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.
The other week, I had an opportunity once again to browse through the book by Don Matsuda “Electronic Troubleshooting”. This was a book that I was fortunate to have bought - and read - many years ago when I was just starting out on doing electronic repairs professionally.
In the preface to his book Don Matsuda notes that Troubleshooting has its own rules. He notes further that the rules are derived from everyday practice and must be taught from the point of view of practice.
Matsuda starts out the book by mapping out a “game plan” for troubleshooting. The game plan is divided into 5 phases.
Phase 1: The Outside
Starts the troubleshooting approach from outside of the equipment under complain. Identify the symptoms. Make the preliminary inspection of the equipment by operating it and evaluating its performance. Decide whether the trouble is in the equipment or the environment. Interview the user to get a background of the problem from his standpoint. Let him know of any problem you see or foresee be before you start out work on the equipment.
Phase 2: The Gateways Into The Circuits
This is when the troubleshooter goes deeper into the root of the problem and begins an approach into the “gateways into the circuits” . Two of these gateways are the input and output devices. Troubles in electronic equipments can be caused by a malfunctioning input device and symptoms will show up at the output. Or the output device itself is defective. Symptoms can be clues that our four senses can pick up or can be measured by our test instruments. An analysis of the symptom is a step towards finding out the right cure.
Phase 3: Working Your Way In
This involves getting down to the nitty-gritty of instrumentation work. DC voltages have to be measured and bad voltages pinpointed. Trace a signal and identify the stage in its path where it deteriorated.
Phase 4: Nailing Down The Bad Part
This involves locating the defect to the bad part. After pinpointing the defective section a test of its components should reveal the defective part/s.
Phase 5: Cooking
This is what is sometimes called 'burn-in test'. After the bad parts have been replaced the equipment is operated for about 20 minutes or longer depending on the type of fault.
In the book, Matsuda also presents some general strategies in troubleshooting. These are:
Trial and Error; Divide and Conquer. If something does not work, try another solution. Cut up problems into little ones and deal with them one at a time. Follow up on one of your hunches, if it turns out to be wrong, at least you have eliminated it as a possibility. Troubleshooting is not a cut-and-dried process. But keeping the game plan in mind will keep us from getting sidetracked.
Questioning and Thinking Things Over . Question all leads you come across. Poll all possibilities.
Playing the Percentages. Troubleshooters pay special attention to parts or sections in an equipment that have a high failure rate. Parts and sections under great heat, electrical and mechanical stress have the highest failure rate.
The Divide-in-Half Procedure; Divide and Conquer. Mentally divide a circuit in half and test each half. When you identify the bad half divide it again and test. This dividing and testing is repeated until the defective part is located. This is faster than testing the parts one by one. In Steve Litt's 10 Step Universal Troubleshooting Process, the Divide and Conquer procedure is also discussed in detail.
Doing the Easy Things First. “When the odds seem pretty equal – and sometimes when they're not – it is best to do the easiest thing first.... At least you will quickly eliminate a possibility and feel better about taking on the more difficult and time-consuming alternative.”
The book then continues by giving valuable discussions and tips on doing voltage tests: what bad voltages tell, which part must be causing the problem. Also very informative are the sections and chapters on checking transistor circuits, amplifiers, power supplies, high voltage circuits, oscillators, high frequency circuits, TV, linear and digital ICs.
Also, the section discussing the “Nature and Frequency of Parts Failures” gives valuable tips on what parts fail most often. The book reveals that the part that fails most often are transistors. Capacitors come in second place as the part that has the highest failure rate. Resistors come third. And then coils and transformers.
In the last chapter of the book which deals with troubleshooting intermittent tough dog troubles, Matsuda advices to get hold of a piece of discarded electronic equipment that a pro technician has wisely turned down and get ready for some real troubleshooting. “However, it is a challenge and a real learning experience, even if you fail.”
In troubleshooting as in life, failure teaches as much as success.
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.
In a CRT-type television set there are three sections where high voltage(HV) arcing can originate:
the flyback transformer where high voltages used by the cathode ray tube(CRT) are generated.
the deflection coil or yoke assembly.
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:
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.
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.
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