Audio LG lf-u850a doesnt work Power Supply problem - SOLVED - Page 1
Forum section: electronics / beginners

author: LIQUIDSNAKEFOX
author_rank: Newbie
timestamp: November 29, 2022, 07:16:39 pm
content: Hello, I'm super new to this, but I'm learning.I have a power supply from a stereo which I have not been able to make it work yet.The model of the power supply is LF-K9350/850 SERIES MP 050607 6870R7815ABwhat i did in order:1.I changed the capacitor and another component (a6252) which had very visible physical damage. (blown up)2. An optocoupler was shorted, but when it was removed from the board it was fine, and the short persisted. I trace it and found 2 failing diodes (ZD902 33V and ZD906 9.1V) replaced them and bye bye short.3. Even with the above, I still do not get voltages in the cold zone, and the largest transformer shows continuity in all its outputs. I unsoldered it and that doesn't happen anymore. but I see this transformer with a bit "weird" and "blown" part.Could it be a faulty transformer?*I don't have an oscilloscope (And I dont know how to use one)Thanks in advance for any advice and all your help!

author: jwet
author_rank: Frequent Contributor
timestamp: December 02, 2022, 06:00:30 am
content: First of all be very careful working on these types of supplies. They take raw line voltage and rectify and have voltages stored in large high energy caps of over 200v often. This can be lethal.Judging by the parts that got fried, it sounds like the supply went out of regulation and took out most of the secondary components- opto, diodes, zener. Which you've replaced.You need to determine if the driver is still driving the power device that switches the main transformer. Carefully probe the transformer and see if its is getting a large AC voltage on its primary winding. If not, I would suspect the switching device- the one on the heat sink behind the transformer. Since this has 4 terminals, this is likely a power device and controller.Good luck and do be careful.

author: LIQUIDSNAKEFOX
author_rank: Newbie
timestamp: December 02, 2022, 07:00:33 pm
content: On the big transformer I get 159VBut also, there is no voltage between pin 5 and 3 on A6252.In addition to that, the C905 330uF 400v capacitor the voltage is not fixed (it shouldn't fluctuate at all, right?), it gives me 159v, but it fluctuates a bit up and down

author: jwet
author_rank: Frequent Contributor
timestamp: December 03, 2022, 12:34:46 am
content: The 159 should be pretty constant. Look at the AC voltage on the switching node of the transformer. The fluctuations could be from the switch firing. A scope would help here.I'll look at the A6252 and get back to you.

author: LIQUIDSNAKEFOX
author_rank: Newbie
timestamp: January 04, 2023, 11:13:14 pm
content: The input voltage is stable.Now, what I have seen in the EER4242 transformer, is that the secondary pins are shorted, and if I remove the transformer from the board, the short disappears, both on the board and on the component...This is normal?I will attach a picture of the shorted points

author: LIQUIDSNAKEFOX
author_rank: Newbie
timestamp: January 13, 2023, 04:25:30 am
content: SOLVED!I dont know why, but the A6252 that I replaced wasnt working.I put another one and that was it, stand by voltaje!