Troubleshooting - Sony WEGA KV27FV15 Training Manual

Troubleshooting

One of the most common failures in switching power supplies are the
converter transistors or in the case of this power supply, IC601. IC601
actually contains two identical transistors. They are assembled in this
package so that their gain and other characteristics are closely matched.
Generally when these transistors fail, they fail by shorting C-E in one or
both of the transistors. This causes the fusible resistor to open due to
excessive current draw. The fusible resistor in this case is R607. The
symptom in this set is that you hear only one relay click on initial power up
instead of three. Usually you would hear two, the power relay followed by
the degaussing relay. This would be followed by another click about ten
seconds later that indicates the degaussing relay is turned OFF. You can
check the voltages around IC601 to troubleshoot this problem. There
would be no voltages present since R607 would be open due to the shorted
transistor.
You should always check the horizontal output for shorts when the con-
verter transistor has failed. It is also a good idea to unload the power
supply and bring the line voltage up slow using a variac.
A A 2W C has sis P o w er S up p ly O u tp u t V o ltag es
V ariab le A C In p u t
30V ac
40V ac
50V ac
60V ac
P rim ary (H ot G round)
O scillator
V oltage
80 V p-p
100 V p-p
130 V p-p
150 V p-p
17
When performing this procedure, set your variac to zero. Jump the relay
contacts. Warning: By jumping the relay contacts you have defeated
the protection circuitry of the power supply. Plug the unit into the
variac and bring the AC voltage to 30 VAC. You should have stable oscil-
lation at this point. Check to see how much current is being drawn from
the AC line. The current drawn should be minimal since the supply is
unloaded. Check the voltages at the points in the table below. The oscil-
lator voltage and frequency are measured at T603/2. The secondary
voltages can be easily checked at CN641 and CN642. Increase the line
voltage by ten volts and check each item in the table again. Repeat this
procedure at 50 and 60 VAC. Warning: It is not recommended that the
power supply be operated unloaded above 60VAC.
Another problem that may occur is that the switching never starts. To
troubleshoot this type of problem, remove one lead from C615. When
you power the set ON, you should have a DC voltage of half the Con-
verter B+ at T603/2. This is where the switching waveform would nor-
mally be present. If the voltage is lower at this point, suspect an open
related to IC601-2 or a leaky component related to IC601-1. If the voltage
is high at this point, suspect an open related to IC601-1 or a leaky compo-
nent related to IC601-2.
F u lly u n lo ade d (C N 641 and C N 642 un p lu gg ed )
S econda ry
O scillator 135
F re quency
145 kH z 48.7 V 14.5 V
145 kH z 63.0 V 18.5 V
147 kH z 63.2 V 20.0 V
U nstable 69.0 V 22.2 V
(C old G round )
12 9 5
5.0V -3.6V
5.8V -3.7V
6.3V -3.8V
6.8V -3.8V
A ud io
B +
4.5V
5.8V
6.1V
6.9V