5196106330

Follow this w i ring diagram when assembling the PCB. Take care with the orientation of the capacitors and transistors.

The trimmer capacitors, headphones Socket and trimpot can now be mount-ed. The trimpot we used is of the verti-cal mounting type mounted horizontal-ly, as the horizontally mounting trim-pots with a knob are difficult to get. The middle leg of the trimpot is directly mounted on the PCB, and the outer legs via wire links.

The battery elips are home-made and take a bit of craft work. They are madę of tinned copper wire, artistically bend into two circles: one circle is the battery contact, the other is mounted to the PCB with the screw. The battery elips are a bit fiddly to build and a couple of miniaturę fingers wouid come in handy.

The next step is to wind the coil on the ferrite rod. The ferrite rod we bought from Dick Smith was supplied with a coil, but we didn’t use it. Per-haps you have an old ferrite rod laying about, the only important specification is the size. The coil is wound with 60 tums of 0.4mm enamelled copper wire.

Parts list

1 PCB coded 87mc9, 47x106mm 1 compact cassette box (Maxell, TDK etc.)

1 6-position switch (DSE S-2050 or similar)

1    3.5 mm stereo PCB mount phono Socket (Jaycar)

2    2.5 mm screws, nuts and washers

1 1.5V battery (AA)

Semiconductors

1    ZN414 AM radio chip

2    BC549 NPN transistors 1 BC557 PNP transistor

Capacitors

6 trimmer capacitors 9.8-60pF (brown)

2 47nF disc ceramic

1    0.1 uF tantalum

2    1uF tantalum

6 disc ceramic in rangę 82-390pF, see text

Inductors

1 Ferrite rod aerial. rectangular 55x13x5 (DSE L-0520)

3    metres of 0.4 mm enamelled copper wire 0.4

Resistors (0.25W, 5%)

1 x 1k2, 2 x 10k, 2 x 100k, 1 x 1M

1 100k trimmer potentiometer (Tandy)

The wire ends of the coil can be fixcd to the ferrite rod with a bit of glue or sticky tapc. The wire is enamelled and the ends have to be scraped clean with a bit of sandpaper or a knife (take care not to cut it).

The ferrite aerial rod with coil is mounted to the PCB with two bits of in* sulated wire through the holes in the PCB. An ordinary reef-knot will secure the rod (don’t solder the ends together as this will act as a shorted turn). You can now solder the wire ends of the coil to the PCB, according to the wiring diagram.

Testing and adjusting

By now the. K7 should be ready for testing and aligning. Ali the testing should be done with the headphones on (in) and plugged in. Firstly put in the battery. Mind the polarity of the battery- the “+" points at the potentiome-

Hertz and beer

This year we are celebrating the centenary of radio. It was in 1887 that the bright German physicist Heinrich Rudolf Hertz experimentally proved that Maxwell had becn right when he predictcd the existance of electromagnetic waves somc 23 ycars earlier. Hertz did his experiments in his laboratory in Karlsruhe, Germany. He uscd a spark-gap transmitter and a loop antenna with a gap as receiver. His very basie radio set wouid nowadays probably de-stroy the input amplifier of your FM tuner, but at the time it was good enough to generate and pick up radio waves over a -distance of a couple of metres. Hertz published his sensational findings in a series of papers between 1887 and 1889.

Hertz*s discoveries inspired many of the contemporary physicists. Some of those scientists came up with appreciable results and the invention of wireless communication was claimed by sevcral people likc Sir Olivcr Lodge from England and Alexander Popoff from Russia. However, at the end of the race it was the young Italian Gugliclmo Marconi from Bologna who was awarded in 1909 with the Nobel Prize for Physics. Not without reason, for by 1909 Marconi had done a lot of pioneering work. In 1896, at the age of 21, Marconi managed to telegraph messages ovcr a distance of 2.5 kilometres. In 1899 hc bridged the English channel and two years later an Atlantic radio link was laid between Poldhu, Cornwall and St. John’s, Newfoundland.

Now, a hundred years later we find the word Hertz in the dictionary and I fear that most people have forgotten that it used to be the namc of a clever physicist of the past.

During those hundred years of radio, a lot has happened. The valve has becn invcnted, the transistor, the Computer has becn introduced and at the moment we are decorating the sky with satellites.

History is being madę all the time and who knows, perhaps at this very . moment, a bright young tcchnician is on the vergc of inventing something that will have an impact as great as the invcntion of radio.

For the time being, as we are celebrating today, I wouid like to say: Cheers Hertz, zum wohl!!

61

ELECTRONICS Australia. September 1987