Sony KDP-57XBR2 Training Manual page 13

3. RA-3/3A Power Supply Troubleshooting
No AC Current Is Read: You have a supply that will not start. This is not common, but can occur. Bring the AC
voltage up to 40 volts. It is recommended to not exceed this level just in case you accidentally cause the
oscillator to start while probing around with the scope and meter. The first important step is to measure the
voltage at the collector of the upper transistor. There should be a DC voltage present that is approximately twice
the AC voltage due to the doubling circuit. The next point to read is the collector of the lower transistor. Since
both transistors are biased slightly above cutoff, one half of the voltage level that is read on the upper transistor
should be here. Both transistors are identical and so are their bias resistors. Consequently, this comprises a
voltage divider. This is an excellent test to see if the bias resistors are good and both transistors are able to
conduct. If the voltage is significantly higher or lower at the mid point, possible causes are an open bias resistor,
shorted feedback coupling capacitors, shorted C612 (this would cause the same voltage to appear at both
collectors), or a defective IC.
If the DC voltage is being divided properly, there is likely a problem with the feedback circuits, an open winding
or solder connection problem at either transformer, or C602 could be open. It is possible that feedback capacitors
C620 and C621, or R617 and R619, are open since they handle significant current during their operation.
AC Current Rapidly Rises With Low AC input: This is where close attention will be required. The original cause
of the switching IC failure is still there and needs to be located. Since you have checked for static shorts on all
the lines, there is a possible dynamic load problem and the power supply needs to be running in order to locate
it. By keeping things under control and monitoring AC current, the failure of the switching IC can be prevented.
This test can be safely performed even if the horizontal output transistor is shorted.
The cause of the over-current problem needs to be located and it will be in one of two places: either the primary
side or the secondary side. Since problems in horizontal and high voltage circuits are the most common, a handy
trick is to disable the horizontal section by unsoldering the horizontal output transistor and re-applying AC voltage.
If the current problem is gone (the current will not go much higher that 250ma with the horizontal circuit
disconnected), the over-current source has been exposed and will be dealt with in the section on the deflection
circuits.
If excessive current still exists, its simply a matter of disconnecting secondary lines and trying to re-apply power.
If disconnecting secondary lines does not resolve the issue, the problem is in the primary circuits. The switching
transistors are either overlapping (conducting at the same time) or there could be a problem with T604 or T605.
In almost all cases, one of the feedback coupling capacitors (C620 or C621) is leaky. This is more likely to be
found in older units or ones that have incurred a large line transient. Watch the waveform at T604 pin 2. It should
be a relatively symmetrical square wave with a 50/50 duty cycle. Any evidence of ringing could indicate a shorted
winding in either of the transformers.
The Soft Start Circuit Check: As mentioned earlier, this circuit can cause switching transistor failures. Since the
supply was brought up slowly with the variac, the soft start has already been performed. The set has been
running for a while now and you are confident that everything is OK. The back cover is put back on and just to
check one more time, the set is plugged in and turned on and the power supply fails again. The set may turn on
just fine but be returned anywhere from a few days to even weeks later with the power supply gone again. While
other problems could be causing this, it is a wise idea to check the soft start circuit on any power supply in which
you have just changed shorted switching transistors to avoid that dreaded call-back from showing up. Here is a
very quick way to check the circuit:
Bring the unit up slowly to full AC with the variac. Set the meter to read AC current, if necessary. While leaving
the variac at full 120VAC, turn the unit off with the remote or front power button. Wait about two or three seconds
and turn it back on while closely watching the AC current. It does not matter what the current level is since this
will vary from model to model and the quality of your incoming line voltage. What you should see in about three
seconds is a rapid increase in current of about 200 to 300ma from the initial current reading. That was the soft
start circuit at work. If the current does not jump, find out why the circuit is not working. Suspect a dried-out
electrolytic capacitor C673, a shorted B-E junction or an open Q653 or shorted Q656.
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