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 allow 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.

This is an RF project and this calls for even more care during soldering as sloppiness during construction can mean low or no output at all, low stability and other problems. Make sure that you follow the general rules about electronic circuit construction outlined above and double-check everything before going to the next step. All the components are clearly marked on the component side of the P.C. board and you should have no difficulty in locating and placing them. Solder first of all the pins, and continue with the coils taking care not to deform them, the RFC’s, the resistors, the capacitors and finally the electrolytic and the trimmers. Make sure that the electrolytic are correctly placed with respect to their polarity and that the trimmers are not overheated during soldering. At this point stop for a good inspection of the work done so far and if you see that everything is OK go on and solder the transistors in their places taking grate care not to overheat them as they are the most sensitive of all the components used in the project. The audio frequency input is at points 1 (ground) and 2 (signal), the power supply is connected at points 3 (-) and 4 (+) and the antenna is connected at points 5 (ground) and 6 (signal). As we have already mentioned the signal you use for the modulation of the transmitter could be the output of a preamplifier or mixer or in case you only want to modulate it with voice you can use the piezoelectric microphone supplied with the Kit. (The quality of this microphone is not very good but it is quite adequate if you are interested in speech only.) As an antenna you can use an open dipole or a Ground Plane. Before you start using the transmitter or every time you change its working frequency you must follow the procedure described below which is called alignment.

Parts List

R1 = 220K
R2 = 4,7K
R3 = R4 = 10K
R5 = 82 Ohm
R = 150Ohm 1/2W x2 *
VR1 = 22K trimmer

C1 = C2 = 4,7uF 25V electrolytic
C3 = C13 = 4,7nF ceramic
C4 = C14 = 1nF ceramic
C5 = C6 = 470pF ceramic
C7 = 11pF ceramic
C8 = 3-10pF trimmer
C9 = C12 = 7-35pF trimmer
C10 = C11 = 10-60pF trimmer
C15 = 4-20pF trimmer
C16 = 22nF ceramic *

L1 = 4 turns of silver coated wire at 5,5mm diameter
L2 = 6 turns of silver coated wire at 5,5mm diameter
L3 = 3 turns of silver coated wire at 5,5mm diameter
L4 = printed on PCB
L5 = 5 turns of silver coated wire at 7,5mm diameter

RFC1=RFC2=RFC3= VK200 RFC tsok

TR1 = TR2 = 2N2219 NPN
TR3 = 2N3553 NPN
TR4 = BC547/BC548 NPN
D1 = 1N4148 diode *
MIC = crystalic microphone

Note: Parts marked with * are used for the tune-up of the transmitter in case you have not a stationary wave bridge.

Adjustments

If you expect your transmitter to be able to deliver its maximum output at any time you must align all the RF stages in order to ensure that you get the best energy transfer between them. There are two ways to do this and it depends if you have a SWR meter or not which method you are going to follow. If you have a SWR meter turn the transmitter on, having connected the SWR meter in its output in series with the antenna, and turn C15 in order to tune the oscillator to the frequency you have chosen for your broad casts. Then start adjusting the trimmers C8,9,10,12 and 11 in this order till you get the maximum output power in the SWR meter. For those who don’t have a SWR meter there is another method which gives quite satisfactory results. You only have to build the little circuit in Fig. 2 which is connected in the out put of the transmitter and in its output (across C16) you connect your multi-tester having selected a suitable VOLTS scale. You tune C15 in the desired frequency and then adjust the other trimmers in the same order as it is described above for the maximum output in the multitester. The disadvantage of this method is that you do not align the transmitter with a real antenna connected in its output and it may be necessary to make slight adjustments to C11 and C12 for a perfect antenna match.
Do not forget to align your transmitter every time you change your aerial or your working frequency.
WARNING: In every transmitter there are present apart from the main output frequency various harmonics that usually have a very short range. In order to make sure you haven’t tuned on one of them do the tuning as far as possible from your receiver, or use a Spectrum Analyser to see your output spectrum and make sure that you tune your transmitter on the right frequency.

Warning