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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

TV field
strength meter

This TV field strength meter is based on a tuner module salvaged from an old Sharp TV although the circuit can be adapted to most VHF/UHF tuner modules used in TVs and VCRs. The pin assignments are usually stamped on the cover of the tuner module or may be found on the bottom of the PC board.

The Sharp tuner is an older model digital tuner, with a 75-ohm input for VHF and a 300-ohm input for UHF. Modern tuners now have a single 75-ohm input for the VHF & UHF bands. Old VCRs can be picked up cheaply at flea markets and at kerbside rubbish collection times.

Click for larger image

The IF (intermediate frequency) pin produces a 45MHz signal (in the USA) and the IF output level varies with the incoming signal level. In fact, the frequency of the IF signal doesn’t matter as it’s simply rectified by diodes D1 & D2. Trimpot VR3 provides a sensitivity adjustment for the meter.

The circuit is powered by a transformer with 8V and 30V AC secondary windings. The 8V AC is rectified by bridge BR1 and filtered to provide the nominal 12V DC rail for the tuner module while the 30V AC is rectified by bridge BR2 and fed to potentiometer VR1 is to provide a tuning voltage range of 0-30V DC. Potentiometer VR1 is connected to the AGC input to serve as a gain control.

If you need more gain to obtain reasonable deflection of the meter, transistor Q1 can be added, as shown.

As presented, the circuit will give a useful indication of TV signal strength and can be used as an aid for best antenna orientation.

Henry Bowman,

Parsons, Tennessee. ($50)

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