TV Power Supply Unit

[Markus1]

Re: TV Power Supply Unit

At 11.5V on VDD pin 9the startup cap is charged up through the internal current source, that is below the startup voltage requirement which is around 14.5VDC.
Page 6/20 of the spec sheet I provided: It is good to read the description on how the IC works.

I did go through it. I think I've got an overall understanding of its working. So the voltage (11.5V) comes from internal current source connected to the Drain. Its level is monitored against the two thresholds:
VDDon to start switching and turning off the internal startup current source,
VDDoff to stop switching and turning on the internal startup source.
And it continues. Feed back from the Opt-isol provides regulation.

So the startup cap connected to the VDD pin may have leakage resistance or the internal current source inside the IC is bad.

I will go with the replacement of the cap C126, and also another one added to pin 4 as per modification (mentioned in earlier post). If no improvement I will try to replace the chip (IC103 Viper22A).
Will report on Monday.

[Markus1]

Re: TV Power Supply Unit

Good news. TV is up and running.
Total of 3 caps in the start-up circuit were replaced: C127 47uF, C126 47uF (refer attached circuit diagram), and also a 22 uF cap (part of modification to the board, not shown on the diagram). All caps rated at 50V.
It appears that the 22uF cap was the culprit. It was inserted (as part of the modification) between pin 4 of the IC103 and Gnd. I do not have an ESR meter but when measured with a capacitance meter this cap has shown only about 10 uF.
After these 3 replacements the old TV came alive and is doing well.

Big thank you to all who responded to my post, and to selldoor and budm in particular. Your advice, instructions and directions for proper diagnostic procedure was much appreciated. It is a fantastic forum with great expertise and also willingness to share the knowledge and to help not only seasoned TV technicians but electronic enthusiast as well. I look forward to a next project.

[budm]

Re: TV Power Supply Unit

Thanks for the good news and the repair follow-up, now when you look at the SMPS ICs, they basically operate in the same principle, and if you can get the spec sheet it will help a lot since the circuits in the application notes will be real close to what you have.

[pr0cess0r]

Re: TV Power Supply Unit

Tahnk you for this post. i changed the 22uF cap and the set work. Its wont always turn on so i will change all caps now that i know it good!