Fluke 289 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

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.

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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Specifications subject to change without notice.

Printed in U.S.A. 12/2008 3392461 A-EN-N Rev A

Modification of this document is not

permitted without written permission

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