SMPS Repair, standby voltage not present - Page 1
Forum section: electronics / beginners

author: Godzilla
author_rank: Contributor
timestamp: December 19, 2024, 05:52:06 pm
content: Hello, im trying to repair an LG mini hi-fi system SMPS, pictures attached.I think the problem is that the standby voltage is not present. The small transformer has a switching IC: STR2A153D (not sure if i can post the DS). The VCC is present but its a saw tooth as in the attached picture. Nothing is present on the secondary side. What should i check? The AC rectifyng part seems to be OK...

author: Harry_22
author_rank: Frequent Contributor
timestamp: December 19, 2024, 08:52:16 pm
content: Hi!Please measure Vcc and FB/OVL (see the attached picture).

author: Gyro
author_rank: Super Contributor
timestamp: December 19, 2024, 09:06:29 pm
content: I would test / replace the small electrolytic capacitors below the STR2A153D in the top side photo. One of them (referred to as C2 in the datasheet) is the reservoir cap for the STR2A153D VCC supply, and is often implicated in startup issues when it dries out.

author: Godzilla
author_rank: Contributor
timestamp: December 20, 2024, 08:07:47 am
content: Quote from: Gyro on December 19, 2024, 09:06:29 pmItested it and it measure 22uF as it should, not sure about the ESR... Im gonna replace it and see.Quote from: Harry_22 on December 19, 2024, 08:52:16 pmVcc is the one on the picture attached, im gonna post FB soon.

author: Godzilla
author_rank: Contributor
timestamp: December 20, 2024, 11:19:03 am
content: Changing the VCC cap fixed it, apparently even if it measured well it probably had a very big ESR... Do you guys suggest to add ceramic caps in parallel 10uF 50V or similar...

author: Gyro
author_rank: Super Contributor
timestamp: December 20, 2024, 07:23:32 pm
content: That's good news. No it shouldn't be necessary, it's not that highly stressed, having a parallel lower value ceramic could give you some interesting effects as the electrolytic ages again. The factors are small can size (low electrolyte volume) and being fairly warm due to location. If you want to improve reliability, pick a decent quality long life rated capacitor and increase the voltage rating as a way of increasing can size and so, electrolyte volume.

author: Godzilla
author_rank: Contributor
timestamp: December 26, 2024, 12:32:59 pm
content: Apparently it was a lie, after some hours it died again, same way. After some tries where it simply started working again randomly after i started measuring things in the circuit with the multimeter, i noticed that putting MMU leads between GND and D/ST pins seems to fix it for some time. Can it be a defective IC or maybe it need some kind of coupling that is missing and that the MMU privedes somehow...

author: Godzilla
author_rank: Contributor
timestamp: December 28, 2024, 10:39:29 am
content: Quote from: Gyro on December 20, 2024, 07:23:32 pmI changed almost every electrolytic in the circuit but at the end it still dies after some hours .The circuit attached should be very close to what i have C9 is a 10nF film cap, should i try change it?

author: Gyro
author_rank: Super Contributor
timestamp: December 28, 2024, 11:24:26 am
content: Sorry to hear that you're still having problems. When you say "after some hours it died again" I assume you mean several hours in standby rather than operating. I suppose it's possible that the switcher IC itself has degraded, such things do happen. Dry solder joints are always possible, particularly on transformer pins where there is greater thermal mass involved during automatic soldering.No, don't touch C9. That is a noise suppression capacitor between primary and secondary sides and need to be safety (Y) rated, you should see it labeled appropriately. It won't have any effect on SMPS operation.

author: Godzilla
author_rank: Contributor
timestamp: December 28, 2024, 12:59:54 pm
content: No I meant operating, i tried to resolder all the transformers joint and its under work again now. I will update you in case.

author: Godzilla
author_rank: Contributor
timestamp: January 06, 2025, 07:16:53 pm
content: At the end it died again, I ordered a new IC and I will try to swap it...