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
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.
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
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.
Parsons, Tennessee. ($50)