Help and guidance to repair switching power supply - Page 1
Forum section: electronics / repair

author: partyanimallighting
author_rank: Newbie
timestamp: July 28, 2023, 06:12:52 am
content: Hi all, let me start by saying that my knowledge is limited as far as tracing and resolving problems especially with power supplies. I replaced a faulty power supply in a 5R moving head with a brand new equivalent (120VAC input, 36VDC output to mainboard, 12VDC to cooling fans and 380VDC to lamp ballast to strike the lamp) and, upon start-up there was a pop and the SMD fuse on the power supply blew out. I believe that the ballast may be faulty and this caused the power supply to blow out. I disassembled the power supply and I removed the blown fuse and bridged it with a thin wire filament and powered up and the substitute "fuse" blew out again. My knowledge is vague when it comes to power supplies and poking around in them so I'll be grateful for any assistance/guidance that anyone is willing to offer as I'm not to sure where to start and what to test. Picture below.

author: wn1fju
author_rank: Frequent Contributor
timestamp: July 28, 2023, 11:35:38 am
content: First of all, what is all that crud on the bottom of the board (top, center)? Take a toothbrush with some alcohol and clean that area thoroughly. Take a magnifying glass and a strong light and inspect all the traces and solder joints in that area after cleaning.With the power supply disconnected and off, I would take an ohmmeter and start looking for shorted components. Usually, electrolytic capacitors on the secondary side are the prime suspects, but check all the diodes and transistors. Note that some parts may be across transformer windings and may indicate a short even though they are OK. If in question, lift a lead of the suspect component and check again.There are hundreds of online "repair guides" for switch mode power supplies. I like this one:https://www.repairfaq.org/sam/smpsfaq.htmFinally, I always cringe when a self-proclaimed novice attempts a SMPS repair. Poking around with a supply that is powered up can KILL you. If you are unsure of exactly what you are doing, STOP.

author: Grandchuck
author_rank: Super Contributor
timestamp: July 28, 2023, 12:22:33 pm
content: Is it crud or an artifact from the flash used to take the picture?

author: partyanimallighting
author_rank: Newbie
timestamp: July 30, 2023, 08:52:24 am
content: Hi all. The "crud" is actually reflection from the flash as I opted for the clearest picture possible (at night) and the PSU is brand new so there's no crud buildup from exposure. wn1fju, I don't necessarily enjoy poking around live power supplies either so I'll be extra careful as I have always been. There will be questions as, once I receive guidance, I'll be following instructions as you guys pass them on to me. I'll start by testing all the caps and see what's what with those. Will I get accurate readings testing the four secondary caps mounted or should I lift them off the board individually?

author: Grandchuck
author_rank: Super Contributor
timestamp: July 30, 2023, 02:08:07 pm
content: The short is likely near the input. The large electrolytic capacitors or the 810 rectifier could be shorted. Please do be careful!