Trying to repair standing desk power supply that went bang w/ no visible damage - Page 1
Forum section: electronics / repair

author: computers_and_cats
author_rank: Newbie
timestamp: March 27, 2025, 10:13:56 pm
content: I have watched a lot of videos over the years from your usual sources but am out of my element. This power supply is from a standing desk I have. One day the desk was going up and stopped followed by a bang sound. There is nothing visibly scorched or damaged and no bad smells coming from the PSU. In the past the desk would occasionally hit some sort of overcurrent limit going up causing it to stop and go back down. I tried to do some troubleshooting and this is what I have found:* Black rectangular input fuse is good.* Voltage output from full bridge rectifier is 168VDC which is apparently normal.* The input side of the blue transformer shows around -100VDC on both pins. No AC voltage detected by my meter and I don't have an oscilloscope I could use. Not sure if that is right or wrong but ChatGPT indicates the mosfet could be stuck on.* Checked mosfet and then removed mosfet to check out of circuit. Drain-to-Source and Gate-to-Source show continuity and Gate-to-Source shows 5 ohm. Everything is indicating this is bad.* Both transformers show continuity on their input and output coils.* I see no voltages on the DC output half of the PSU* ChatGPT also said check gate drive circuit but not sure which chip that is.The mosfet is a Toshiba K14A65W. I don't have any on hand and am hesitant to spend $10 on one for no reason if the issue is something else. I closest thing have is a K3568 I could steal from my scrap pile. Its specs are wrong in a few different directions though. RDS(on) and output capacitance being the bigger issues from what I gather. Obviously everything in life is a risk but could I substitute a random close enough N-channel mosfet just to do a no load power on test?

author: MathWizard
author_rank: Super Contributor
timestamp: April 01, 2025, 09:23:28 pm
content: Any luck with it ? Negative 100V sounds low and backwards at the blue 'transformer' , and it looks like the full rectifier voltage goes right to the trans.To me it looks like there's also a Standby power, using that yellow 'transformer' and probably the 8-pin DIP next to it, which is probably an off-line switcher IC, with a power mosfet inside it. So was there any 5V STBY on the output side ? It might also be 'making' the low-voltage supply on the HOT side, like 12V. So the STBY rail would usually have to be working, before the main SPMS would startup. And if the STBY was missing at the output, 1 of the opto-coupler's should make an error signal, like being pulled high or low.I'd check it all over unplugged, and see what that 8-pin DIP and low-voltage/STBY rail is doing first, before trying another MOSFET.But for an idle test, if you haven't tried already, I'd say the 500V mosfet should be ok, there's lots of other protection diodes around there too, and I don't see any boost-converter stage, so I'd try it.If you had an old computer PSU for parts, you might find a better match MOSFET. But shipping aside, a new suitable MOSFET should be $1-2.

author: thm_w
author_rank: Super Contributor
timestamp: April 01, 2025, 09:38:45 pm
content: I would say try the K3568 as well, see what happens.I'm surprised the fuse didn't blow if the FET was blown, thats odd.

author: tom66
author_rank: Super Contributor
timestamp: April 01, 2025, 09:55:35 pm
content: If the fuse did not blow, either the transformer winding did, or the current sense resistor did. Current sense resistor is most likely.Also, it's often the case that the controller IC and/or gate resistor bites the dust when the FET fails, so it's worth checking those before you power it up. In most cases you can do a resistance test to see if the controller is good.

author: computers_and_cats
author_rank: Newbie
timestamp: April 01, 2025, 10:06:30 pm
content: Thanks I will make note of that as well. All the coils show continuity so I think that is good. If a transformer blew I am kinda hosed. I will have to start checking components from the sound of it.

author: computers_and_cats
author_rank: Newbie
timestamp: April 01, 2025, 10:11:15 pm
content: I haven't touched it since this post. I will have to check the voltage through the path out the rectifier. I used a known ground point for the negative probe and the positive probe on the transformer.I will have to double check for standby power. Past the isolation gap I didn't see anything for voltage. I will have to do some more digging and probing.I checked a handful of random PSUs in the scrap pile but they all seem to use multiple lower rated mosfets in their design. I might see if I can work on reverse engineering the PCB since it is a simple single layer board.

author: wilfred
author_rank: Super Contributor
timestamp: April 02, 2025, 12:07:01 am
content: If you don't see signs of where the bang came from I'd keep looking. There would almost certainly be something that has physical damage. Whether it is a scorch mark from arcing or a cracked package where the smoke came out. Even if you repair a fault on the power supply that might only be secondary damage following a short elsewhere. I'm thinking a motor, limit switch or just a pair of wires that got caught during movement. Can you try another power supply?

author: DonKu
author_rank: Regular Contributor
timestamp: April 02, 2025, 12:16:11 am
content: Quote from: thm_w on April 01, 2025, 09:38:45 pmSome wise guys would say the MOSFET is there to protect the fuse.

author: computers_and_cats
author_rank: Newbie
timestamp: April 02, 2025, 01:23:08 am
content: It needs a fun mix of 5V, 5.7V, 12.7V and 33V. I would have to see if I could rig something up. I suspect the 33V is for the motors. I will have to check harder for signs of the bang. Also forgot to mention it pissed off my battery backup when the power supply died.

author: tom66
author_rank: Super Contributor
timestamp: April 02, 2025, 07:24:04 am
content: I think these are the current sense resistors, check these are not open circuit.

author: computers_and_cats
author_rank: Newbie
timestamp: April 03, 2025, 03:48:58 am
content: Looks like tom66 and wilfred were right. I decided to go for the low hanging fruit and check those resistors. The damage was hidden in the shadow of the filter cap. I don't think that resistor is supposed to have speed holes. Pulled the other one and it is also scorched. I might see if I can salvage some .12 ohm resistors from the junk pile and try a close enough substitute mosfet to power this up. From there I will have to check what MathWizard suggested then buy the right parts for a final test.

