TRANSISTOR FAULT FINDING
With R1 o/c there is no voltage on the base, no base bias and no
base current.
The transistor is OFF and there is no collector current and no
voltage drop across R3.
The collector voltage will be the same as the supply volts, +9.
There is no current through R4, so there is no voltage drop
across it.
The voltage at both ends is the same, zero volts.
There is no potential divider action and the full +9 volts is applied
to the base by R1.
This causes a high base current to flow, which in turn causes a
high collector current to flow.
There is a large volts drop across R3, causing a low collector
volts.
There is a large volts drop across R4, causing the emitter volts to
rise.
The base is still forward biased, so base current flows.
The base current will be higher than normal, because all the
emitter current goes to the base and none to the collector.
Since there is more current through R1, there will be a higher
voltage drop across it, and the base voltage will be lower.
Since the emitter voltage is always about 0.6 volts lower than the
base, it will read 0.1 volts.
We would expect the collector voltage to read zero, but we are
actually reading the base voltage via the base/collector junction
and it will read about 0.6 volts lower than the base.
There can be no base or collector current.
Since the current through R1 is less than normal, there will be a
lower voltage drop across it and the base voltage will be higher
than normal.
Since there is no current through R3 there will be no voltage
dropped across it.
The voltage will be the same at both ends, and the collector will
be at the supply voltage.
We would expect there to be no voltage across R4, but the meter
connects the emitter to zero volts and some current flows through
the meter.
The voltage indicated would depend upon the type of meter.
The emitter is connected to zero volts and there is an increased
emitter/base forward bias.
Base and collector currents increase.
There is more current through R1, a higher voltage drop across it,
and a lower base voltage.
Since the collector current is higher there will be a larger voltage
drop across R3.
The collector voltage will be lower than normal.
The emitter voltage will of course be zero.
If the capacitor goes open circuit, the dc voltages will remain at
their normal values.
However, the gain will be much reduced.