Hi I have a DC 30v 5a bench power supply QW-MS305D.
I need help pls .
Only 6 months old not used much as a hobby I repair laptops n phones.
Bought it from ebay and they said only 30 day warranty.
So spent weeks lately trying to repair it. I obv bought a better more expensive replacement unit.
But just annoying me that the faulty part I cannot sourse anywhere on Google or you tube .theres no part no on it . I
It is a 30v 5a -1507 only markings on it. And it has 13 pins 6 input and 7 on the output side which has a 2 3 2 pins 3 outlet power supply that feeds 3 big tracks on the back . But they are SHORTED. So all 3 tracks joined short circuit .
Cannot find a replacement driving me crazy



I traced t

 

have a DC 30v 5a bench power supply QW-MS305D.
I need help pls .
Only 6 months old not used much as a hobby I repair laptops n phones.
Bought it from ebay and they said only 30 day warranty.
So spent weeks lately trying to repair it. I obv bought a better more expensive replacement unit.
But just annoying me that the faulty part I cannot sourse anywhere on Google or you tube .theres no part no on it . I
It is a 30v 5a -1507 only markings on it. And it has 13 pins 6 input and 7 on the output side which has a 2 3 2 pins 3 outlet power supply that feeds 3 big tracks on the back . But they are SHORTED. So all 3 tracks joined short circuit .
Cannot find a replacement driving me crazy would appreciate a part no v much or a compatible . It is a switch mode modified to linear as bought. Even emailed the manufacturer . Nothing
David

 

That is the high frequency switching transformer.
What do you mean by "they are SHORTED"?
They are the windings of the transformer which will show as very low resistance on an ohmmeter.

 

I'd guess that switching power transformers like that are custom items.

How did you determine that the power supply had failed?

How did you determine that the transformer was the failing part?

I'd think it unlikely that the transformer would fail. Searching
on that power supply I found "a" manual:

https://www.image-tmart.com/imgs/PDF/QW-MS-series-user-manual.doc

Which talks about not connecting anything to the power supply while
turning it on or off. That's sounds like the power supply is a poor
design and does out-of-bound things on power up/down.

There's also this page on someone who's fixed one:

https://neil.fraser.name/news/2020/04/26/

He replace a power MOSFET. I'd think that a lab type power supply
should be designed so that it's near impossble to blow out the pass
element like this so again I'm not impressed with the power supply design.

Also he notes that the output voltage was 63V after the failure, again
not something I would want powering my circuits...

 

That is the high frequency switching transformer.
What do you mean by "they are SHORTED"?
They are the windings of the transformer which will show as very low resistance on an ohmmeter.

Hi Mr chips thankyou for responding
Yes im aware that some componants csn show short circuit on a buzzer continuity and still be ok . I check with ohm meter too as buszzer sounds at below 1 amper 0.67 something like that.
Anyhow to answer your question
The windings have 3x output s
7 pins in a 2 pin 3 pin and 2 pin
The picture I took of the underside board I point to these holes that show the 3 different power rails . With the switching transformer pins are all 0.06 ish with each other which made me think that the mosfets thermisters and everything were shorted out ?
I replaced them still same problem .
Changed all parts on those rails .
Before considering taking out the transformer and the power tracks were no longer all linked as one !!
And testing the 7 pins as I said 0.06 /7 between all pins. I hope that is a bit clearer.trying to take short video clip but even 20 seconds is too big a file to upload?
Many thanks dave

 

tried to show in photos the transformer no part number etc 7 pin 3 outlets in a 2 3 2 conf feeding three different power rails .
All 7 are SHORTED together ohm meter resistance 0.7 ish
However in another photo my test Leads have 0.5 ish ohms so the short is about 0.1 ohm ??
Also spied 30v dc to them in another photo and the same 30.7 v across all 3 output windings . But no current being drawn
1 more pic of the 3 main tracks that it feeds but as all linked together all parts on that circuit ie mosfets diodes show as shorted and were originally replaced as thought that was most likely fault .or a cap etc

 

That's a Switchmode PSU Transformer, you won't be able to buy one online,.

When the Transformer was in the psu , what was happening , do you have a scope ??

 

I'd guess that switching power transformers like that are custom items.

How did you determine that the power supply had failed?

How did you determine that the transformer was the failing part?

I'd think it unlikely that the transformer would fail. Searching
on that power supply I found "a" manual:

https://www.image-tmart.com/imgs/PDF/QW-MS-series-user-manual.doc

Which talks about not connecting anything to the power supply while
turning it on or off. That's sounds like the power supply is a poor
design and does out-of-bound things on power up/down.

There's also this page on someone who's fixed one:

https://neil.fraser.name/news/2020/04/26/

He replace a power MOSFET. I'd think that a lab type power supply
should be designed so that it's near impossble to blow out the pass
element like this so again I'm not impressed with the power supply design.

Also he notes that the output voltage was 63V after the failure, again
not something I would want powering my circuits...

Hi michael8
Many thanks for supply
I put some more pics explaining
A long explanation to Mr chips tired as he sent back also. If you can access the thread .

