Panasonic TX-37LZD80F repair, intermitent relay click, low standby 5V, photocoupler

[Chungalin]

This LCD TV had an intermittent failure consisting of unstable 5V standby. When standby 5V drop to 3V, relays click and TV resets. Sometimes it does this from cold state.

I tried to heat all components related to 5V standby (switching regulator IC7000 SanKen STR-A6159, diodes, photocoupler PC7000 Panasonic CNC1S171R "3171 R", shunt regulator IC7001 Mitsumi MM1431AT "1431A"), with no changes in behaviour. Tried in cold state and hot state, apparently no differences. Vibration and knock test negative.

I changed STR-A6159 with STR-A6151, no improvement. Since at that moment I couldn't identify photocouplers and shunt regulators (Mitsumi and Microchip logos are very similar!), I swapped PC7000 with PC7001, and IC7001 with IC7005. No problems after that.

I found a latin repair forum talking about a similar issue in TC-32LX85X and they fixed it by replacing Panasonic photocouplers.

http://www.dtforum.info/index.php?topic=83770.0;all

Then, another latin blogger with same issue stating that he couldn't confirm that photocoupler was "really" bad, but that failure was definitively related to it.

http://13norte.blogspot.com.es/2012/...-enciende.html

Sometimes I've found what I call "creeping metallization" to occur between legs of IR receivers or tactswitches, causing intermittent or permanent failures. I haven't observed this fenomena in this case, but I've cleaned both photocouplers before resoldering. Let's see how long it lasts.

PSU is TXN/P10NDA, and PCB marking is TNPA4467. Chassis LH69.

[budm]

Re: Panasonic TX-37LZD80F repair, intermitent relay click, low standby 5V, photocoupl

You should check the resistors network that drive the LED side of the OPTO, they usually use very high value resistor to drop the line Voltage to drive the LED in the OPTO.

[tom66]

Re: Panasonic TX-37LZD80F repair, intermitent relay click, low standby 5V, photocoupl

I had a bad optocoupler cause an intermittent 7-blink error on my Panasonic plasma TV (TX-P42ST50B). I've also seen some older LG plasma TVs and one LCD TV model that suffer from failed optocouplers causing a variety of symptoms including low power rails or cycling power.

It's a surprisingly common fault.

[dskall]

Re: Panasonic TX-37LZD80F repair, intermitent relay click, low standby 5V, photocoupl

What is best way to test optocoupler

[budm]

Re: Panasonic TX-37LZD80F repair, intermitent relay click, low standby 5V, photocoupl

CTR.

https://www.renesas.com/en-eu/produc...ology/ctr.html


http://www.edn.com/design/test-and-m...ocoupler-s-CTR

[attainteddragon]

Re: Panasonic TX-37LZD80F repair, intermitent relay click, low standby 5V, photocoupl

i have a rough method for quick checking.

you use 2x dmm's
if you diode test the diode side, you will see a resistance on the led side as it conducts.

it's not definitive, but it works for a quick rule it out test.

[dskall]

Re: Panasonic TX-37LZD80F repair, intermitent relay click, low standby 5V, photocoupl

Is it true that optocouplers degrade and get out of tolerance?

[Th3_uN1Qu3]

Re: Panasonic TX-37LZD80F repair, intermitent relay click, low standby 5V, photocoupl

Is it true that optocouplers degrade and get out of tolerance?

Yes. Optocouplers use infrared LEDs, which, like all other phosphor-based things that emit light, get dimmer with age.

[Chungalin]

Re: Panasonic TX-37LZD80F repair, intermitent relay click, low standby 5V, photocoupl

Agree with everything, but I’ve returned the TV without replacing any photocoupler, and the only thing I’ve done is unsolder both photocouplers and resolder them in swapped position. Same for shunt regulators. Remember it was an intermittent failure. I’ve insisted to customer to bring back the TV in case of failing again soon, so we’ll know if photocouplers are really unreliable or not.