Troubleshooting - Sony KV-32FV16 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.
AA2U Chassis Power Supply Output Voltages
Variable AC Input
*Oscillator begins to be unstable at this point.
Resistance Measurements
Another method that can be used after a power supply repair is to take a
resistance measurement on the connectors that connect to CN641 and
CN642. The points measured should be the 135, 12, 9 and Audio B+
pins. These points should measure similar to a capacitor. The resistance
Primary (Hot Ground)
Oscillator
Voltage
80 Vp-p
30Vac
120 Vp-p
40Vac
150 Vp-p
50Vac
60Vac 150 Vp-p
45
When performing this procedure set your variac to zero and jumper 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 oscillation 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 oscillator voltage and frequency are measured at T603/2. The sec-
ondary voltages can be easily checked at CN641 and CN642. Increase
the line voltage by ten volts and check each item in the table again. Re-
peat 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 Converter B+
at T603/2. This is where the switching waveform would normally 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 component re-
lated to IC601-2.
Fully unloaded (CN641 and CN642 unplugged)
Secondary (Cold Ground)
Oscillator 135 12
Frequency
145 kHz 29.7V 2.7V
145 kHz 39.5V 3.7V
147 kHz 46.1 4.3V
147 kHz* 56.2V 5.3V
measurement is low and slowly rises. Any steady low resistance mea-
surement is an indication that there is a short or excessive load in one of
the circuits that is fed by that power supply line. This problem will need to
be fixed before the power supply is reconnected and powered up.
9 Audio B+
.49V 18.9V
3.4V 21.3V
3.9V 21.3V
4.6V 22.4V