author: tom66
author_rank: Super Contributor
timestamp: April 03, 2025, 08:28:53 am
content: I would strongly suggest testing the controller IC before you replace any power electronics components, otherwise the FET may just pop again immediately... been there, done that, got the T-shirt.To do so, clear pictures of the controller IC, the 8 pin chip near the MOSFET, will be required.Also, it looks like the FET that failed is the one for the main rails on the power supply, I would guess the 33V for the motors, so it will be under stress when the desk rises and falls.

author: IanJ
author_rank: Unknown
timestamp: April 03, 2025, 09:31:49 am
content: Can you give us the make/model number of the PSU?Can you give us the part number of the 8-pin DIP IC's on the top side....one of them is likely the switching controller IC. Possibly a UC3842A perhaps?Appears you have the large unregulated DC, but do you have the standby supply?Here's a repair I did which offers some insight as to how switchers work:https://youtu.be/ZpuusBJhh4c?t=734The standby supply can be as simple as a dropper resistor and capacitor, and easy to test. Once you get that confirmed you can start exploring elsewhere, but I'd suggest testing components in-circuit with the power off (and discharged) first. There's plenty you can go after without powering up.Last but not least.......Please be safe when working on these PSU's, there are very large and dangerous voltages on-board even when unplugged. Be careful.Ian.

author: BradC
author_rank: Super Contributor
timestamp: April 03, 2025, 09:38:31 am
content: I always power up with a dim bulb tester when doing PSU repairs. It has saved a few $$ worth of semis over the years where an unrestricted power up would have smoked the new one also.

author: computers_and_cats
author_rank: Newbie
timestamp: April 03, 2025, 09:07:11 pm
content: Replies here don't work the way I am used to and still learning how to use the forum so just going do this all in one comment.tom66Ok got pictures of two of the ICs. IC4 has too much conformal coating on it to read but that is on the output side anyways.IC1 (Top) is labeled J39K78 OB2334AP. Appears to be an "OB2334 High Performance Current Mode PWM Power Switch"IC2 (Bottom) is labeled J3965A OB5269CP. Appears to be an "OB5269 High Voltage Green Mode PWM Controller"* On IC2 when I was previously probing around I was seeing -63V on several of the pins. Something tells me that is very bad based on the datasheet.Yes the fet that blew appears to control the 33V rail on the board.IanJICs are listed above with pics attached. Model info off the unit:Standing Desk Control System JCB36N2B-110 by Zhejiang Jiecang Linear Motion TechnologyPSU PCB: Linkpower APCB1180I believe large unregulated DC and standby supply would be accurate. The display and controller are always on and waiting for request to change desk height. I included a picture of the brains of the operation. I will definitely check out that video.Also yes trying my best to be careful thanks. Probing while live is intimidating but I am remembering to be very mindful of where and how I touch this.BradCThanks for the tip. I will have to make one of those before applying power again.

author: tom66
author_rank: Super Contributor
timestamp: April 03, 2025, 10:56:15 pm
content: OB2334AP can be disregarded for now. It contains an internal FET and we suspect the fault is with the main 33V supply, with the external FET.For OB5269, test resistance (POWER MUST BE OFF!) between pin 5 and 6, and pin 5 and 4. I would like to see readings in the several kohm range, if not more.I suspect the negative voltage readings are due to you using the wrong ground, not an actual fault. Can't see how a fault could lead to large negative voltages here.

author: computers_and_cats
author_rank: Newbie
timestamp: April 03, 2025, 11:19:37 pm
content: Between pin 5 and 6 I get 69.5KBetween pin 5 and 4 I get 69.5K alsoI was referencing to wall ground for those negative voltages which was probably wrong.

author: tom66
author_rank: Super Contributor
timestamp: April 04, 2025, 10:07:28 am
content: Use the negative of the high voltage electrolytic capacitor (brown one marked 200V on your board) for ground, not wall ground.But for now, I can see your controller IC isprobablyOK. So you can probably get away with just replacing the FET. Use the light bulb recommended by others in series to reduce the risk even more. Good luck.

author: MathWizard
author_rank: Super Contributor
timestamp: April 06, 2025, 07:05:35 pm
content: So I know FET's might go off with a bang, but what about the big current sense/limiting resistors that burned, do they actually pop sometimes too, like tom66 said? Can they locally heat up inside enough to blow apart ? I know they are probably not regular resistors, they are fusable/flame retardant or something. But yeah 120V or whatever basically into some sub 1 ohm resistor, yeah that can be a huge current. Enough to trip a breaker I'd think, or close to it, and quicker than the fuse would burn out. But I guess all the wiring adds up and there's more resistance than I think, back to the breaker.

author: tom66
author_rank: Super Contributor
timestamp: April 06, 2025, 07:48:24 pm
content: Those resistors absolutely can go open circuit, what will happen is the FET shorts so full line voltage gets put across them (well a bit across the FET and a bit across the resistors). Depending on the resistor they can survive the momentary 1000W+ dissipation this causes but in this case it looks like they bit the dust too. It really depends on how long the fuse/circuit breaker takes to open. So good point they should probably be replaced if they measure open circuit.

author: thm_w
author_rank: Super Contributor
timestamp: April 07, 2025, 11:19:28 pm
content: Quote from: MathWizard on April 06, 2025, 07:05:35 pmThe bulk capacitance on the input will dump into the resistor and FET, I can't see what value it is 200V maybe 300uF?Someone can test if thats enough to blow up a resistor on its own. That might happen before the 5A fuse has time to heat up.Would be an interesting test to run.