Bench Power Supply - Page 1
Forum section: electronics / repair

author: Mad Mart
author_rank: Contributor
timestamp: March 28, 2016, 01:29:47 pm
content: I bought a budget bench power supply last year and the other day whilst testing some vehicle voltage stabilisers it just stopped working. I have established that there is 230V AC coming into the unit but there doesn't seem to be any low voltage DC at all. I can't seem to find a schematic on-line so I was wondering if anyone on here can help a relative newbie?The power supply is a QW DC POWER SUPPLY QW-MS3010D but it is also badged under other names/numbers. It's a 30v, 10a model.

author: mij59
author_rank: Frequent Contributor
timestamp: March 28, 2016, 02:26:38 pm
content: First to check is the transformer, secondary voltage(s) present ?Post some photo's of the pcb(s), and the inside of the power supply.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 28, 2016, 02:38:42 pm
content: If nothing happens when you turn the unit on it's normally time to replace the primary fuse.

author: Mad Mart
author_rank: Contributor
timestamp: March 28, 2016, 03:46:21 pm
content: The transformer (centre top below three caps) has 14 pins, 7 per side. Is this what I should be checking first? Where is the primary fuse on the photo?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 28, 2016, 04:25:54 pm
content: There is no input fuse visible in the picture. There may be one inside of the input connector, possibly accessible from the back, or they just left it out.It's a switched mode supply, so not that easy to repair. The first thing would be to carefully check up to the primary side rectifier. Just be careful the big caps are discharged.

author: mij59
author_rank: Frequent Contributor
timestamp: March 28, 2016, 04:49:53 pm
content: Assumed it was a linear power supply, but it's a switch mode power supply, so testing the secondary voltage of the transformer is not the first step.Be very careful, most of the circuit is connected to mains, high voltage present.You could check the mains voltage on the mains plug, and the working of the mains switchPlease post a picture of the other side of the pcb.

author: Mad Mart
author_rank: Contributor
timestamp: March 28, 2016, 06:09:26 pm
content: There is a fuse on the incoming connector.The black wire is the 230V incoming mains from the on/off switch on the front of the unit. There is definitely mains voltage getting this far.This is the best pic I can get without desoldering wires.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 28, 2016, 07:41:09 pm
content: Looking for 3010D power supply returns a lot of information, unfortunately all for linear power supplies. It looks like you are the first (?) with a switch mode oneand a complicated one even. I think you should start reading about SMPS's and try to find out what went wrong, or just throw this one away and get another one for about 60 euro, or maybe even a somewhat more decent one. I like to fix stuff also but SMPS's are a PITA even with schematics, and I doubt if you can find one. If it's not something very simple like a blown fuse (without any further damage) it will be a frustrating task.

author: Seekonk
author_rank: Super Contributor
timestamp: March 28, 2016, 08:13:55 pm
content: Often there is a secondary fuse to prevent user just installing a larger one. From the sound of things this is just a random failure. If nothing lights up, I suspect the problem is around the small IC center bottom. This converter / transformer supplies power to the control electronics. Read out the part number of this chip. Data sheet will give typical schematic.

author: Tim F
author_rank: Regular Contributor
timestamp: March 29, 2016, 03:30:49 am
content: I have two of those supplies, well actually the 30V 5A model - 305D. With one of my units the termination to one of the secondary windings on the transformer came loose so when set to certain voltage ranges and the faulty secondary winding was selected the unregulated supply voltage would plummet until the supply dropped out of regulation and the output voltage got so low that it would select a different secondary winding on the transformer which happened to be functional. Then unregulated supply voltage would then shoot up enough for output voltage to come up enough for it to select the faulty winding again and the process would repeat over and over.I removed the outer insulation on the transformer and found that the transformer has aluminium secondary windings. They had just loosely twisted the copper wiring with the aluminium windings (because you can't solder them). I crimped them properly which fixed the issue.Another problem I found was that the unregulated voltages were way too high and the pass transistor was burning up a lot of power because of a 20-25 odd volt drop between unregulated supply and output. This is probably a combination of using a different transformer than the circuit was designed for and the increased mains voltage in Australia (normally 240-250Vac).I reversed engineered the circuit and changed all the set points (by changing resistors) where it switches in different transformer taps so now the worst case is about 12-14V drop.If your unit does not regulate at any set voltage then you might have blown the pass transistor(s), a fuse or the current shunt (0.15ohm 5watt resistor in my units). I think the only difference between the 305D and the 3010D is that the 10A model has two pass transistors, two current shunt resistors and a bigger transformer. My 305D has unpopulated locations on the PCB for the extra components.edit: scratch that, looks like you've got some kind of SMPS version, so my experience probably isn't useful unfortunately. First thing to check would be the switching transistors, mounted to that aluminium heatsink.

