USING
TDA 8172 à VERTICAL OUTPUT / QFJAF 6818 à HORIZONTAL OUTPUT / AN 7522 & AN 17821A à AUDIO OPUTPUT / NOM 8370-B-6NF à SYSTEM CHROMA JUNGLE / KA5Q1265 à POWER REGULATOR / TDA9850 à AUDIO DECODER / ET-5A5E-AF108M à TUNER
POWER REGULATOR CIRCUIT - CLICK ON IMAGES TO ENLARGE
Set got for repair under dead condition. The owner said that he heard a 'click' noise inside it while he was watching a program, and gone dead. That's only he knew.
Checked the set. Fuse found opened. [F501 3.15A/250V]. Surely there must be some short circuit within the power regulator primary section. Checked all the components related to primary power rectifier section. On checking, 2 main rectifier diodes [out of 4] {VD503 & VD505} found shorted.
Replaced all the 4 diodes. Checked the resistance across C507-220Mfd 400V. Found very low. Seems to measure below 20Ohms. Surely there must be an other shorted component at the main rectifier section. Desoldered out the power regulator IC [KA5Q1265]. Now measured the resistance across C507. Found high. Confirmed that the STR, ie KA5Q1265, has damaged.
According to schematic diagram of the set, pin number 1 of KA5Q1265 is connected to main rectified voltage [at about 375VDC with 220VAC input], and pin number 2 is connected to ground. Measured the resistance between pin 1 & 2 of this IC. Found very low. Reversed the meter probe polarity, the same effect. It is shorted internally. That was the reason for fuse failure. An over voltage or voltage spike at the input voltage can cause this. Can't predict, what might be.
When power regulator sections are troubleshooted, extreme care should be taken in checking each and every component soldered on to it. Even a ceramic capacitor can poise serious problem, and cost more for components.
With this type of SMPS power supply circuit, it will be best to check each and every suspected component out of circuit. Here, special care should be given in checking the capacitor [C512 - 1000pF/2KV]. Even though this capacitor is physically small, its function when the SMPS works is great. It is called a freewheeling capacitor, and most important one. It is best to replace this capacitor along with the power regulator, as if this capacitor has gone out of value, or have any crack over its body, it will surely damage the new power regulator, no doubt. Also check the solder terminals of this capacitor, as well as all the other components in circuit Re-solder all the suspected solder terminals by applying a little more solder, without making any solder bridge short in between adjacent solder terminals. Always replace all the four mains rectifier diodes together, even though one or two of them has got damaged. It is a good practice.
The components that need special care, in this type of SMPS circuit are, F501- PS501(Posistor), VD503,504,505,506 - C515(AV400V2200Pf connected between Cold Ground and pin-8) of SMPS transformer, C503, 504, 505 &506 - connected in parellel to main rectifier diodes, C515A (470Pf/AC400V) - connected to pin-4 of SMPS transformer to Hot Ground. In other words just keep in track with all the connected components for damage. Check them with utmost care, A capacitance meter is recommended to test capacitors.
Never use an over-rated fuse in place. If protection fuse must blow up, there should be a reason for that. Rectify the reason first, before replacing it.
Enlarge the above schematic, and see all the circuit. Click on it once. You can enlarge it once more by an other click.
Horizontal output transistor blows off as soon as the set is switched ON, after replacing it. Coming up by next post.
