Help a newb fault find a DC power supply - QW-MS3010D DC - All fixed! - Page 1
Forum section: electronics / beginners

author: TURSTY
author_rank: Newbie
timestamp: July 13, 2025, 11:20:42 pm
content: Hi guys,I'm a green newby on the path to teaching myself basic electronic circuit repair, purely because I find it somewhat interesting.I have a VHF marine radio that's making a pretty bad hiss so was about to get stuck into looking at it, when my DC bench power supply decided to just stop powering up.Its a QW-MS3010D DC Power Supply, and yes, I know its rubbish and it will be replaced with something decent, but for now I would like to fix it as a learning opportunity.So the symptoms are easy....- It was working fine for a number of days, powering a 12v smart relay while I connected it up to my home automation. Was off for a day, then when I went to power up the 12v VHF, it wouldn't turn out at all (the VHF want connected to it at that stage).- Nothing is displaying on the screen and no output at all when switched on- Current limiting lightbulb goes bright, then goes outWhat have I done so far?I couldn't find an exact schematic for this power supply, but the attached MS605D one is pretty similar so should work as something to refer to. I've marked the voltages I can measure at different locationsI have tested the following:AC and input side:- K (the 110v/220v switch) doesn't exist on my PSU - Im in New Zealand so its blanked and labeled 200-240v only- Both Bridge rectifiers (BD1 and BD2) are working - getting 330v DC- Not getting any output on any of the transformers (tested with a multimeter and scope)- Removed Q1 and Q3 MOSFETs and they tested fine in the component tester- Couldn't seem to measure R3 and R4 in circuit, so I took the large capacitors C1 and C2 out and chucked them in the component tester - both measured 1.1% Vloss which I think is ok? R3 and R4 are good - 100k.- R2 and R5 are good. So is R13. No shorts on any of the transformer windings or the caps on the left side of the transformers- U1 on my PSU is a DK-142 not a DK112, but seems to be a similar chip, although I can't find a data sheet for it.- U1 Vcc measures 1.43v - which I think is way to low?- D7 ad D7a test fine. So does R7- I cant measure anything on any of the transformer coils with both a multimeter or a scopeOutput side of the transformers:- I tested as much as I could off T3 - All the resistors and diodes test ok. No shorts on the Caps. I haven't removed Q2 yet- Same for T1 and T2 - all the diodes and resistors seem good.- U4 is the same KA7500B chip - Vcc has 0V- CN2 - the thermal sensor is open circuit when cold (not sure that's supposed to be or not?)- No power on U6 either (LM324N on my unit)Here's where it gets interesting. I took MOSFETs Q1 and Q3 out to test and reinstalled - same problem persisted, but when I took the big Caps C1 and C2 out to test the resistors over them, when I reinstalled them, the PSU fired up and ran again!After turning it off however, it wont start up again and the same fault persists.I'm a bit stumped, but suspect the problem is on the input side of the transformers? Maybe the two capacitors but they seem to test ok. Any suggestions would be greatly appreciated!

author: Seekonk
author_rank: Super Contributor
timestamp: July 14, 2025, 10:17:06 am
content: Repair is not about wild guesses. That is a rather convoluted design and would be difficult without a schematic, but you got one. No display is the key. It is not in the power output section. The supply that powers the control electronics is not working. Removing the caps and started working is a red herring. Has nothing to do with anything other than it might have made a connection work in another section for a while or just a random intermittent. Problem is on the AC side with that little switcher.

author: TURSTY
author_rank: Newbie
timestamp: July 15, 2025, 03:27:43 am
content: Quote from: Seekonk on July 14, 2025, 10:17:06 amThanks for the reply. Yeah I'm definitely still learning so would love to get to a point where I'm 100% confident in the test results I find being abnormal. I spent a lot of time thinking on that section around U1 (the DK112 chip) but was unsure what I should expect to see. I take it the above is enough to go ahead and replace the DK112 chip as then next step?

author: Poroit
author_rank: Frequent Contributor
timestamp: July 15, 2025, 08:00:46 am
content: G'day Tursty,This may help you.

author: TURSTY
author_rank: Newbie
timestamp: July 28, 2025, 01:56:58 am
content: I'm always a fan of closing out threads so people searching can maybe benefit in the future.I changed out the DK112 chip driving the control circuitry transformer and its back in business working as it should. Happy days. Thanks Seekonk.