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Spy Circuits

We needrYt say any morę about construction as you will obvious!y know how to put the kit together.

SETTING UP

Solder the paper elips to the board as shown in the photo and bend them to suit the sockets on the multimete "sensitivity" control (100k pot) to minimum resistance and switch the Circuit ON. Turn the "Set fuli scalę deflec give fuli deflection on the meter. Now turn the sensitivity pot until the needle just starts to "dip."

At this point the Circuit is the most sensitive as the DC amplifier transistor is just tumed on and any signal app diodes will reduce the voltage appearing on the top of them and turn the transistor off - the needle on the met drop. The Field Strength Meter is now ready for use.

USING THE FIELD STRENGTH METER

This project will help you get the best silt of any transmitter. It will give an accurate readout because it does n« transmitter but registers the strength of the field AT A DISTANCE.

The way it is used is to set up the antenna of the Field Strength Meter in the same piane as the transmitting a best pick-up) and at a distance that just causes the needle on the meter to defiect.

The meter is wired as a "DIP" meter and the needle deflects towards zero as the field strength inereases. Pla« peaked on the test bench, with the antenna out-stretched and bring the receiving antenna so that the needle j Peak the Circuit a smali amount and take your hands away so that they don't upset the reading, and watch the output inereases, the needle dips further. By maintaining the exact same distance between bug and meter, yc bug with another.

It’s the fastest way of determining the output without doing a "field test."

IFITDOESNT WORK

As with all our projects, they work be cause we have actually built them and checked their performance. If yoi the first thing to do is check the value of the components against the overlay on the board.

Two components in the wrong place can make a huge difference and a Circuit like this is fairly critical as the b correct.

Secondly, make surę all the parts are fitted and nothing has been missed. Also make surę all the parts have i neatly and cleanly.

We still get projects sent to us for repair where one or morę leads have not been soldered and obviously the \ work.

Next you can make a few voltage readings. Although they don't tell you too much, it is a fast way of determini the correct DC conditions.

The voltages:

RF Stage:

Collector: 6.1v Base:    5.8v

Emitter:    5.2v

DC stage:

Collector: 0.2v Base:    0.65v

Emitter: 0v

If these check out ok, you should make a few further DC tests. If the meter swings fuli scalę at power-up, you between base and emitter of the BC 547 to see the needle falls to zero. This will show the transistor is workin transistor may be shorted.

Next remove the 47k on the diodę pair. This will also cause the needle to move down-scale and show the bia; working. It is morę difficult to test the RF stage and merely probing around the stage with a meter or CRO, will cause the needle to defiect.

Of course we have assumed you have bought a kit and PC board for the project. The frequency of operation ■ it important that it is built on the correct PC board.

We can not guarantee "bread board" jobs or circuits madę with your own components as so many variables en Things like different markings on capacitors, different RF transistors or signal diodes could make the differenc and failure.

If you know what you are doing, thafs fine - you can use your own components. But if you intend to learn fron take any chances. It's cheaper in the long run to get all the projects in kit form and build them exactly as spec If you get really stuck, don't hesitate to buy another kit and start again. You can come back to the faulty one Ić so important that we don't want you to miss out. With a field strength meter you can carry out experiments the chapter of a book to explain. Here's one:

2011-12-08

mk:@MSITStore:F:\spy circuits.ehm::/SpyCircuits-l.html