Nordmende LCD-TV - defective SMPS (JSK4180-024/A/B)

[carl_lab]

Nordmende TV N3202LBD doesn't switch on from standby.
Standby LED turns from red to blue, but LCD panel doesn't light.

SMPS output "5VSB" (standby supply) is OK (=5V).
SMPS control input "ON/OFF" is 0V on standby and 2.2V on operation mode, but doesn't switch main PS on (12V and 24V).

Then I disconnected "ON/OFF" line from control PCB to power supply.
On controller side it is now 4.8V in operation mode. I'm not sure 2.2V under load is enough to switch the main PS on. (PS: it's enough!)
Maybe "ON/OFF" input is somehow shorted and pulls 4.8V from controller PCB down to 2.2V?

On optical inspection I could not find any suspicious components.

So I must find schematics of this SMPS "JSK4180-024/A/B"...

[carl_lab]

The trick was, not to search by google web search but search for pictures!

I found this one at https://de.scribd.com/doc/208538169/JSK4180-024A-SCH .
At the picture, text is nearly unreadable, but schematic is quite usable ...
Finally I found a way to download a very well readable (zoomable) PDF! 

[carl_lab]

Problem solved:
Optocoupler IC2 (PC817) was defective.
So switching on the 12V+24V main supply was impossible.

 

[dav]

Could you detail the 817 defect? (LED shorted, 817 partially working but uncalibrated...)

[carl_lab]

LED is OK. Phototransistor does not conduct.
I replaced old one by a new optocoupler, which works, so root cause doesn't really matter for me.