Sony KDP-57XBR2 Training Manual page 14

3. RA-3/3A Power Supply Troubleshooting
B+ Regulator Troubleshooting
B+ Voltage Too High: This symptom can be a little troublesome to work with because the unit will be going into
shutdown protect and there is the possibility of damaging components (particularly the horizontal output transistor).
Once again, the main relay is going to be jumped, but this method works quite well if the proper steps are
carefully followed. Make sure that the unit is in the off mode. Turn on the set and let it go into shutdown. It is
important to follow this step. The main CPU must be kept in the "power on memory" mode. If it is not, the relay
on command will not go high and Q656 will hold the soft start circuit on and interfere with your diagnosis.
Jump the main relay RY601. Bring up AC voltage with the variac, while closely monitoring the 135V line. As soon
as 135 volts is reached, bring up the voltage to about 137V. It is OK to go this high since the protect circuit does
not even activate until 139V or more. The purpose of this procedure is to determine whether or not the regulator
stage recognizes the over-voltage condition. Read the voltage at T604/pin 7. There should be at least 12VDC
present. If not, the 18V line has failed coming from T605/pin 2. Failure of this line is rare so it should be there.
Next, read the voltage at T602, pin 8. It should be dramatically lower than pin 7. If it is not, the problem is being
caused by a lack of current through the control winding since IC654 is not pulling down on pin 7. Before you opt
for replacement of IC654, keep in mind that this IC is very reliable. Many have been changed needlessly. This
condition is usually caused by the B+ voltage sampling resistor(s) such as R668. It increases in value and the
regulator is fooled into thinking there is insufficient 135V.
If the voltage at T604/pin 8 is much lower than pin 7, either the control winding is open or IC654 is aware of the
153V being too high and is trying to correct the problem by inducing more current into the winding. Assuming the
winding is not open (extremely rare), all that is left is an oscillator section has drifted so far off frequency so as to
be "out-of-range" of the control circuit. This can be caused in older units by dried out capacitors. Any of the small
value capacitors in the primary oscillator can cause this, so it is recommended to replace all of them to get the
normal frequency back in range.
B+ voltage too low: This is a little easier to work with since the risk the damaging components is not present. We
know that excessive current is not causing the problem since the set is not going into shutdown. Simply read the
voltage at pins 7 and 8 of T604. If they are equal, IC654 is responding properly to the condition. Troubleshoot
the primary circuits for oscillator distortion or frequency shift. If pin 8 is lower than pin 7, and this will usually be
the case, suspect Q653 soft start is constantly on or shorted or IC654/pin 4 shorted.
NOTE:
On a properly working power supply, it is sometimes normal to be unable to reach full B+ level with the
relay jumped. It depends on whether or not the microprocessor is in the "on" state. If it is not, the relay high
command will remain low and Q652 relay drive will be off. This causes Q656 to remain on, holding the base of
Q653 low. This keeps the unit in "soft start" all the time. The purpose of Q656 is to discharge C673 at turn-off so
that there is always a soft start when the unit is turned back on. If the unit happened to be in the "power on" state
when last running, the B+ level will rise to proper potential with the relay jumped. This is very important to
remember when troubleshooting OVP problems as the soft start may kick in and interfere with regulator checks.
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