Samsung TV CS-29Z58MQ: three colored lines on top of picture

I have recently encountered in my repair work a Samsung Slim Fit CRT TV model# CS-29Z58MQ with a problem of three horizontal blanking lines appearing at the top edge of the screen. According, to the customer the problem started with just one line but after some time an additional two lines also appeared on the edge of the picture.
I inspected the board and tried to spot some bulging capacitors with signs of electrolyte leakage which usually cause this kind of picture abnormalities but found none.
Upon closer inspection of the solder side of the main board I saw signs of dry solder on resistors R428 (1.5 ohms, 1W) and on R413 (1ohm, 1W). These two resistors are connected in series with the Flyback transformer's 16.5V supply pin to the vertical output IC LA78045. The resistors are located at the rear edge of the main board beside the flyback transformer.
After resoldering the dry solder on the resistors the three colored lines on top of the picture disappeared.

Daewoo TV DTQ-2072 repeated blowing of fuse

Where can the problem be if the main fuse in the TV power supply blows fuse repeatedly?
This was a problem I have encountered with a Daewoo TV model# DTQ-2072. The fuse blows even when the IC regulator  STR-59041 has been replaced. To isolate the cause of the problem, I replaced the fuse again and disconnenected the PTC thermistor on board location RH02.
I plugged the set to AC outlet and powered up the set from standby. The picture and sound came up but the picture has color smears(dynamic misconvergence) on the sides of the screen - a result of disconnecting the thermistor which passes momentary current to the degaussing coil.
An ohmmeter check on the thermistor shows that it has developed a higher resistance. Opening the plastic case of the PTC thermistor revealed that the thermistor discs are burnt.
Replacing the PTC thermistor eliminated the  color stains on the picture and solved the problem of  blowing of fuse.

JVC TV AV-J21MX: No power problem

This TV that came in for repair was found to have a Horizontal Output Transistor D1878 (on board location Q522) with a short between the collector to emitter. When the defective transistor was pulled out of circuit there was still a short measured from the B+ line to ground. The deflection yoke was unplugged from the TV's main board but still the short was present.


Tracing back towards the power supply it was found out that a shorted RU3A diode on board location D921 is shorting the B+ line to ground.

Replacing the transistor D1878 and diode RU3A restored power and normal operation to the set.

Philips Projection TV dark horizontal lines on picture

A Philips 43PP7445/69 Projection TV came in for repair with complaints of black horizontal lines on the picture. At first it seems that an interference in the video circuits is causing the problem. But when I entered the convergence alignment mode the problem showed itself as horizontal jittering which can be seen in the vertical lines of the convergence alignment cross.

By swapping boards the problem was isolated to the ACS Panel which controls the convergence alignments.

Troubleshooting Philips CTV 21PT3932: Black OSD problem

Symptom: Black on-screen display(OSD)
Cure: Replace diode BZX79-C3V9 on board location 6601.
Repair Details:
Set used the L9.1A chassis.
Picture and sound is good; the only thing obviously wrong was that the OSD was black.
When the +3.3V supply going to the microprocessor was measured actual voltage reading was at about +5V. This indicated that the +3.3V supply regulator circuit was defective. The +3.3V supply was restored to its specified voltage output when the BZX79-C3V9 was replaced. Normal color was also restored to the OSD.

Philips TV L01.2 chassis problem: criss-crossing white lines on a dark raster

TV troubleshooting is about reading the lines on the screen. And in an electronic repairer's work troubles appearing as strange lines on the screen or picture of the TV crop up from time to time. These types of trouble in a CRT TV are most often cause by a problem affecting the scanning or deflection circuits. An example of these TV troubles manifesting as weird lines on the picture was tackled in one of my previous post where a capacitor in the vertical output circuit has decreased in capacitance and caused black lines to flicker across the picture.

The other day as I was flipping through a notebook that I keep where I jot down details of troubleshooting work that I have done I came across a record of a weird lines on the picture problem. Initially the symptom is 'no picture, dark raster'. Increasing the G2 control on the flyback transformer revealed strange lines that break white and zigzag across a blank raster.
I was also able to keep a camera shot of the symptom which you can see below.



The lines tell of trouble that lies in the vertical scanning circuit. The lines are symptomatic of excessive vertical scan distortion and fold-over. The TV chassis contains a TDA8172 vertical output IC which does not feel warm when you touch it. This would mean it is not overdriven or overloaded. Voltage measurement on the vertical output IC indicate normal supply voltages. What is obviously abnormal is the voltage on the output pin 5 which measures +5V instead of 0V. Replacing the IC did not provide the cure to the problem.

I decided to scope the input signals to the TDA8172 vertical output IC and discovered that the Vdrive+ signal on pin 17 of the UOC TDA9580 was generating a rectangular wave pulse instead of a sawtooth pulse. Both the Vdrive+ and Vdrive- The UOC was diagnosed to be defective since the sawtooth pulse is supposed to be generated from inside the UOC. A normal picture was restored to the set when the UOC was replaced.

Vertical deflection problem: horizontal gaps on picture

Coming at the tail-end of 2008 a day before New Year's Eve, a repair technician came to me for help in the repair of a 21-inch TV.
There was a problem in the picture of the TV which looks like black horizontal stripes and gaps on the screen. The picture below would describe to you more about the symptom.


















And here's another photo:



According to the technician, he has already replaced the TDA8172 vertical output IC. Unfortunately, the trouble still persisted.
The vertical output IC is obviously running hot as you can get singed just by touching its heat-sink for just a few seconds.

