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6-27
6.7.6 Digital Eel ICs
1. The device is operating correctly if the output high
state is - 0.81 to - 0.96V and low state is - 1.65
to -1.85V.
2. The input must show the same two levels as in
step 1.
6.7.5 Capacitor
1. Shorted capacitors have OVacross 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.
3. Leaky capacitors will often have a decreased volt-
age across their terminals.
6.7.4 FET Transistor
1. No gate current should be drawn by the gate of an
FET transistor. If so, the transistor is defective.
2. The gate-te-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.
6.7.3 Operational Amplifier
1. The
11+"
and
II -"
inputs of an operational
amplifier will have less than 15 mV voltage differ-
ence when operating under normal conditions.
2. When the output of the amplifier is connected to
the
I I -
"
input (voltage follower connection), the
output shouId be the same voltage as the
I I
+ "
input voltage; otherwise, the operational amplifier
is defective.
3. If the output voltage stays at maximum positive,
the
I I
+ "
input voltage should be more positive
than
II -
"
input voltage, or vice versa; otherwise,
the operational amplifier is defective.
6.7.2 Diode
A diode (except a zener) is defective if there is greater
than one volt (typically 0.7 volt) forward voltage
across it.
forward voltage across its base emitter junction
should be off (no collector current); otherwise one
of the transistors is defective.
6.7 TROUBLESHOOTING INDIVIDUAL
COMPONENTS
6.7.1 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 emitter
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 with less
Figure 6·34. Auxiliary Output Amplifier
Waveforms
0.0 Vdc
U10 PINS 1,2,6 AND
7-----
-5.0V
+5.0V
+2.5V
-2.5V
U9PINS2AND3
~--~--
/;
~--~--
I I
I
U9PIN6
~_-=
Figure 6·33. AM level Shifter Waveforms
U11 PINS 2 AND 3
- - 0.0 Vdc
U11 PIN 6
/\...--7'.... - - - -
-2.5V
/ '\L- ~ --
-5.0V
AM
Full cw
Full ccw
Off
AM
Triangle
AUX GEN MODE
WIDTH/dF/dM
AM CARRIERLEVEL/NULL
SUPPRESSEDCARRIER
FUNC (Main Generator)
AUX GEN FUNC
Setting
Control
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Troubleshooting
