Application Note

2 Fluke Corporation Troubleshooting electrical problems in high-end TVs
the heat sink). Set up the 289 DMM to record
the temperature at ten second intervals and start
recording. Turn on the TV and allow the set to
warm up to its normal operating temperature,
about 20 to 30 minutes. After about 15 minutes,
the temperature should be around 45 °C to
55 °C. Figure 1 shows the screen shots of the
temperature recordings for a normal amplifier
and an amplifier that is running too hot.
3. Troubleshooting intermittent turn-on
problem in projection television.
An older projection TV had a problem where it
intermittently would turn on and have sound,
but no picture. If you turned off the set, waited
about two minutes, then turned the set back on,
you could get a picture about one time in three
tries. The problem was that one of the switching
power supplies did not start up every time the
set was turned on.
Using the 289 DMM, the problem was traced to
the startup circuit where there was a defective
electrolytic capacitor. The DMM was connected
across the supply for the controller IC. When
the controller IC was enabled, the supply would
drop below the minimum supply voltage needed
for the controller to operate. Figure 2 is a screen
shot of the supply voltage for the controller IC
with the TV turned on. The screen shot is a
recording made after the capacitor was replaced.
Before the capacitor was replaced, the volt-
age dropped below 12 V, below the low supply
threshold, shutting the IC down. The Zoom fea-
ture enables the technician to examine the volt-
age levels as the supply is being enabled and
turned on.
4. Verification of power supply input over
voltage shutdown:
On some projection television sets, the power
supply is designed to shut down if the line volt-
age in above a certain level. It is important to
verify that the power supply does shutdown if
the line voltage goes above this set level. Using
the recording function, of the 289 DMM, the test
can easily be done.
The TV connects to the power line through a
Variac (variable auto-transformer). Set the 289
DMM up to record in ac volts at one second
intervals. Turn the TV on with the line voltage
set to approximately 120 V. Connect the 289
DMM across the line and start the recording.
Slowly increase the line voltage until the TV
turns itself off or shuts down. Do not exceed
140 V if the TV does not shut down; there is
something wrong with the shutdown circuit in
the TV.
Figure 1. Left Screen Shot shows an amplifier that is running too hot. Right Screen Shot shows an amplifier running at normal temperature.
Figure 2. Screen shot showing the controller IC supply voltage as the
supply is being turned on. Before the supply is enabled, the voltage is
about 24.75 V and drops to approximately 17 V.