Capacitor in power supply problem? - Page 1
Forum section: electronics / repair

author: negativ3
author_rank: Regular Contributor
timestamp: July 14, 2020, 08:56:47 am
content: Hi all,I am repairing an amplifier (2.1ch cheapo PC type) power supply and I think I have identified the mains rectification smoothing cap as having an issue. When I measure across it, I get 315V DC which seems correct, but only at the exact moment of measuring the voltage does the supply output start up. Does this indicate a bad cap in your experience? It is rated 450V, 47uF and reads 44uF out of circuit.ThanksAndy

author: tautech
author_rank: Super Contributor
timestamp: July 14, 2020, 09:43:18 am
content: SMPS I presume ?Depending on just where you're placing the other DMM probe it could be the VCC supply storage cap or the HV DC dropper resistors as there's just enough current passing through you DMM to satisfy the IC's UVLO and kick it into action for the kickback winding on the primary side to take over and keep it running.Identify the IC and get the datasheet for it and study how they are used and it should become clear.

author: negativ3
author_rank: Regular Contributor
timestamp: July 14, 2020, 11:20:44 am
content: Smells like an SMPS but I'm struggling to find the switcher...MOSFET on the top of the board and a small SOIC on the bottom with marking 848L11 I cannot as yet decipher.For sure the cap is ok, changed it out and still not functioning.Any hints appreciated.ThanksAndy

author: tautech
author_rank: Super Contributor
timestamp: July 14, 2020, 11:49:01 am
content: Quote from: negativ3 on July 14, 2020, 11:20:44 amLooks pretty small, can you confirm the package by measurement ? SOT-23/6 ?You say 848L11, all on one line ?Have a hunt for a resistor dropper for the HV DC. Maybe 2 or 3 in series for a total of normally 500k+.Check any measure correctly.

author: negativ3
author_rank: Regular Contributor
timestamp: July 14, 2020, 12:09:37 pm
content: The hunt has begun, see picture attached.Than you.Andy

author: Seekonk
author_rank: Super Contributor
timestamp: July 15, 2020, 12:19:29 am
content: Could you explain that. Does the output power bump or does it work fine after it starts?

author: negativ3
author_rank: Regular Contributor
timestamp: July 15, 2020, 04:21:07 am
content: It seemed to be working fine, now I have a dead short across the main cap location, after removing the cap, mosfet and transformer the short persists.the mosfet and transformer measure ok out of circuit.The main fuse may have provided protection but it did give me one hell of a suprise

author: negativ3
author_rank: Regular Contributor
timestamp: July 15, 2020, 02:06:10 pm
content: Yea, so a stray leg of a clipped resistor was found under the dut. Totally my fault and a lesson learned many years ago but relearned again today.The MOSFET, line inductor, and fuse were toasted. Replacements on the way and a scrupulously clean work surface has been arranged.

author: Rudi
author_rank: Newbie
timestamp: April 19, 2025, 07:34:45 pm
content: Does anyone have a Datasheet of this PWM controler 848L11 Sot 23-6?I have the same board as above but the controller will not start.If I put 16.3 Volt manually on it, it starts pulsing.

author: Rudi
author_rank: Newbie
timestamp: April 20, 2025, 10:20:04 pm
content: I ordered SG6848T, a guy in another forum had the same problem and replaced 848L11 with SG6848T