Shorted Switching Transistors; Regulator Troubleshooting - Sony KDP-57XBR2 Training Manual

8. RA-4/4A Power Supply Troubleshooting
Two rapid relay clicks followed by a third click one to three seconds later: If this condition occurs, the unit is going
into protect mode. The timer LED may assist you on this problem as this unit utilizes self-diagnostics although
not all protect lines are monitored. In this case, go to Chapter 9 for troubleshooting the protect circuits.

Shorted Switching Transistors

The greatest concern in this situation is what caused them to short. One important clue to watch for is whether
all four have failed. Since only two are on at any time, a failure of all four is a warning sign that transients may
have caused the failure. It could have been caused by other areas but line transient damage is likely to have
damaged more than just the transistors. Since the switching transistors are not biased in an AB class like
previous supplies, they are easier to troubleshoot.
Jumping the main relay and applying low AC voltage with a variac does not work well in this power supply design.
It will not start oscillating until 70VAC or more is applied. Since the soft-start circuit limits initial current by up to
50% and the FET drivers are able to withstand large current surges, direct shorts on any of the secondary lines
will not cause them to short. The supply will attempt to start and then the unit will go into over-current protect
before they can be damaged. Starting the unit with full AC power and observing for shutdown is necessary while
watching the initial AC current draw. This chassis operates at current levels of approximately 800ma, depending
on the AC voltage level.
Excessive current: In this case, the unit will shut down and AC current will peak at 2A or more. Something is still
wrong. Since it is natural for technicians to always check for shorts on secondary lines, the excessive load is
probably dynamic or, although remotely possible, one or more of the switching transistors are on when they
should not be. It gets down to disconnecting secondary lines at this point starting with the 135V being the most
likely cause. Do not forget to take resistance readings across the ± rails. Do not lift components on the 135V line
located before L6106. This will open the regulation loop and cause the supply to output maximum voltages.
If lifting secondary lines does not get rid of the problem, the cause is a short in one of the two transformers or one
of the driver ICs (IC6002/6004) is the cause. An effective way to test the drive and oscillator IC circuits is to
disconnect the drive signals to the gates of the switching transistors and short VCC switch Q6011 (not the main
relay). This will run IC6003, IC6002 and IC6004. Connect a dual trace scope to pins 1 and 7 of each of the drive
ICs and verify that square wave pulses are present and are 180 degrees out-of-phase with no overlap in timing.
If there are no pulses, look for DC voltage. If either of the above conditions is present, the problem has been
found. If both drive ICs have overlap or no pulses with DC, check IC6003 pins 9 and 10. If they have overlap or
output DC, troubleshoot the oscillator.
Very little or no current: The power supply is not running. This is a scenario likely to be encountered if the
switching transistors have been damaged from line transients. The gates of the transistors are very high impedance
and the transient has probably damaged one or more of the protect components. Notice the number of protect
diodes in this area. Leave the new switching transistors disconnected from the drive signals and short VCC
switch Q6011. The oscillator and drive ICs can now be serviced starting with the IC6003 and moving forward.

Regulator Troubleshooting

As illustrated in Figure 8-6, the regulator stage is quite simple in design. IC1601 monitors the 135V line. It acts
as an inverter so any rise in the B+ will cause the output to go lower and vise-versa. Controlling the brightness
of the LED inside the photo-optical coupler will vary the DC voltage at pin 1 and 6 of IC6003. The more the
voltage is pulled down, the higher the frequency the oscillator will run.
Excessive B+: This will cause the set to shut down and flash the timer LED in sequences of three. One way to
quickly isolate the cause is to short pin 2 of IC1601 to ground. Under normal conditions, this will cause the B+
level to drop dramatically. This is a quick way to verify that IC6005 and IC6003 are functioning properly and this
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