[Solved] FATEK FBs-14MA PLC Repair: SMPS Output Voltage Issue at 10V Instead of 24V

Dear Friends,

Working on Service & Repairs of the PLC ( FATEK Brand) Model FBs-14MA, with DEAD POWER SUPPLY BOARD ( Bottom PCB).

Successfully, I have managed to make repairs by replacing the NEW SMPS IC NCP1203D6 with the related defective SMD parts and the burnt Bridge Rectifier and FET, and voila, it's OPERATIONAL again.

But, for the another same PLC, after replacing the NEW SMPS IC NCP1203D6 with the related defective SMD parts and the burnt BR and FET, the SMPS voltage outputs only 10V DC, while the actual output voltages are 24V DC. The suspected Optocoupler PC817 is also replaced with a NEW one, but the output volts are the same e.g. 10V DC.

Need some help to figure out the actual cause for this 10V output instead of NORMAL 24V DC output.

Your help shall be appreciated. Pictures are attached for reference.

Regards

Are there any electronic components on the print side of the circuit board, as no such photo has been posted by a colleague? It is difficult to help from a distance, can you please draw a schematic? On the secondary side, in the optocoupler adjustment circuit, is there an LM 431 IC, the line that controls the voltage on the main power supply output line?

What is the component highlighted in red?

Does this power supply only have one voltage or more?
@Telo makes a good point - check the TL431 and its surroundings.

>>21292912 >>21292912
Hi Telo,

Thanks for the prompt reply..
There are no parts on the bottom side of the PCB, as reviewed the picture attached of the bot side of the PCB for Power Supply Board.
As you suggest, I guess the TL431 / LM341 SMD IC in SOT23-3 pack is the cause of this issue , since the 24V DC is the only output available, as normal operation. I don't have the schematic of this board also.

Thanks for the suggestion. Will try to replace it with the available in TO-92 pack, as SOT23-3 pack is not available here.
Thanks again.

Added after 1 [minutes]:

>>21292964
Hi KARWO,

Thanks for the prompt reply..
There are no parts on the bottom side of the PCB, as reviewed the picture attached of the bot side of the PCB for Power Supply Board.
As you suggest, I guess the TL431 / LM341 SMD IC in SOT23-3 pack is the cause of this issue.

Thanks again

Yes solder the new TL431 it is the one responsible for the regulation.

Thanks for the help & support.

Lisek replies and some efforts finding the defective part (TL431 SOT23 SMD IC) replaced with new part & resolved the issue.

Thanks to all.