5196106327

The K7 is very compact: even the headphones fit into a standard compact

cassette box.

(02) via capacitor 06. Resistor R5 biascs transistor 02 and the parallel capacitor 07 filters off the higher fre-quencies, which contain a lot a noise etc. Finally the aniplified AF signal is fcd into the headphones.

The Circuit is provided with a stereo phono socket so that people who want to use their own stereo headphones will hear the sound from two sides instead of one. The sound from such a simple receiver is obviously mono, even if you are tuned to a stereo AM station.

The PNP transistor 03 acts as an clectronic On/Off switęh. When the headphones are plugged in then the base Circuit of 03 is dosed and the transistor is turned on. The voltage drop ovcr the emitter-collector is only about 100mV.

Selecting the frequency rangę

The resonant frequency of the tuning Circuit is determined by the ferrite rod coil LI and the parallel conncction of the trimmer capacitor Cl and capacitor C2 (when the switch SI is in position 1).

The trimmer capacitors have a rangę of 9.8-60pF. To get the radio tuned into the right section of the AM band, you have to add the fixed parallel capacitor C2 into the Circuit. Table 1 gives the values for the parallel capacitors C2, C4, C6, C8, CIO, C12 for the required frcqucncy ranges.

Frequency	C2, C4
rangę kHz	etc. pF
1200 — 1600	0
950— 1200	56
800 — 950	120
725 — 800	180
675 — 725	220
625 — 675	270
600 — 625	330

Table 1: Values of parallel capacitors for the diflerent freguency ranges.

Nccdlcss to say, select the values you need for C4-C12, depending on the particular stations you want to tunc in.

It is advisable to determinc the values of these capacitors before purchasing the componcnts or actually assembling the PCB. The ceramic disc capacitors are very chcap and it might be worth to buy a few cxtra values, in case you change your mind on the rangę.

Construction

The printed Circuit board (PCB) for the K7 project has quite an odd shape, to suit the cassettc box. Two holes are provided so that you won’t have to re-move the two plastic projections (the tapc blocking mechanism) from the cas-sette box. The two other cut-outs are mcant for the battery and the ferrite rod coil. The PCB is coded 87mc9 and measures 1()6 x 47mm.

Before assembling the PCB, you have to modify the cassettc box. The me^: chanical details of the modifications are given in Fig.3. The hole for the headphones socket has to be drillcd, then a File uscd to work out the spaccs for the switch and the potentiometer.

• Most cassette boxes have little plastic bits at the insides. To make the PCB Fit, thosc bits have to be taken out. This is best donc with side cutters.

Make surę that the PCB easily fits in the cassette box. If the PCB has been cut off too large, then it will havc to be pruncd down with a filc.

The order of assembling is the same as for most other PCB*s. First the wire links, then the resistors, the capacitors and finally the transistors and the AM radio chip.

The 6-position switch has to be mounted before the trimmer capacitors. The switch is not dircctly mounted on the PCB, but via smali wire links. The best approach is to solder the wire links first to the PCB, and then bend and cut them so that they can be soldcred dircctly to the switch.

• • - i i m

Fig.3: This diagram shows the modifications required for the cassette box_t to suit the K7 PCB.

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ELECTRONICS Australia, September 1987