Application Note

3 Fluke Corporation Troubleshooting electrical problems in high-end TVs
After the TV shuts down or turns off, stop the
recording and save the session. Select Trend
and Zoom to observe when the line voltage
jumps two to three volts within one or two sec-
onds. The voltage at this point is the voltage
level where the TV shuts down. Figure 3 shows
the transition point just as the TV shuts down at
a line voltage of 132 V.
5. Tracing cause of failure of 3-terminal
regulator used as post regulator off of
an off-line switching supply.
In one instance, a three terminal, five volt regu-
lator (TO-220) mounted on a small heat sink
ran much hotter than normal. The solder on the
pins and circuit board had changed color, and
the paper-phonolic board turned a dark brown
around the part pins. The case of the regulator
had cracked, and it was replaced. After the new
regulator was installed, the temperature of the
regulator was checked using the 289 DMM and
a temperature probe. The thermocouple was
connected to the metal tab of the regulator and
289 meter, set to record at 5 second intervals,
recording started.
After about five minutes, the temperature of the
metal tab was about 55 °C. The input voltage
to the regulator was 14.5 V when it should be
around 12 V. The output level of the switch-
ing supply is regulated by sensing the voltage
level at the output of the control/sense circuit,
which generates a voltage proportional to the
Figure 4. Screen shots showing temperature of 3-terminal regulator IC before capacitor replacement, left, and after capacitor replacement, right.
Figure 3. Screen shot showing the voltage level of 136 V where the
TV shuts down.
voltage across a sense winding of the power
transformer. The problem was traced to a faulty
capacitor. Once the capacitor had been replaced,
the input voltage dropped to 12.03 V, and the
temperature of the regulator dropped to 45.2 °C
at five minutes and 46.9 °C at eight minutes.
Screen shots of the temperature recordings are
shown in Figure 4. The left screen is before the
capacitor was replaced, and the right screen is
after the capacitor was replaced.
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Printed in U.S.A. 12/2008 3392461 A-EN-N Rev A
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