But in a nutshell amps showing and adjustable . But absalute 0 0.00 volts
So cannot drive the current. Then lost the amps also but display 00.00 00.00
So investigated and found that the 3 main power rails were shorted together!!see pics . 3 outlet supply never thought it was the transformer .
Did some tests on the mosfets etc on the heat sink that are both on same rail and found them both to be connected to the 3 power tracks so thought great bought replacements and the same so received the new and old out of the board and they were not shorted !!!
So disconnected anything that was connected between rails until the only thing left was the transformer so unsoldered it . And the 3 rails were separate again
Tested the output pins 2 3 2 pin config and all showing 0.6 0.7 ohms , but my test leads as shown in pic 0.5 ohms so the transformer 0.1 ohm so the short between the windings must be close to the outlets otherwise there would be more resistance.
Many thanks .
Will look at info you sent
Obv I bought a better power supply from Amazon so easier to complain to if malfunctions.
It was just niggling me that I couldn't figure it out so kept going back to it
. It is a load of rubbish and should have binned it
But I get a bit OCD over things I cannot repair.
Laptops and phones pretty straight foreward . But 1st time on a power supply.
And all the obv faults like blown caps or bridge rectifier or any other componants with a part number . A doddle
But no it had to be a problem no one else on YouTube or wwweb had an answer.
So thankyou
Dave

 

That's a switching power supply transformer so it isn't designed to run
at 50/60 Hz, but at 20 KHz or higher.

https://en.wikipedia.org/wiki/Switched-mode_power_supply#Transformer_design

The "less copper in the transfomer" means that the wires in the tranformer
are short and don't have much resistance. It's likely that the measurment
is showing the normal resistance (+ meter offset) and the transformer
is fine.

 

The perils of eBay and 30-day warranties.
A quick search on this item threw up user problems.
Even new at £39 would put me off
A decent bench power supply is worth paying more for, to protect you and anything attached to it.
I wouldn't spend any more time on it.
My 40-year old Thurlby is still going strong, and a newer one an EL-R series is a reliable budget PSU

 

The perils of eBay and 30-day warranties.
A quick search on this item threw up user problems.
Even new at £39 would put me off
A decent bench power supply is worth paying more for, to protect you and anything attached to it.
I wouldn't spend any more time on it.
My 40-year old Thurlby is still going strong, and a newer one an EL-R series is a reliable budget PSU

Hi Phil
Thanks for reply . But an ohm reading of 0.01 ohms is def shorted. When it's attached to the board it joins the 3 supposedly different main power tracks together. As I've said .
I have already bought another power supply more money better reviews and from Amazon so trust the company.
I know I should throw in bin but I'm like a dog with a bone if repairing something that doesn't have a part number or isn't available anywhere.
A bit OCD. I'm afraid
Regards
Dave

 

That's a Switchmode PSU Transformer, you won't be able to buy one online,.

When the Transformer was in the psu , what was happening , do you have a scope ??

Hi Dave sorry 4 delay
Thanks for your interest .
When the transformer was in the psu
No vantage no current
Tracked the voltages on the board and had between 2 and 10 volts ac
Nothing on the Dc voltages
I changed the bridge rectifier .
No difference . A few caps
And other things that were on the 3 power rails that were all short circuit
Changed the power mosfet on the heat sink .
And still no power on dc .
Took out the transformer and cool all 3 power rails were separate and everything on them were showing correct results ie no shorts on mosfets etc see my above pics and explanation .
Also the 3 pin middle rail is connected to the neutral track ?
So as it stands with that there's not 3 power outlets as one goes to neutral .?? Confusing
Cannot get schematics anywhere .
Other similar p supplys with different name . Have big heavy main transformer and the electronic componants do the work of changing and managing the Dc voltage /current . If I knew what the outlet voltages were on the 3 rails I could chuck transformer in bin and make my own power supply mini pcb and link to the appropriate tracks .
Other week I'm just going to make it into a digital clock display .lol
David

 

Hi Dave sorry 4 delay
Thanks for your interest .
When the transformer was in the psu
No vantage no current
Tracked the voltages on the board and had between 2 and 10 volts ac
Nothing on the Dc voltages
I changed the bridge rectifier .
No difference . A few caps
And other things that were on the 3 power rails that were all short circuit
Changed the power mosfet on the heat sink .
And still no power on dc .
Took out the transformer and cool all 3 power rails were separate and everything on them were showing correct results ie no shorts on mosfets etc see my above pics and explanation .
Also the 3 pin middle rail is connected to the neutral track ?
So as it stands with that there's not 3 power outlets as one goes to neutral .?? Confusing
Cannot get schematics anywhere .
Other similar p supplys with different name . Have big heavy main transformer and the electronic componants do the work of changing and managing the Dc voltage /current . If I knew what the outlet voltages were on the 3 rails I could chuck transformer in bin and make my own power supply mini pcb and link to the appropriate tracks .
Other week I'm just going to make it into a digital clock display .lol
David

You ever figure it out? Could it be the optocoupler? ........(complete guess on my part I tend to be the toss every part at it till it works or the costs exceed the price of a new one)

 

I got mine (exact same one) repaired about 2 months ago now.
It's been in constant use. No issues.

I had connected it up backwards to a battery and POOF (magic smoke)

07/22/2021 07:40
Replaced all 4 current and voltage adjustment potentiometers

07/22/2021 12:08
Seems to be working Now after replacing D21

07/22/2021 12:08
CURRENT - COARSE Potentiometer needs cleaning or replacing

07/18/2021 05:10
Connected everything and tested. Seemed ok, then D21 immediately caught fire when current was adjusted up at all.
Turned out to be a bad R1SM Diode on the 12v- input rails.

07/11/2021 07:02
Both IRF840 Mosfets had very high On state Drain to Source resistance so both were replaced.

07/07/2021 09:14
Found a track on the PCB that was damaged near the 12v power in. Was blown apart. All fixed.

06/27/2021 10:55
P75N75 Out For Delivery today June 27th

06/20/2021 10:20
Found shorted P75N75 Mosfet Ordered P75N75 Mosfet