Sencore TF151 - FET Troubleshooting Chart; Checking FET Gate Leakage (IGSS); IGSS Leakage Measurement

r
TROUBLE
Short, gate to Source
Short, gate to Drain
Short, Source to Drain
Gate open
Source open
Drain open
N Channel set for P channel
or P channel set for N channel
Gate and Source leads
interchanged
Gate and Drain leads
i ntercha nged
Source and Drain leads
interchanged
BIAS switch in wrong position
FET TROUBLES
AS APPEARS ON TF1Sl
Meter can be zeroed but no Gm reading
Meter can be zeroed, but needle vibrates
rapidly and changes on ly slightly when GAIN
button is depressed
Meter can be zeroed but needle vibrates
rapidly - no Gm reading when GAIN button is
depressed
Meter can not be zeroed and needle wanders
around
Meter can be zeroed but no Gm reading when
GAIN button is depressed
Meter can be zeroed but no Gm reading when
GAIN button is depressed
Meter can be zeroed but needle vibrates
rapidly. Meter reading is negative when GAIN
button
is depressed. Leakage position gives
full scale deflection of meter .
Meter can be zeroed but Gm reading is in the
negative direction
Meter can be zeroed but Gm reading is in the
negative direction
Normal operation
No Gm reading on enhancement types.
Depletion types may zero and give a Gm
r eading, but
meter pointer vibrates ra-
pidly indicating excessive current drain
from power supply.
CHECKING GATE LEAKAGE (lGSS) OF FIELD EFFECT TRANSISTORS
Gate leakage (lGSS) is a very important parameter in an FET. It is similar to . th e
grid leakage in a vacuum tube and can cause somewhat the sam e type of problems
in the circuit. In IF Circuits, it can upset the gain of the system, and caLise AGC
problems. The leakage actually lowers the input impedance of the FETandcan cause
unbalance of bridge amplifiers using the FET . The TFl Sl can check the gate leakage
of a junction or insulated gate rET and can also check both gates of a dual gate FET .
The leakage should be checked out of circuit as the circuit impedances may cause a
leakage reading on the meter, even though the FET has no leakage.
1. Connect the Source, Drain, and Gate leads as described before.
2. Set the function switch to the LEAKAGE (lGISS) position on the Field Eff ect
Transistor side of the TFl51 and the TYPE switch to the desired type,
'N' or 'P' Channel.
3. Read the leakage in microamps directly on the red LEAKAGE scale of the
meter.
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