Sencore TF151 - Checking Regular Transistor Leakage (ICBO); Leakage Measurement Procedure; Testing Field Effect Transistors (FETs)

TROUBLE
Base to
Emirter short
Base to Collector short
Emitter to Collector short
Collector and Base leads
interchanged
Emitter and Collector leads
interchanged
Base and Emirter leads
inter cha nged
AS APPEARS ON TFlSl
TFlSl will BETA CAL, but TYPE switch must be
in wrong position. Beta reading will be all the way
to
left or greater than infinity with meter pointer
vibrating.
TFlSl will BETA CAL, but when GAIN burton is de-
pressed, meter indication will not change .
If there is a dead short, TFlSl will not BETA CAL.
If there is a low resistance short the TFlSl may
BETA CAL, but the meter needle will vibrate ra-
pidly when checking beta.
TFlSl will BETA CAL, but meter reads to right
(a beta of less than one).
TFlSl will BETA CAL, but meter may read to right
(a beta of less than one) indicate a very low beta
figure. A few transistors may read the same as
they are made to have the Emitter and Collector
leads transposed.
Transistor will BETA CAL as oppOsite polarity tran-
sistor and no Beta reading is obtained when GAIN
button is depressed.
Sometimes if a transistor is shunted with another transistor or diode directly across
one of the junctions, it may indicate as an open or shorted transistor. If there is any
doubt, disconnect one or two of the leads and recheck the transistor. In this case,
the base lead is the best to start with.
If a transistor checks defective in-Circuit, but good out of Circuit, be sure to check
the other components in that circuit for defects, especially capacitors and diodes.
It is a good idea to also check the other components in the circuit when the tra nsistor
checks bad both in and out of circuit. Very likely, the transistor defect was caused
by the failure of some other component.
CHECKING REGU ~ AR TRANSISTORS FOR LEAKAGE (ICBO)
The leakage between the base and collector (ICBO) is one of the most important para-
meters of the transistor. This must be measured out of the circuit to prevent the
circuit components from affecting the reading.
1. Connect the E, B, and C test leads to the transistor as previously described .
2. Set
the Function switch to the LEAKAGE (lCBO) position on the regular transistor
section and the Type switch to the desired type, NPN or PNP.
3. Read the leakage in microamps directly on the red LEAKAGE scale of the meter.
TESTING FIELD EFFECT TRANSISTORS
The Field Effect transistor is fairly new in the electronic industry but its use has
become quite wide spread. It is unlike the regular or bi-polar transistor that op-
erates on a current principle and has its gain measure in current (beta). The 'FET
is very much like a vacuum tube, it has high impedance input, is voltage operated,
and its gain is measured in micromhos. The chart below gives a comparison be-
tween the differeht devices.
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