Service manual

STP 11-25R13-SM-TG
E - 2
E-2. Collector Voltage.
The two extreme cases are full V
CC
voltage at the collector and zero collector voltage. Both can
prevent signal output, but for entirely different reasons.
a. Zero collector voltage means that the DC path for collector current to the V
CC
supply is open.
Look for an open collector load resistor or an open decoupling resistor (if there is one present in the
circuit).
b. Full V
CC
present at the collector means that the collector circuit is not open.
(1) In many amplifiers with a collector load with very little DC resistance, the collector voltage is
often practically equal to the full V
CC
voltage.
(2) The reason for full V
CC
voltage on the collector could be a shorted collector load resistor. If the
collector load resistor is good but there is no voltage drop across this resistor and full V
CC
voltage is
present on the collector, then the most common causes are as follows:
(a) A defective transistor.
(b) An open emitter circuit.
(c) The bias voltage is such that it has reverse-biased the transistor.
E-3. Base Voltage.
When the base voltage is such that it reverse-biases the transistor, there is no collector current and
therefore no output signal. Generally, this is due to a short or an open in one of the components in the
base circuit, but it could be a direct result of an open or a short in the emitter-base junction.
E-4. Emitter Voltage.
With emitter bias, the emitter resistor can be open or shorted, or its bypass capacitor may open or
short. When the emitter bypass capacitor shorts, the emitter voltage becomes zero if the capacitor is tied
to ground. If the emitter bypass capacitor opens, then degeneration occurs with a resultant loss of gain.
The most obvious trouble would be an open emitter resistor. With the emitter circuit open, collector
current cannot flow and there is no output. The collector voltage now equals the full V
CC
value.