Which optocoupler to buy?

[mpadilla]

I must say it's great the amount of guidance that's available in this forumn!

I have this LN32A550P3FS, that when plugged in the standby light blinks steady. Following on guidance from this forumn and others, I opened up the tv and found on visual inspection some capacitors were bulging. I replaced the capacitors minding polarity. I put the tv back together with same results of standby light blinking. By the way all voltages are correct after replacing capacitors except for the standby voltage. The board stamp says it should be 5.2 but it measures as 4.19. The power supply is BN44-00208A (PSLF171501B). I did more research and found this zip file and this post that basically state a certain optocoupler needs replacing. However, this zip file and post were for a slightly different model. But I believe the problem might be the same. I see in the schematic that the optocoupler is tlp781. When I search (digikey and others) many different types come up.

Can someone possibly point me to the correct one to buy?

[fzabkar]

Re: Which optocoupler to buy?

I would check capacitors CB852 (the 5V filter) and CB802. CB802 filters the auxiliary supply for the PWM controller IC.

Also measure the voltages between pins 1 & 2 and 3 & 4 of PC804S, but be mindful of the HOT side.

[budm]

Re: Which optocoupler to buy?

From your link:
http://www.electronicspoint.com/thre...2#post-1603408
'Problem: optocoupler PC801S defective - replace. Then replace the RP809, RP810, RP811, RP812, RP816! Good luck! BART'

That opto PC801S has nothing to do with low 5V STBY, that OPTO is used for turning on/off the rest of the power supply when the power supply get the PS-ON control signal from the main board.
Opto PC804S is the one used for voltage regulation feed back to main tain 5V output.
So chekc what fzabkar has suggested.

[mpadilla]

Re: Which optocoupler to buy?

I will test and get back. Thank you for the feedback.

[budm]

Re: Which optocoupler to buy?

BTW, this series is know for bad C102 47uF 10V SMD Tantalum cap on the main board, one of the symptom is LED blinking continuously.
For example:
https://www.badcaps.net/forum/showth...911#post616911

http://www.shopjimmy.com/catalogsear...t/?q=LN32A550P

[mpadilla]

Re: Which optocoupler to buy?

Thank you for pointing that out budm. Of course this tv has that exact board. I'll replace that cap on the main board and report back.

[mpadilla]

Re: Which optocoupler to buy?

Ok, I was super tired the first time I measured and it came out that the stby voltage was low. However, today I have had rest. I went back and re-measured everything. Here's the results. PS looks good to me based on measured values? I'm thinking now all I need is C102 as budm mentioned. Can you provide feedback?

CNM801
1 - B/L ON/OFF
2 - ADIM
3 - PWM
4 - GND
5 - SCHEMATIC SAYS 24V DETECT; BOARD STAMPED DET. 5V: MEASURED 5.5V

CNM802
1 - PWR-ON/OFF
2 - N/C
3 - ST-BY5.2V: 5.3V
4 - GND
5 - GND
6 - GND
7 - 12.8V: 12.3V
8 - 12.8V: 12.4V
9 - GND
10 - GND
11 - GND
12 - GND
13 - 5.3V: 5.2V
14 - 5.3V: 5.3V
15 - 5.3V: 5.2V
16 - 5.3V: 5.3V
17 - GND
18 - GND
19 - 12V: 12.1V
20 - GND
21 - 12V: 12.1V
22 - 12V: 12.1V
23 - N/C
24 - N/C

CNM803
1 - 24V: 24.7
2 - 24V: 24.7
3 - 24V: 24.7
4 - 24V: 24.7
5 - 24V: 24.7
6 - GND
7 - GND
8 - GND
9 - GND
10 - GND
11 - N/C
12 - B/L ON/OFF
13 - ADIM
14 - PWM

[budm]

Re: Which optocoupler to buy?

At this point go ahead and replace that c102 cap, if you do not have it, you can use 100uF 10~16V LOW ESR lytics cap for testing by putting it in parallel with C102, notes: the bar marking on the C102 is the POSITIVE leg of the cap.

[mpadilla]

Re: Which optocoupler to buy?

Thank you for the advice! Unfortunately, out of all the components I have, I don't have a 100uF 10~16V LOW ESR lytics cap for testing.

I'll just order the C102 cap linked to digikey in the linked post. Thanks budm! I'll report back in a few days with my results.

[mpadilla]

Re: Which optocoupler to buy?

Worked like a charm! After soldering the new c102 cap, TV is functional again. Thank you again for the guidance!

[budm]

Re: Which optocoupler to buy?

Thanks for the repair follow-up.

[japlytic]

Re: Which optocoupler to buy?

Care should be taken in substituting the optocoupler since I have read in Power Integrations Application Note AN-14, the Current Transfer Ratio (CTR) of the optocoupler should be within a certain level (and not have an uncontrolled value) suitable for the controller; otherwise, the power supply could oscillate, or for too low CTR, excessive photodiode currents are required to properly control the controller duty cycle, and if CTR is too high, the shutdown latch or other protection features on the controller could be triggered during startup or load transients.