Service manual

STP 11-25R13-SM-TG
P - 3
c. Other components, including R3 and R5, could cause trouble as well.
(1) If resistor R3 were to open, then the base of Q2 would be less positive than before. This would
cause Q2 to approach cutoff and the collector voltage of Q2 would increase.
(a) This would cause transistor Ql to become more forward-biased, thus causing its voltage drop to
decrease. More of the input voltage would be passed on to the load device, so the output voltage would
increase.
(b) The same sequence of events would occur if R5 were to short. As with any short, a burnout
could occur in another component, possibly R3 or R4 since R5 is in series with those resistors.
(2) If R5 were to open, then the symptoms would be opposite to the ones in paragraphs (1) (a) and
(b). The base of Q2 would now be more positive and Q2 would approach saturation.
(a) The collector voltage of Q2 would decrease and the forward bias of Ql would decrease. Thus,
Ql would drop more voltage and so the output voltage would have to decrease.
(b) If R3 were to short out, then the symptoms would be the same as the ones for an open R5, with
the addition of a possible burnout of another component, such as R4 or R5.
d. Either of the transistors could also cause the circuit to malfunction. The exact symptoms would
depend on the exact location of the malfunction, an open or a short.
(1) For example, consider a shorted emitter-base junction in transistor Q2. If this were to happen,
the emitter current of Q2 would flow toward R4 and R3 instead of toward the collector of Q2 and then
through resistor R2.
(a) As a result, R2 would drop less voltage and the base of Ql would be more positive than before.
This would drop less voltage and the base of Ql would be more positive than before.
(b) This would increase the forward bias of Ql and the voltage drop of Ql (E
Q1
) would decrease.
The output voltage would be higher as a result.
(2) On the other hand, a short in Q2 from the emitter to the collector would cause different
symptoms. The collector voltage of Q2 would decrease as a result of the short. This less positive
collector voltage would be applied to the base of Ql and the forward bias of Ql would decrease. Thus, Ql
would drop off more of the input voltage and the output voltage would be lower than before.
(3) Now look at an example of what would happen if an open occurred in Q2. If the open were to
be in either the emitter lead or the collector lead, the path for current flow through Q2 would be
interrupted.
(a) Therefore, resistor R2 would also get less current than before. So, the collector voltage of Q2
would have to increase and this would cause the base of Ql to become more positive.
(b) This would increase the forward bias of Ql and E
Q1
would decrease. As a result, the output
voltage would increase.