
kept wet and from time to time you can wipe the hot tip on them
to
remove all the residues that tend to accumulate on it.
DO NOT file or sandpaper a dirty or worn out tip. If the tip
cannot be cleaned, replace it. There are many different types of
solder in
the market and you should choose a good quality one
that contains the necessary flux in its core, to assure a
perfect joint every
time.
DO NOT use soldering flux apart from that
which is already included in your solder. Too much flux can
cause many problems and is
one of the main causes of circuit
malfunction. If nevertheless you have to use extra flux, as it
is the case when you have to tin
copper wires, clean it very
thoroughly after you finish your work.
In order to solder a
component correctly you should do the following:
- Clean the
component leads with a small piece of emery paper.
- Bend
them at the correct distance from the component’s body and
insert the component in its place on the board.
- You may
find sometimes a component with heavier gauge leads than usual,
that are too thick to enter in the holes of the p.c.
board.
In this case use a mini drill to enlarge the holes slightly. Do
not make the holes too large as this is going to make
soldering
difficult afterwards.
- Take the hot iron and
place its tip on the component lead while holding the end of the
solder wire at the point where the lead
emerges from the
board. The iron tip must touch the lead slightly above the p.c.
board.
- When the solder starts to melt and flow, wait till
it covers evenly the area around the hole and the flux boils and
gets out from
underneath the solder. The whole operation
should not take more than 5 seconds. Remove the iron and leave
the solder to cool
naturally without blowing on it or moving
the component. If everything was done properly the surface of
the joint must have a bright
metallic finish and its edges
should be smoothly ended on the component lead and the board
track. If the solder looks dull, cracked,
or has the shape of
a blob then you have made a dry joint and you should remove the
solder (with a pump, or a solder wick) and redo
it.
- Take
care not to overheat the tracks as it is very easy to lift them
from the board and break them.
- When you are soldering a
sensitive component it is good practice to hold the lead from
the component side of the board with a pair
of long-nose
pliers to divert any heat that could possibly damage the
component.
- Make sure that you do not use more solder than
it is necessary as you are running the risk of short-circuiting
adjacent tracks on
the board, especially if they are very
close together.
- When you finish your work cut off the
excess of the component leads and clean the board thoroughly
with a suitable solvent to
remove all flux residues that may
still remain on it.
- The construction should not present
any difficulties if you follow the circuit diagram carefully and
place the components in their
place as it is outlined on the
component side of the p.c. board.
Solder first of all the
pins, then the resistors, the capacitors making sure that the
electrolytic are connected correctly with respect to
their
polarity and finally insert the transistors in their places and
solder them very carefully as overheating during soldering
can
destroy them.
When all the components have been
soldered make a careful inspection of the circuit and if you are
satisfied that there are no
mistakes make the following
connections:
- Supply voltage at points 3 (+12 VDC) and 4
(-).
- Input signal at points 1 (signal) and 2 (common).
-
Outputs 5,6,7 (signal) and 4 (common).
As it is only two
of the outputs are connected initially. To enable the third you
should also connect the positive leads of C7 and
C9
together.
