Sony BVM20F1U PSU Short After Recap - Page 1
Forum section: electronics / repair

author: Mantrox
author_rank: Newbie
timestamp: February 27, 2020, 02:19:45 pm
content: Hey folks, i'm completely out of my depth here, and just trying to go step by step in terms of troubleshooting, while getting some more skills and knowledge on the matter.Just to be clear, i have no knowledge about eletronics, hence why my approach to some of this may seem inadequate.Recently my Sony BVM20F1U monitor engaged the "Overload" alert, and stopped turning on the tube.I was told that, when it comes to the "Overload" problem, the issue usually lies on the PSU and within it, the components that fail more often are the electrolytic capacitors.With this, i decided to replace all the thru hole electrolytic caps, and document the procedure to help the community.Bought one of the cheaper soldering stations, documented the cap values and mapped them on the board view i got from the service manual.I changed every cap. All went well, even though my soldering skills leave a lot to be desired.Yesterday i mounted the PSU back in, turned the monitor on, and... nothing, silence.I took it out again, opened the PSU and re-checked that all the cables and daughter boards were correctly connected. They were, so i tried again, this time leaving the psu in the ON position for a bit longer, maybe 10 seconds.This time, i heard a loud bang. No smoke, no fire, no smell. A similar sound to when you short a capacitor.I dismounted the PSU again and found that a positive trace from the largest cap (C28 - 270uF 450V) shorted to another trace which belongs to one of the daughter boards mounted on the PSU.I am looking for help to try and understand why this happened, and what could have caused it.What my next steps in the troubleshooting process should be, and if it is even worth the time, or did this short damage the PSU beyond repair.I'm very eager to learn more about how all of this works.I'll add to this post the relevant information about said PSU.Board schematic, photos, whatever else i can find.PSU SchematicComponent Function ChartBoard Photos Component SideBefore RecapAfter RecapBoard Photo Solder SideQuadrant 1Quadrant 2Quadrant 3Quadrant 4Short CloseupCap Map & List W\ESR Measurements Of Old Caps I Removedhttps://docs.google.com/spreadsheets/d/1UrVHBQClCox3K6WayTCZapE6uFr7cD4cfokzMBOBfuo/edit?usp=sharingThank you all in advance for any help on this issue.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 27, 2020, 03:44:26 pm
content: Switch mode power supplies can be a real pain in the neck to get working - when they blow, it is often more than one component that ends up getting destroyed. If you are not careful, you end up destroying new components as fast as you can solder them in.I would start by checking all the high power semiconductors, and anything connected to these semiconductors. I would begin on the primary/mains side and work my way through, checking everything.

author: Mantrox
author_rank: Newbie
timestamp: February 27, 2020, 04:05:23 pm
content: By high power semiconductors do you mean, transistors? Or do i have to find a way (and tools) to test every component that falls under that category?I was thinking about getting one of those multifunction tester with a ziff socket from amazon and starting with the transistors.Since the short, went to a daughter board's trace which has resistors\caps and 1 IC, would i have to test each of those individually?Do i do anything to the damaged area other than cleaning?Does that white line in the PCB divide the high power from the low power portions?Sorry for the eletronics 101 questions but i'm really not familiar enough with all the terminology.Thank you for taking the time to respond.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 27, 2020, 04:23:13 pm
content: Start by checking the physically biggest transistors and diodes, since they carry the most current and are most likely to blow.When/if you find a broken one, you look on the diagram to see what other devices are connected to it, and check those parts as well.

author: Mantrox
author_rank: Newbie
timestamp: February 27, 2020, 04:33:11 pm
content: Thank you. I'll report back and document the results of my tests.Is this something i can do in circuit or would i have to necessarily desolder every component for test? I assume the latter.

author: SilverSolder
author_rank: Super Contributor
timestamp: February 27, 2020, 04:45:33 pm
content: Needing to desolder depends on the situation. For example, if you have 10 components in parallel, and one of them is shorted, it can be hard to figure out which one is shorted without desoldering something.There are various tools that you can use to trace where a current is going when you are trying to find a short. But desoldering things is the easiest to understand!

author: Mantrox
author_rank: Newbie
timestamp: February 27, 2020, 04:51:02 pm
content: Got it.I'll go slow and steady, desoldering, to make sure.

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 27, 2020, 07:10:41 pm
content: If you shorted pin 1 of CN5 and the adjacent large trace it can't of harmed much. Fuses, thermistor diode bridge, fuse resistor.Check you haven't left other bridges elsewhere though.

author: Mantrox
author_rank: Newbie
timestamp: February 27, 2020, 08:33:21 pm
content: Quote from: shakalnokturn on February 27, 2020, 07:10:41 pmIt was indeed pin 1 of CN5. I'll post a clearer picture, after i clean up that section of the pcb.Edit:Here's the short with less dirt and a bit better focus.https://imgur.com/a/so4rqdcQuote from: shakalnokturn on February 27, 2020, 07:10:41 pmWhat do you mean by "left other bridges"?I didn't touch the CN5 part of the pcb, or most of the psu, i just changed the capacitors.Another thing is, before changing the caps the PSU was turning on the monitor and gave an alert when turning on the tube.After changing the caps, it didn't even turn on.What could have gone wrong in the capacitor swap that would create this problem?

author: WattsThat
author_rank: Frequent Contributor
timestamp: February 28, 2020, 12:49:32 am
content: The likely faults that come to mind would be a solder bridge followed by having installed a capacitor backwards (incorrect polarity).

author: SilverSolder
author_rank: Super Contributor
timestamp: February 28, 2020, 01:08:59 am
content: A "solder bridge" is what happens when you accidentally solder two adjacent tracks together.If an electrolytic capacitor is installed backwards (reverse polarity) it will be damaged - it is important to get that right.

author: Mantrox
author_rank: Newbie
timestamp: February 28, 2020, 09:38:06 am
content: Quote from: WattsThat on February 28, 2020, 12:49:32 amQuote from: SilverSolder on February 28, 2020, 01:08:59 amI wanted to make sure i didn't misunderstood the meaning of solder bridge.I did inspect the board several times after changing the capacitors and didn't find that any of their new solder points was close to making contact with something they shouldn't.I also made the cap map, with the polarity according to the board, and the board itself had guides for the polarity which i checked on every cap insertion.But it does sound like that's the problem.I'm gonna check everything again.Thanks.Edit:Oh, one more question.If i eventually find a faulty component and replace it; what do i do with the area where the short happened?Do i have to scrape the burned trace?Re-tin the CN5 pin 1?Or just clean it and let it be?Edit nº2:Another thing, cap number 17, acording to the schematic should be a 470uF 35V. But i had a hard timing finding stock of those and ended up putting in a panasonic 470uF 50V cap.That's not a problem, is it?I read that you have to be carefull about capacitance, but as long as it's more voltage and not less, the swap should be fine.