An R.F. Transistor Testor

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An R.F. Transistor TestorBY FRED BROWN,* W6HPH

I F you do any solid-state experiment ing youhave probably accumulated a ciga r-box full ofmiscellaneous tran sisto rs of all descriptions including some un -marked or undecipherabletypes. This gadget lets you determine in a matter of seconds whether an unknown transistoris aud io, r.f., o r v.h. f, It will test either NPNo r PNP bipo lar transistors at the fl ick of aswitch and is just as adept at testing junct ionFET's. Co nstruction time would normall y beone or two evenings, and if junk-box parts aremade use of, the cost can be held to a fewdollars.The pr inciple of opera tion is very simple :the transistor to be tested is placed in a frequency-switchable oscillato r circui t and therelat ive strength of oscillation is indicated onthe meter. Since any transistor that can osci llate a t a given frequency will also be able toamplify at that frequency, measuremen t o f thehighest oscilla tion frequency (F n. . . ) is also amea sure of the highest amplification frequency.Norm all y, o f course, transisto rs are ' used atfrequencies well below F m . . since at even on efourth this frequency a maximum power gainof only 12 db is ava ilable. Thus only a roughindica tion of the upper frequency limit isneeded ; in this instrument the freque ncy rat iobetween switch positions was made about 3 toI . Continuous frequency coverage could havebeen provided, but at the expense of unnecessarily complicating the instrument. With sixswitch posi tions an over-all frequency range of190 to I is covered.- 1950 Sunset Drive. Vista, CA 92083- -

Front view of the d . tran sistor tester showingcontrol locat ions. Slide switches are used for OnOIf, 8i-Polar/FET. a nd PNP/NPN se lection, whilea 6-posit ion rota ry switch is used for frequencyselection. Between the slide switches and thefreq uency switch is a socket for the transistorunder test.

CircuitAs can be seen from the diagram , the transistor to be tested is placed in a rat her conventional tuned-collector oscillato r circuit whichcan be switched in frequency by means o f SI'The transistor is biased by mean s of resistorsRI , R:, a nd R 3 Th e values of the se resistorswere chosen to make the co llector current fa llbetween I and 1.5 rna almos t irrespective of

tr ansistor cu rrent ga in, Or whether the transistoris a Germanium or Silicon type. Practically anytransistor worth its salt will have substantialpower ga in at one mil co llecto r current. Junction type FETs are accommodated by simplyswitching out the forward-bias resisto r, R I Thela rge values of the three biasing resistors prevents damage to any tran sistor that should happen to be plugged in the wrong way or opera tedwith the wrong polari ty. The po la rity. reversingswitch, S'l> permits measurement o f either NPNor PNP (o r N-channel o r P-channel ) tran sistors.Th e r.f. voltage on the emitter (o r source)of the oscillator is coupled to the IN914 diodeby the 500 pf capaci tor and the rect ified voltageis ind icated on the 0-1 rn a meter.Feedback on the two highest frequency positions is provided by the 5 pf capacito r connected between the collector and emitter. Onthe four lower frequencies additiona l feed backcapaci tors are switched in between co llectorand emitter and between emitter and ground.

ConstrudionTh is part icular un it was built in an emptyfilm box measuring about 3 by 3 by 9 inches.With ca reful layou t you could get by with abox about half tha t volume. It is important thatr.f. lead s be kept sho rt so the tran sistor socketshould be mounted close to the frequency selector switch.This plastic box provided no chassis "g round"so a 3 inch square piece of tin plate (cut froma tin can ) was placed under the top panel. Thisplate extends over to the tran sistor socket andall "ground" connections are soldered directlyto it.For good v.h.f, performance the .02 mf basebypass capacitor and the 5 pf feedback capacitor should have leads no longer than JI.z inch.The .02 mf disc capacito r between the supplyvoltage and ground should also have short leadsand should be mounted at the low end of 1.,-

Apr il, 1975 CQ 35


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500

1500

, wide, 1%"

l .

%" long., fo rm., form.[, - 62 ph, Miller 4630 or eq uiv .

Fig . l -S chematic of the d . tra nsistor te ster. All ca pacitors are d isc ceramic. Values less tha none a re in mf, va lues greater tha n one are in pf. Resistors are lA wa tt.

OFF 5. 511 I~ 8' POLAR O FE T l , 15 l ,: : 9v. I- I S, l .I ! l .I 22 !1. R, ~ f - -2K 91130

Transistor l ,R, und test rJ- liq0K C/ ;7102 Xl 10 3901100, ~ 390 IE , ,R, , S. ,- ~ __ _____ _ -1 IK 5500 l l -Ha irp in loop, # 18 bore wire, \>'2K long.

~ L- 313 t- 1N914 lz-3 turns # 18 ai r wound, %" Ld.,0 1 mol \.. .001 4 -2A turns # 18 e c1osewound on %'4 - A9 tu rns # 26 e c1osewound on %'-" l s-2Aph, Miller 4626 o r equiv.The highest frequency coils can be mounteddirectl y on the switch termina ls. Lead lengthson the J mHz and 3 mHz co ils are not important and they can be mounted anywhere convenient.

Ad justmen tWhen construc tion is completed. place aknown-to-be-good bipola r transistor in the socketand set the NPN-PNP switch to the appropriateposition . It's probably best to start a t a lowfrequency position of the frequency selectorswitch. \Vhen the unit is turned on the metershould indicate about half scale. I f it doesn't.check the d .c. voltages on the transistor. Thecollector voltage should be 9 volts and the emi tter voltage about 2.5 volts with respect toground. The base to emitter voltage will be

about .6 volts for a silicon transistor and .2volts for a germanium transistor. I f you don' tmeasure these values you either have a bumtransistor or you have made a wiri ng mistakesomewhere.When the circuit is oscillating as indicatedon the meter. you can check the oscillatorfrequency. This is probably best done with agenera l-coverage communications receiver, butremember to beware of images and harmonics.I f the frequency comes ou t too low you can

raise it by removing turns from the appropriatecoil. If the frequency is too h igh, you can lowerit by shunting the co il with a sma ll capacito r.Your communica tions receiver will permitfrequency checks up to 30 mHz. For the 100mHz position you can use an f.m. radio tocheck frequency. The 100 mH z frequency canbe easily ad justed by spreading or squeezingthe coil. L ,.To check the highest fre quency position youcan use a TV set, 190 mHz will wipe out channel 9. The hairpin loop, L" can also be adjustedby squeezing or spreading. On this highest frequency switch position adjust the 3 to J3 pftrimmer across the IN914 diode for maximummeter reading.With careful construction you might be ableto shorten L , to the point where the highestfrequency is 300 mHz ra ther than 190. I t is

questionab le that this would be wise, however.because transistor lead lengths plus socket inductance would make the test unfair at such ahigh freque ncy; even 190 mHz is pushing thingsa bit.\Vith a good v.h.f. transistor the meter shouldread about the same on each po sition of thefrequency-selec tor switch. If the meter readstoo high on one switch position i t can be low-[CO l i t i ll lied 0 11 page 66]

36 CO April, 1975


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An R.F. Transistor Testor