Repair Briefs from Other Sources


1.7) Dead 5 year old Sony TV

(From David Klementovic)

5 year old Sony TV
Failed to power on. tic-tic-tic from the PS.
Had to be an overload or shorted component of some sort.

My client asked me to repair his 5 year old Sony television which failed to power on. I put it up on the service cart, plugged in the line cord and tried to power it on. I heard a tic-tic-tic sound coming from the switching power supply board F. A quick check of the power supply was done with the ohmmeter but nothing obviously shorted or open.

Then I removed plug F61 from the power supply board, pressed the power on button on the set and the switching power supply began to work, I could hear the audio amplifiers working through the speakers. Something in the television chassis was loading the power supply down. I metered the brown wire on plug F61 (B+ supply) and found it shorted to ground. Then I tested the horizontal output transistor Q503 and it was shorted.

I inspected the flyback and other high voltage components and seeing nothing else shorted replaced Q503 (2SD1941) horizontal output and Q502 (2SC2688-lk) horizontal driver with exact replacements. Now, I know that frequently horizontal output devices are over-stressed by some other defect in the chassis so I prepared for this possibility by removing B+ jumper wire JW-41, inserting a 5.6 ohm 10 watt resistor in its place and clipped my voltmeter leads across the resistor. This gave me the ability to monitor the current being consumed by the flyback transformer.

I powered up the set slowly using a variac and an isolation transformer, the picture was good. I adjusted the variac for 125 volts AC, set the customer picture controls on the Sony to midway, (brightness, sharpness) and the Trinitone to LOW. I tuned to a music video channel on cable in order to observe the raster changing frequently from dark to light, and recorded a voltage drop across my 5.6 ohm resistor of 2.2 volts with a dark raster to 3.98 volts with a white raster.

After 10 minutes of operation, the voltage across the resistor began to climb even though the raster was fairly dark at the time. At 5 volts the right side of the raster began to shrink, and after about the loss of 3 inches of raster from the right side I powered the set off, alas there was more trouble in this set for me to debug. I inspected for problems with the scan-derrived B+ voltages, pin cushion correction circuitry, deflection yolk and retrace tuning capacitor, nothing found defective! The troubleshooting breakthrough came when I replaced a 0.47 mfd electrolytic capacitor (C525) which coupled the horizontal drive signal into the base of the horizontal driver transistor and C537, a 4.7 mfd electrolytic capacitor in the horizontal driver circuits. These two capacitors had weakened and had resulted in the eventual failure of the horizontal output transistor. I checked the horizontal waveform on the base of the horizontal output transistor and it looked OK. So I removed the 5.6 ohm resistor, soldered B+ jumper JW41 back in the circuit and tested the set for 6 hours.

The Sony worked good and has continued to work good for my client for 3 months now.

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