Panasonic TC-L42E50 Dead

[neilc6]

This TV was dead with no standby light. Panasonics typically will blink diagnostic codes for troubleshooting. Luckily, a service manual is available for this TV online.

Power supply board doesn't have legend but schematic has pinout. 5VS standby should be at pin 5 on PS and pin 9 on main. I measured an unstable 2.7V - 3.5V. Disconnecting the connector, it is a stable 5.3V on the power supply side.

Power supply schematic (pg 52) shows standby voltage coming from the transformer, D7407 and to a 5V regulator IC7502. D7407 tested ok.

On the main board side, the 5V standby (STB5V) feeds the HDMI ports, an analog ASIC IC5000 and feeds a few pins on the main processor. There are two decoupling caps on this line at the ASIC C5000 and C5012 which I checked for shorts. Resistance on this rail measured 26K so not shorted.

Possibly the 5V regulator IC7502 is not putting out enough current. Thinking of possibly providing the 5V standby separately to see if the TV turns on.

[neilc6]

Re: Panasonic TC-L42E50 Dead

Changed the 5V regulator and get the exact same symptoms. Problem is likely on the main board and the analog asic.

[budm]

Re: Panasonic TC-L42E50 Dead

What happen to the 5VSTB if you only have cable between main board and power supply board connected but nothing else connected to the main board?

[neilc6]

Re: Panasonic TC-L42E50 Dead

Disconnected everything else from main board and 5V still fluctuates. May disconnect hdmi section from 5VSTB and check since it is isolated through a resistor on the main.

[R_J]

Re: Panasonic TC-L42E50 Dead

The power supply only supplies 6v to IC7502 in standby mode, so I'm not sure you will get a steady 5.5v at its output. Is the 6v on the input stable? Are you getting a steady stb3.3v standby on A10 pin5 (page32) ?

[neilc6]

Re: Panasonic TC-L42E50 Dead

Input to the 5V regulator is 8.5V with main disconnected and unstable 2.8-4V connected. Enable pin to the regulator is 3V. STB3.3V is reading 0V at connector A10.

[R_J]

Re: Panasonic TC-L42E50 Dead

I would suspect the power supply should give a steady 6v for the regulator in standby, Have you checked the vcc voltage of IC7301, it could be low due to a weak C7310 cap.

The 5v regulator does seem similar to a RICOH R1524x series (R1524H050B-T1-FE)

[neilc6]

Re: Panasonic TC-L42E50 Dead

VCC to IC7301 is 12.7 volts jumping +/- 20mV with either the main board connected or not.

The 5V regulator substitute I ordered was Ricoh R1514H050B-T1-FE (150mA). There is a 200mA version and is slightly lower than the 5.3V original one.

Guess I can check C7514 and C7310 on my ESR tester.

[R_J]

Re: Panasonic TC-L42E50 Dead

If the vcc for the ic is stable around 13v that is likely ok. I would think that if the regulator only outputs 5v at 150ma it wouldn't load the 6v input down to 3~4v
How is the 16v and 24v lines? are they really low as well? I would guess they would be 8v & 12v in standby, if they are close, then maybe C7514 is bad and not holding up the 6v in standby

[neilc6]

Re: Panasonic TC-L42E50 Dead

16V line is 11.7 plugged and 11.3 unplugged. 24V line is 18.5V plugged and discharges slowly unplugged.

C7514 tests fine. 2100uF and 0 ohm ESR.

[dick_barton]

Re: Panasonic TC-L42E50 Dead

Is IC7502 a 6V regulator as per the schematic?

[neilc6]

Re: Panasonic TC-L42E50 Dead

Is IC7502 a 6V regulator as per the schematic?

IC7502 is a 5V (5.3V) regulator. In standby the input voltage should be around 6V and goes up to 12V according to the schematic when the PS turns on. But CE may shut the regulator down when on as well.

Looks like the lower standby voltages for the main are generated by the ASIC on the main board.