Application Note

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Application Note

Troubleshooting electrical

problems in high-end TVs

Using logging and TrendCapture in the Fluke 289 DMM

From the Fluke Digital Library @ www.fluke.com/library

These troubleshooting steps feature the Fluke 289 DMM

because of its onscreen trending capabilities. TrendCapture

helps troubleshooters spot anomalies faster by showing results

on-screen as soon as recording sessions are complete. Follow

correct safety procedures for testing live circuits. Use a safety-

rated, fused digital multimeter (DMM) and test a known voltage

source first to make sure your meter is operating properly.

1. Testing over voltage or supply overload shut-

down in televisions:

Most televisions sets that use CRT’s have shut-

down circuits that turn off the TV when the high

voltage goes too high. These same sets also

have circuits that detect excessive supply voltage

or load currents for several low voltage supplies.

It is often a real challenge to find an intermittent

shutdown problem because most TV’s do not

have built in circuits that indicate which supply

is intermittently overloaded or has intermittent

excessive voltage.

You can use the Fluke 289 Digital Multimeter

(DMM) to find the power supply that is caus-

ing the shutdown problem. Connect the meter

across the HV/deflection supply, monitor the

voltage level, and record the voltage level over

a period of time. You can also set the meter in

MIN-MAX mode, wait for the shutdown to occur,

and observe the maximum voltage. If you don’t

see an over voltage problem, test another supply.

For some supplies, you may be able to access

the current sense shunt, and then you can easily

connect the meter across the resistor to monitor

the current in MIN-MAX mode.

2. Testing for cause of repeated failure of con-

vergence amplifiers in projection televisions.

Projection television sets that use three projec-

tion CRT’s have circuits that dynamically modify

the deflection of the electron beams in the three

tubes to converge the three colors displayed on

the screen. The convergence is done by driving

secondary deflection yokes with hybrid power

amplifiers. These amplifiers are usually pack-

aged two or three in a hybrid assembly.

The amplifiers are fairly reliable, but occasion-

ally there may be a set where the amplifiers

fail in less than a year, and the replacement

amplifier fails in six to nine months. Using a 289

DMM and a temperature probe, you can conduct

a simple extra check to determine if a repair

is really complete and the TV will not have a

repeating failure.

After a hybrid amplifier assembly has been

replaced, connect the temperature probe to the

amplifier assembly and heat sink (at the junc-

tion of the body of the amplifier assembly and

Summary of content (3 pages)

PAGE 1
Troubleshooting electrical problems in high-end TVs Using logging and TrendCapture in the Fluke 289 DMM Application Note These troubleshooting steps feature the Fluke 289 DMM because of its onscreen trending capabilities. TrendCapture helps troubleshooters spot anomalies faster by showing results on-screen as soon as recording sessions are complete. Follow correct safety procedures for testing live circuits.
PAGE 2
Figure 1. Left Screen Shot shows an amplifier that is running too hot. Right Screen Shot shows an amplifier running at normal temperature. 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.
PAGE 3
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 seconds. 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. Figure 3. Screen shot showing the voltage level of 136 V where the TV shuts down. Figure 4.