capacitance, measuring the length of
a coaxial cable, and the list goes on.
No physical connection is required.
You just have to bring the sense
coil in close proximity to the circuit
Basically, the dipper is an LC
oscillator that’s tuned by a variable
capacitor. The sense coil, which is
traditionally plugged into a socket at
the top of the meter, determines the
frequency sweep of the oscillator.
When the coil is placed close to a
tuned circuit, you adjust the frequency using the variable capacitor.
When the frequency of the dip oscillator matches the frequency of the
circuit under test, the energy is
transferred from the oscillator to the
passive circuit. In effect, there is a
dip in the output power of the
There are many dipper designs
implementing every sort of
oscillating device from vacuum
tubes to transistors to tunnel diodes.
The simplest is a one transistor
circuit built around an MPF102 FET
(Figure 7). It’s tuned for a range of
2 to 60 MHz, but small changes in
the values of the tuning capacitor
and coil can extend that range up
Construction is straightforward,
but the connecting wires should be
kept short and to the point to avoid
Calibration is traditionally done
by marking the dial using tank
circuits of a known frequency.
This technique will put you in the
ballpark. For those of us who
have graduated to the digital
age, Figure 8 has a digital
Coil information is shown in
Table 1. PVC water pipe — found
at any Home Depot — makes an
excellent coil form. The tap is
counted from the bottom of the
coil and the winding length is
critical. Spread the windings
apart, if necessary.
Lastly, the wire is enamel
coated magnet wire, not plastic
Q. Can you please find updated
numbers for the following
obsolete transistors: HEP724,
HEP739, SK3011, SK3003 ... 2N1524,
MPS3708, TIS- 59? I feel that the
TIS- 59 can be replaced by a 2N3819
or a MPF-102, but that’s only a guess.
A. A few years back, there were
no less than four major semiconductor makers who offered a line
of replacement transistors, including
Motorola (HEP), GE (GE-), Sylvania
(ECG), and RCA (SK). These
parts were originally intended for
the replacement, hobbyist, and
experimental markets and — in time
— found applications in industrial
repair and maintenance departments.
Today, only NTE (www.nteinc.
com) survives. I was able to find all
but one of your transistors on their
cross reference website (http://
Form) — and that’s only because the
TIS- 59 is listed as a TIS59 (originally
from TI). Yes, the hyphen matters in
the search engine. Once you have
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