Repairing Power supply won't turn on (with "Sense" line ?) - Page 1
Forum section: electronics / repair

author: Mozee
author_rank: Contributor
timestamp: May 11, 2018, 02:49:00 pm
content: Hello ,I have this audio amplifier power supply, It outputs different voltage and has "Power on" line and "Sense" line as shown in the attached picture,Now the 3.3 and 5 volt rails are not providing any power my guess is that it is due to the "sense" and "power on" lines , I have little knowledge about power supplies so help will be appreciated here. I want to know how does this know of power supplies sense the need to turn itself on,It reminds me of the ATX power supplies (the desktop computers PS) where we had to add a resistor between the white wire and ground (can't remember exactly how!).Thanks in advanceEdit:I uploaded new Images of the PCB, Here you go:https://ibb.co/mgRaBJhttps://ibb.co/g0Zj5d

author: LateLesley
author_rank: Frequent Contributor
timestamp: May 11, 2018, 05:26:15 pm
content: The P_ON/OFF will probably need jumped to ground to power it up. I'm not suggesting you do this at this point though. The sense wire will be a feedback wire to the regulating circuit to set the output voltage.I suspect that there will be more components underneath the board. Any chance of taking the board out of the plastic carrier, and getting a photo of underneath? I think it needs a once over before it's powered up.And remember to short the caps before taking it out, especially on the primary side, so you don't get any nasty surprises. (All with mains removed of course, we don't want it powered yet!)

author: lordvader88
author_rank: Frequent Contributor
timestamp: May 11, 2018, 06:02:22 pm
content: Troubleshooting & Repairing Switch Mode Power Supplieshttps://readerspage.files.wordpress.com/2016/07/smpsrepairebook.pdf

author: Jwillis
author_rank: Super Contributor
timestamp: May 12, 2018, 08:17:57 am
content: The power on is just that it switches the unit on.The sense line is used to sense current requirements.When it senses the requirements are low the supply will go to low out put state for more efficiency.When the the unit senses a higher requirement it boosts the power as required. This is done through a power management chip or it may be in the PWM it self or can be done with a FET or transistor by detecting a load. Best to find the datasheets for the individual chips to see what they do.I can see three opto couplers but can make out the 8 pin .Likely the PWM but not sure. And schematic might be helpful.If you can trace where the sense line leads that may be of help to figure the circuit out. A clear image of the bottom traces and description of the input output lines there might help as well.Switch mode supplies can be a little daunting but not impossible to understand.edit // sorry can't make out 8 pin

author: Mozee
author_rank: Contributor
timestamp: May 12, 2018, 11:25:05 am
content: Thank you all , I added new Images of the PCB's back! Couldn't attach them directly due to size limit

author: LateLesley
author_rank: Frequent Contributor
timestamp: May 12, 2018, 05:52:04 pm
content: Ah ha! And someone's been there before! Can see their handiwork. We need to check out SMIC45, it looks like it has some history. Can we get the number off that component?

author: Mozee
author_rank: Contributor
timestamp: May 12, 2018, 06:01:11 pm
content: Quote from: LateLesley on May 12, 2018, 05:52:04 pmHmmm, Which one do you me? Which component? sorry for my stupid question

author: Jwillis
author_rank: Super Contributor
timestamp: May 12, 2018, 07:00:07 pm
content: wait sec.Pin 15 (-VP) on the connector. What is that ? Is that a termination or a broken/burned off wire?

author: LateLesley
author_rank: Frequent Contributor
timestamp: May 12, 2018, 09:55:32 pm
content: Nice spot Jwillis. Looks like the wires for pin 14,15 are chopped. Looks intentional.@OP I mean the black transistor-looking thing, next to 3 electrolytics, nearest the power connectors. If you look at the black writing on the board, you'll see all the components have labels SMC for capacitors, SMR for resistors, SMD for diodes, etc, then a number. The transistors/FETs/ICs seem to be labelled SMIC then a number. the numbers help identify which component it is.