Sencore TF151 - Page 21

1l1e amount of AC signal applied to the transistor by Tl is controlled with the llETA
CAL control R 1. It is adjusted for each tranSistor under test so that the average
collector current will be .2 milliamps. 2 milliamps. or 20 milliamps depending upon
the setting of the FUNCTION switch. 1l1e . 2 rnA is for low saturation type tran-
sistors. the 2 rnA for low power transistors. and 20 rnA for power transistors for
a more accurate check. The average collector current is indicated on the meter and
is adjusted for the proper value by setting the meter to full scale with the llETA CAL
control at the lev el selected by the FUNCTION switch. When the GAIN button is
depressed. the meter is transferred to the base circuit to measure the average base
current. In the Xl ranges. the meter has the same sensitivity and will measure . 2.
2. or 20 rnA of base current. In the X 10 range. the meter is 10 times more sensitive
and will measure one tenth of the collector current as full scale base current. Zener
diodes CHI am.J CH2 are used to keep the applied Ae signal from changing due to line
voltage variations. Capacitors C3 and C4 in the collector and base circuits provide
the "short circuit" to the AC signal that is required in making the AC beta measure-
ments. 1l1e TYPE switch S2 controls the meter polarity and applies the correct
polarity of voltage to the transistor in th e leakage test. Capacitor CS prevents the
transistor under test from developing spurious high frequency oscillations.
Leakage is measured by applying four volts DC between the collector and base and
measuring the current that flows. 111e DC voltage is developed from transformer
T2. diodes CHS and CR6 and is regulated to six volts by zen er diode CR7. The four
volts for the leakage test is divideddown from the six volt supply by R6 and RS. R4
and CR4 are in parallel with the meter and are used to set up the full scale deflection
of 5000 microamps on the 200 microamp meter. 1l1e nonlinearity of the diode and
series resistor allow the meter to read directly for the first one half uf the scale;
1l1en the diode begins to conduct and shunt some of the current around the me ter giving
the nonlinear Leakage scale on the meter. 1l1is reduces the need for switching to
various current ranges and reduces the confusion . 1l1e leakage section is the same
for both regular transistors and field effect transistors. 1l1e function switch selects
the test so that the voltage is applied to the different elements of the transistor. and
the TYPE switch selects the correct polarity of voltage to be applied as well as the
correct met er polarity .
1l1e
field effect transistor test section is independent of the regular transistor test
section . 1l1e field effect transistor or FET must be checked just like a vacuum tube.
for Gm. The unique system used in the Sencore TFl51 consists of a bridge arrange-
mentconsisting of TRI. TR2. RIO. Rll. H12. and R14. 1l1e transistors act like a
switch and are turned on or off by the AC signal applied from the transformer T2.
With this system connected between the drain and source of the FET. Rll. the Gm
ZERO controL is adjusted so that equal currents flow in the bridge and the meter
reading is zero. 1l1e impedanceof the bridge system is kept very low and the voltage
applied to the drain of the FET under test is held constant to conform to the Gm form-
ula so that accurate Gm measurements can be made. '!lle signal developed by the
other secondary of T2 is a constant amplitude square wave formed by diodes CRI0.
CRII and resistor 1\16. 111 e signal is divided down by R17. R18. and R19. R17 is
adjusted so that the signal app.1ied when the function switch is in the Gm Xl range is
.3 4 volts peak to peak. In the Gm XIO range. the signal is taken from a pr e cision
resistor and is . 034 volts peak to peak to gi ve a ten times Illultiplier up to 50.000
micromhos. When the square wave signal is applied to the gate of the rET under
test. it causes it to conduct heavier on one half of the cycle than the other. 111is
causes an unbalance of current in the bridge circuit and the meter will read the un-
balance. 1l1e amount that the FET will conduct is dependent upon the Gm of the de-
vice and therefore the meter can be calibrated in Gm. Switch S6 selects either
NORM or reverse hias for depletion type FETS or POS or forward bias for enhance-
ment type rETs for proper testing . 1l1e three prong line cord is connected so that
the case is automatically a t ground potential 10 conform to Underwriters Laboratories
specifications. " 111is is also done to help prevent damage to the insulated gate type
rETs when checking them out of circuit.
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