Based on the symptoms there appears to be non-linearity in the vertical scanning as indicated by the gaps in the picture. A slight fold-over in the top edge of the picture can also be noticed.
Since the vertical output IC has already been replaced, I set my sights on the minor components in the vertical deflection circuit.

According to the technician he has already tested the resistors and capacitors located around the vertical output IC using the ohmmeter of an analog multimeter and found no open or changed value resistors. He tested the capacitors' charging and discharging action also using the analog ohmmeter.
Digging in to the guts of the TV, I set my sights on the capacitors first as they are known to be the passive components with the highest failure rate - second only to ICs and transistors. Capacitors are better tested with a capacitance meter, not to mention the ESR meter, I skipped testing the capacitors with the ohmmeter. Testing the capacitors around the vertical output with a capacitance meter I found one mylar capacitor whose measured capacitance was way off its rated value. As indicated on its package its value should be 220nF/63V. However, this capacitor only registered 3.2nF on my capacitance meter. Apparently, this capacitor has decreased in capacitance to, approximately, less than 2 percent of its rated value.
Getting another capacitor with similar indicated value from my parts bin, I tested it on the capacitance meter to verify the rating before using it to replace the original capacitor which has decreased in capacitance.
After installing the new capacitor, the TV set was again turned on. Now we see a clean picture - the black gaps and lines have completely disappeared.
So now we have identified the cause of the problem to be the 220nF/63V mylar capacitor located a fraction of an inch to the right of the TDA8172 IC.
Tracing the connections on the printed circuit board and a review of the TDA8172 datasheet, it was found that the capacitor was connected to the output pin of the vertical output IC through a 1.5 ohms resistor. The capacitor's other pin is connected to ground pin of the vertical output IC.

Related articles by Zemanta

• The PCB as a heatsink (makezine.com)

TV high voltage arcs

In a CRT-type television set there are three sections where high voltage(HV) arcing can originate:

1. the flyback transformer where high voltages used by the cathode ray tube(CRT) are generated.
   
2. the deflection coil or yoke assembly.
   
3. the CRT itself where the highest voltages are applied.
   

The HV arcs can be colored yellow, orange or red. But the arcs with the highest voltages are those that are blue in color.

The most common source of high voltage leak and arcing is the flyback transformer. Aside from producing the scan-derived supplies where some sections of the TV get power, the flyback transformer also provide the high voltages used by the CRT.

Typical voltages for color CRT's ranging in size from 14" to 34" are:

Anode : 16kV - 30kV
Screen or VG2: 100 - 400V
Focus: 300 - 600V
Cathode: 170 - 200V

Such high voltages can produce arcing. Especially if the flyback transformer has developed HV leaks. HV arcs can jump from the case of the flyback when the plastic case and internal epoxy-type insulation has developed cracks.

The flyback's ferrite core can also produce intermittent arcs that jump to surrounding heat sinks. When this happens intermittent blanking of raster will occur due to dropping of high voltage.
Most of all, the EHT cable's rubberized insulation can deteriorate and develop pin-prick cracks where the HV can arc towards the direction of the CRT, deflection coils, wires and aluminum heat sinks.

HV leaks can sometimes be remedied by dipping the flyback in insulating varnish and letting it dry for a few hours before re-installing. EHT cable can be wrapped with HV electrical tape to cover leaks. If arcing is still observed the best solution would be to replaced the flyback module.

There will also be arcing if the areas surrounding the flyback's anode cap is is covered with dust and dirt. Cleaning the anode cap and the areas of the CRT around the anode button or connector will solve the problem.

Sometimes arcing will occur inside the CRT itself when there is shorting in the internal grids or electrodes. The CRT seal can also develop cracks and air will have entered into the tube which is supposed to operate in a vacuum. When this happens the tube is considered 'gassy' and a blue arc can be seen inside the neck of the CRT. Then, there will be no other solution but to replace the CRT.

The video below shows what a HV arc will look like inside the neck of a 'gassy' CRT.

TV deflection coil defect

Modern TVs are designed to shutdown when problems are present either in the power supply or its loads to prevent the set from incurring further damage.

This protection feature was exhibited in a 14-inch TV which was brought in the shop for repair.

The set appears to power-up but will shutdown even before a raster is visible on the screen. A soft hissing sound can also be heard during the one or two seconds before shutdown.

The B+ supply voltage going to the horizontal output transistor's collector was measured but drops in that short moment before shutdown.

The scan-derived supplies going to the CRT panel such as the G2 and video output circuit supply voltages are both dropped.

Because of the hissing noise heard before shutdown the flyback transformer was suspected to have a high voltage leak that may be causing arcing noise. But a few tries to power-up the set reveals the source of the noise to be somewhere around the neck of the CRT. However, arcing was not visible inside the CRT neck. So the next possible source of HV arcing or leak is the deflection coil.

Next step was to unplug the deflection coil connector. When the set was powered on B+ voltage did not drop and there is a palpable rush of high voltage to the CRT. Its filament was also noticed to be lighted. This meant that a short or leak was removed when the yoke was unplugged.

When the yoke was disassembled the source of arcing was uncovered. Looking inside the yoke reveals burnt marks in the inner windings of the deflection coils.

When a deflection yoke burns it is usually at the outer edges of the windings or in the area in between windings as shown in the photo below. Notice the blackened edges of the winding on the left. In this case, the coil also reeks of a bitter smell.

Now that the cause of the trouble has been identified the only problem now is where to find a replacement part for the burnt deflection coil.