Section 6 Troubleshooting; Factory Repair; Troubleshooting Tables; Transistor - Wavetek 182A Instruction Manual

6.1

FACTORY REPAIR

Wavetek maintains a factory repair department for
those customers not possessing the necessary per­
sonnel or test equipment to maintain the instrument. If
an instrument is returned to the factory for alignment
or repair, a detailed description of the specific prob­
lem should be attached to minimize turnaround time.
6.2

TROUBLESHOOTING TABLES

Table 6-1 gives an index of the troubleshooting tables
by indications of common problems. The tables do not
cover every possible trouble, but, when used in con­
junction with circuit descriptions and schematics, will
be an aid in systematically isolating faulty components.
6.3
TROUBLESHOOTING INDIVIDUAL
COMPONENTS
6.3.1

Transistor

1. A transistor is defective if more than one volt is
measured across its base-emitter junction in the
forward direction.
2. A transistor when used as a switch may have a
few volts reverse bias voltage across base­
emitter junction.
3.
If the collector and emitter voltages are the
same, but the base emitter voltage is less than
500 mV forward voltage (or reversed bias), the
transistor is defective.
4. A transistor is defective if its base current is
larger than 10% of its emitter current (calculate
currents from voltage across the base and emit­
ter series resistors).
5. In a transistor differential pair (common emitter
stages), either their base voltages are the same
in normal operating condition, or the one vvith
less forward voltage across its base emitter
junction should be off (no collector current);
otherwise, one of the transistors is defective.
SECTION
TROUBLESHOOTING
6.3.2

Diode

A diode (except a zener) is defective if there is greater
than one volt (typically 0.7 volt) forward voltage
across it.
6.3.3

Operational Amplifier

The " +" and .. - " inputs of an operational
1 .
amplifier will have less than 15 mV voltage dif­
ference when operating under normal conditions.
2. When the output of the amplifier is connected to
the" - " input (voltage follower connection), the
output should be the same voltage as the " + "
input voltage; otherwise, the operational ampli­
fier is defective.
3. If the output voltage stays at maximum positive,
+
the input voltage should be more positive
" "
than the" - " input voltage, or vice versa; other­
wise, the operational amplifier is defective.
6.3.4

FET Transistor

No gate current should be drawn by the gate of
1.
an FET transistor. If so, the transistor is defective.
2. The gate-ta-source voltage is always reverse
biased under a normal operating condition;
e.g., the source voltage is more positive than the
gate voltage for 2N5485, and the source voltage
is more negative than gate voltage for a 2N5462.
Otherwise, the FET is defective.
3. If the device supplying gate voltage to an FET
saturates, the FET has too large a Vgs (pinch off)
for the circuit and should be replaced.
6.3.5

Capacitor

1. Shorted capacitors have zero volts across their
terminals.
2. Opened capacitor can be located (but not always)
by using a good capacitor connected in parallel
with the capacitor under test and observing the
resulting effect.
6
6-1