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  • #10104
    Gary Albach
    Forum Participant

    Downsizing to a condo means giving up all that workspace in the basement and garage, and so I decided to continue reducing the size of some of my test gear. The latest round started with the little signal injector on the right in the photo below. It’s a simple one-transistor blocking oscillator with a 1 kHz output ringing at about 400 kHz (schematic attached). I used a general-purpose transistor and an IF transformer from an old transistor radio but virtually anything should work with some tweaking.

    This project worked so well I pushed on and ended up with both a signal injector and a signal tracer (complete with speaker) in the same size little plastic box as I had used for just the signal injector (the dual-unit is the one in the middle of the picture below). The switch on the right of the enclosure selects either the injector or the tracer function. This time, the circuit needed two transistors as shown in the attached schematic. As an injector they are configured as a multi vibrator generating a 1kHz ‘sort-of-square’ wave full of harmonics, and as a tracer they become a 2-transistor amplifier driving a speaker through an output transformer. An RF input signal is demodulated by the non-linearity of the input transistor. It’s amazing how much performance one can get from a minimum amount of silicon. As with the signal injector, the transistors are general purpose 2N2222s. The speaker came in a bag of 10 for $2.00 (20 cents each!) from AliExpress – it’s only 1/4-inch-thick, probably made for disposable greeting cards! I didn’t pay much attention to component placement and lead dress, so my circuit tends to oscillate at ultrasonic frequencies in its tracer mode. This doesn’t seem to affect its operation as an amplifier for tracing and is only noticeable as a soft white noise in the speaker when there is no input signal. A little fiddling with the lead dress or the bias on the transistors could probably cure this.

    And finally, got around to putting my $5 all-purpose Chinese tester board (ESR, transistors, diodes, resistors, capacitors, inductors, etc.) into a plastic case. I cut the holes with a Dremel tool and finished them with a file. Since I bought this little LCR-T4 board, I see that it’s now available from several places on-line complete with a plastic enclosure. For the price, this is a handy little piece of test equipment.


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