author: mij59
author_rank: Frequent Contributor
timestamp: March 29, 2016, 04:12:02 am
content: Check for mechanical problems like bad solder joints, connectors, etc.You need to do some desoldering, I think the easiest way to get access to the pcb are mains connections.On the first heat sink is a thermal fuse (clixon), it could be a normally closed or normally open type.

author: Mad Mart
author_rank: Contributor
timestamp: March 29, 2016, 07:24:19 am
content: Thanks for your replies guys. Always the same with me, nothing's ever straightforward.I'll remove the PCB and check all the soldering, it could be that simple.

author: speakeasy
author_rank: Newbie
timestamp: March 07, 2019, 05:40:20 pm
content: Thanks for all of this I had a similar supply I had shorted out even with current limiting it killed the components and this got it back up and running for under $20.

author: Chris56000
author_rank: Frequent Contributor
timestamp: March 07, 2019, 06:22:44 pm
content: Hi!https://www.eevblog.com/forum/beginners/maisheng-ms-605d-power-supply/There's some diagrams & schematics here that might prove to be of assistance!Chris Williams

author: wictor
author_rank: Regular Contributor
timestamp: March 08, 2019, 05:26:58 am
content: Check that black NTC near fan.Wictor

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 08, 2019, 07:14:44 am
content: As others have mentioned it's worth checking the thermal switch (klixon).Check the smaller SMPS for the housekeeping is working.Also see this thread as your PS is quite similar:https://www.eevblog.com/forum/repair/maplin-n93cx-(aka-manson-nrp-2050)-smps-fault/

author: cailan
author_rank: Newbie
timestamp: July 09, 2019, 12:44:42 pm
content: i have a QW-MS3010D DC bench power supply that does not output any voltage0Volts and 0Amps i have replaced the 4 transistors with new ones nothing is burned so im out of ideas plz help

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 09, 2019, 06:10:09 pm
content: Do the LED displays light?If not, start by looking at the secondary of the smaller supply built around the DIP8 and smaller transformer at the bottom of your photo.

author: cailan
author_rank: Newbie
timestamp: July 10, 2019, 04:48:20 am
content: yes all of the lights work like the led display and all of that just no voltage on output

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: July 10, 2019, 03:11:41 pm
content: I think I still have one of those. I've had to fix it too. In mine one of the wires that were going from the front panel to the pcb was not soldered properly. It was just sitting in the pcb not connected.Also, check if your potentiometers to regulate voltage and current are ok. They break really easy and even if they look ok, they may not be.Check them with your multimeter for resistance. Look for cracks on them.Check the protection diode on the front, between positive and negative binding post as well as a capacitor that's there...See if you have any cracks in PCB, any pieces broken, broken tracks. Soldering on all the components and finally the output transistors

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 10, 2019, 11:02:16 pm
content: Ouch! Looking closer I really like the design, primary MOS heatsinks facing secondary rectifier heatsink, same with klixon... Not a killer as things are a little insulated just things I would have tried to avoid.Getting back to the troubleshooting, now we know the housekeeping power supply is running, I think it's time to measure the value of the large red/brown capacitor in the primary.

author: cailan
author_rank: Newbie
timestamp: July 11, 2019, 12:15:50 am
content: potentiometers to regulate voltage have been checked and work fine no cracksi dont see a protection diode on the front, between positive and negative binding postsPCB is fineoutput transistors are fine

author: Dacke
author_rank: Regular Contributor
timestamp: July 13, 2019, 06:46:19 pm
content: Quote from: cailan on July 11, 2019, 12:15:50 amThe front panel lights up I'm assuming, so the low voltage supply to the logic is probably working. I would check the gate resistors to the main switching transistors. You replaced the transistors, so I'm assuming they failed at some point. If they did, these gate resistors can often go with them. They're generally small and low value, attached to the gate of the FETs. If these are ok, work backwards towards the oscillator. A scope would be more helpful at this point, but if you don't know what you're doing with a scope on a mains powered switch mode supply, then don't do it.