Amateur Radio Testing Circuits

Fig, L Circuit for testing the oscillating character-ïs+ïcs of unmarked transistors. This circuit is shown with a negative supply for PNP transistors—for NPN units reverse the meter and use a positive supply.

the orbital beam hexode glass envelope anci try to guess which is the grid, filament and cathode lead. At this point enter Frank Jones, Build up and use one of his transistor testers and the door of knowledge is opened. No longer do you care if you can t see inside. Everything becomes orderly. PNPs go into one box and NPN's into the other, The duds are set aside for Psuedo-

Zener use. The sorting process turns into an exhilarating game. Plug them in, test, pull "em out. You don t mind that after squeezing the three little wires between thumb and forefinger for the hundredth time, you have an excruciating cramp in the thumb! But wait; now that these wonderous things are sorted, how do you know which of them is "Power", "RF" "IF" ' Audio". "Switching" and no test? Frantic thumbing through all issues of 73 from January 1962 forward was of little help for a solution to this new dilemma- In fact it was a hinder-ante, You find far too many tube articles which you remember wanting to try! Anyway, after having invested so heavily—at least $10 so you can take advantage of the double bonus—it is incumbent upon you to take the next step to find out which of these little three-pronged rascals will oscillate, A circuit to help sort the unmarked transistors further is shown in Fig, l.3

This is a Clapp transistor oscillator, either crystal or series L-C resonated. As shown, the battery and meter are set up for PNPs, Change the polarity to check NPN's, By using several crystals, for example, 450kHz, 2MHz, 3.5MHz and 7MHz or higher, you can learn which of the 1001 or more bargain transistors in your possession will oscillate, It sure takes a lot of plugging but it s worth it. Imagine the thrill that shivers through your frame as you hear that crystal clear note in the station receiver. It takes you back to the days when—for old-timers-after you sawed iwo hacksaw slots at right angles across the bakelite base of a type 76 triode, between the four prongs, so as to decrease the dielectric shunting, you gof the

Fig. 2. Conversion of the BC-9Û6 frequency meier to detect rf signals down to a J 00 microvolts or so and

tube to work at meters! For the younger man the occasion might be similar to uncorking his first 807 on 10 meters. At any rate, the yield of good oscillators from the packet of 100 bargain items is surprising. Many were vigorous to 7.3MHz. How well they will work in other circuits remains to be learned. It mav be safe to assume that some of them will perform as rf amplifiers; how noisy they are can be learned from trial and error unless one uses the more modern transistor testers which are available at much greater cost than the simple ones referred to in this work-

I lie field effect transistors are indeed fascinating. It boggles the mind to think what would have happened to the radio-radar-sonar systems had the solid state version of the tube arrived first. We'd probably be discovering the vacuum lube about now and that would be a blessing to those of us who need bi focal glasses in order to work with these miniature components. Jim's FET Primer caused me to try the FET s offered by Po 1 y-Paks. Again, the ones received are unmarked except that one is painted an ominous black. The advertisement seems to say that mine are low noise KET's made by Crystalonics. The package has a nice re-print from 73 and also some data which leads me to believe that one of the FETs is a C-610. Since my surplus BC-906E Frequency Meter had a 1S5 tube which went west, it was a logical choice to go FET. The original and modified circuits are shown in Fig. 2.

The FET can be plugged into the tube socket for experimentation, then later soldered in il you so desire. The schematics show he tube socket pin numbers in which to plug the FE~ leads. It is necessary to remember that when looking at the bottom of the FET, the base lead is where the collector lead normally is located on a standard transistor. (Not on alt FET*s. Consult manufacturer's data sheet if in doubt. Ed.)

The absorption wave-meter with the FET performs much the same as 1 he original lube version. One less battery is required and the battery voltage will depend upon the FEH used. The FET which [ assumed to be a C-filO works well with 1.5 volts* When the meter reads 500 micro-amperes full scale, (he measured drain current is 1.5 milliam-percs. As the cavity is tuned through a two meter rf field, the drain current will dip just as it did with the vacuum tube in the circuit.

The sensitivity may be improved by leaving out the 1N34 diode and disconnecting the 1.5 megohm resistor from the end of the radio frequency choke nearest the Hi-Lo switch. These modifications are indicated in Fig. 2 by the heavy lines. Then, with the rf lead from the cavity connected to the gate of the FET, the field effect transistor will act like an old-fashioned vacuum tube with a floating, leaky grid,

With this modification the frequency meter will respond to 100 microvolt signals fed into the cavity by way of the plug-in antenna. The fact' that the FET will perform in this manner is a measure of compensation and a bit of solace to a hard vacuum tube man,

WA6BSO, *'Field Effect Transistor Primer," 7Bt December 1965.

2. W6AJF, "Two Transistor Testers/1 73, Septt-mlxjr 1966.

3. M. S. Kiver. "Transistors/' 3rd edition, 1962, Mc c,raw-Hill Book Company, New York,

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