MY SERVICE EXPEREINCE
Sony KV-2954 MT, 29" Trinitron TV. No picture. No sound. No power. got for repair yesterday. Party informed me, that somebody has attempted to make it repair at home, but failed, even after replacing 5 Nos of horizontal output transistors. The owner showed me all of them, and handed over to me. In an Ohmmeter check, all of these transistors found shorted. [Collector to Emitter short]. These are the only information I've got from the customer. I asked him about the probelm he faced before the TV has failed. He told me that, it was working properly, and suddently the picture increased in width, blurred and within about 2 minutes, it became dead.
Today morning, I began to check the circuits. As the TV is in dead condition, I concentrated in its main power regulator section circuit first. Because, without making this section OK, I can't proceed. Checked all the resistors and diodes and found OK. Disconnected the main output voltage to line out put transformer [135VDC] from the main power SMPS section. Switched on the set, after connecting digital voltmeter across the main input smoothing capacitor. No input voltage at all. Disconnect the AC mains cord, and checked the main ON/Off switch contacts. They were found OK. Concentrated on fuse and surge limiting resistor. The fuse was Ok. but the surge limiting resistor found opened. Unplugged the fuse out from its cartridge and looked for its ratings. It was found that, some body have replaced the original fuse [3.5Amps - 250VAC] fuse, with an other fuse of (10Amps 250VAC]. Too high rating to this set. Anyway, I concentrated to point out the reason for the blow up of surge limiting resistor.
Before proceeding I desoldered out the degaussing posistor [three terminal one] out from the circuit, as a prcaution. If switch ON this set, after degaussing coil disconnected, this posistor will surely get damaged with most Sony televisions.
Checked the resistance across the main filter capacitor [330MFD 450V]. Found too low. Less than 15 Ohms. Something short detected. Desoldered out the main bridge rectifier block. Measured the resistance across the main filter capacitor again. No short was found. Assumed the rectifer block to be short. Checked all the four internal diodes of it, referring the schematic. No fault found to these internal rectfier diodes. All four of them was OK.
Concentrated the circuit parts before rectification point. Checked the AC input to rectifier points. Measured the resistance there, without inserting the rectifier block. A short circuit found. Disconnected the two disc ceramic capacitors that connected in series with AC mains input supply, and measured the resistance to AC input points again. Was found OK. No short circuit. Checked the disconnected ceramic capacitors. One among them found shorted. This might be the reason for the blowing up of fuse and somebody have placed a 10Amps fuse in place of 3.5 Amps, and thus the the surge protection resistor might also opened.
Replaced the surge protection resistor, two of the ceramic capacitors stated above, resoldered the rectifier stalk in place, replaced the 10 Amps fuse with a 3.5Amps one. Checked the circuit resistance again at the AC input power cord points after the set have switched ON with out pluging it into power socket. No short circuit detected.
Output supply from the secondary of the SMPS power disconnected. Connected a 100W filament type bulb as LOT load, switched the set ON. Measured the voltage across the to terminals of the bulb. Found more than normal. Mesured over 142VDC with digital multimeter instead of (135V). In no reson it should't be over than 136VDC. Further check to stabilize this fault needed.
Will continue
