Nuts & Volts - January 2005

Q&A

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 under test.

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 oscillator.

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 or down.

Construction is straightforward, but the connecting wires should be kept short and to the point to avoid spurious radiation.

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 counter output.

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 insulated.

JANUARY 2005

Figure 5

Obsolete Transistors

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.

Kendrick via Internet

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:// nte01.nteinc.com/nte/NTExRef SemiProd.nsf/$$Search?Open 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