Problem with KA3005D clone PSU

[tonny]

Hi!

I have a problem with my PSU that I need help with. Due to use error it was short circuited briefly and now it doesn't work properly.
It's a RND 320-KA3005D unit wich uses same internals like Korad or similar clone-like power supply.

Problem that I have is that device works fine when I power it on but when I activate power delivery option device just goes nuts. Display shows max voltage but there is no voltage that seems to be delivered to the leads. I have done visual inspection of all the boards but there are no visible burned components. It's only the power board that looks a bit strange. One of the transistors has something on the surface (see photo) but I don't know if it's OK and it was already like that when I bought it or not. I can post a video if that will help showing the problem.

I was wondering if someone had any guesses on what can be wrong based on scenario (short circuit) or symptoms (when power is activated device goes crazy)? I have access to a soldering station, digital thermometer as well as multimeter.

I have found following already existing thread on forum https://www.eevblog.com/forum/repair/korad-kd3005p-problem-with-current-metter/ which tries to solve different problem with similar/same PSU and there are some schematics provided for the device so it might be helpful. I'm rather novice so I don't know how to use that schematics in a best way.

Thank you in advance!

I have used device just a couple of times and it was rather expansive purchase for me. It will be sad to throw it away. Also, considering the nature of the product (electronics testing), one would assume that they could think of potential short circuits that might occur and have some protection against them. I tried to contact the manufacturer but haven't got any reply from them. Shop where I bough the device also brushed me off.

[tonny]

I have created a video showcasing the problem: https://youtu.be/JiYRBEcfVwE

[bateau020]

There are quite a lot of PDFs with schematics, procedures and upgrades to be found on the internet of this PSU.
Attached is one PDF I got regarding this PSU. Not sure if I'm allowed to post it, just found it elsewhere here. So if someone thinks I should remove it, tell me.
Schematics are in post #2 of the topic you mentioned.
I suggest just start measuring, and checking suspects after desoldering.
The fault shown in your video is a common one. Multiple people have reported this AFAIK. Shouldn't be too hard: the output transistors are probably to blame.

[JackJones]

It's not a clone, just rebranded. I have the same RND Korad, it's just the same as any other internally.

Shorting the output of a lab supply should not damage it, they are designed to withstand it.

One thing I didn't hear in your video was the relays making a noise. It should be selecting the appropriate taps on the transformer, that would probably be the first thing to check.

Edit: I don't think the relays not switching is the main cause, probably just a symptom. The schematics you linked aren't super clear, but at a glance it looks like U52A takes the feedback from the output to set the correct voltage, I'd start probing somewhere in there.

[m k]

You can start checking how it is through an external connection.
If output is same as it is internallly you know there is a rationality of some sort.

Or just start checking all internal voltages.
With luck one is missing or irregular and is messing the feedback.

[thinkfat]

There are quite a lot of PDFs with schematics, procedures and upgrades to be found on the internet of this PSU.
Attached is one PDF I got regarding this PSU. Not sure if I'm allowed to post it, just found it elsewhere here. So if someone thinks I should remove it, tell me.
Schematics are in post #2 of the topic you mentioned.
I suggest just start measuring, and checking suspects after desoldering.
The fault shown in your video is a common one. Multiple people have reported this AFAIK. Shouldn't be too hard: the output transistors are probably to blame.

I remember Dave made a video about this (or a similar failure), #315 or something if I'm not mistaken.

[tonny]

Thank you for your tips! I have started checking the transistors on the power card and D1047 components doesn't seem to be working well. It supposed to be NPN transistor but one of them was passing current in several/both directions. I removed both of them and readings were different. They are sold in pairs so I ordered 2 of them and replace both of them to see if this will make things better.

[tonny]

Yeah, I have seen that #315 video. It looks very similar. Same crappy PSU as mine I wish I have seen it prior buying. I hope it was just poor quality components that caused it and not some kind of design flaw.
I have even watched the followed up video (https://youtu.be/uecvB4U1fps) and it's good to know that it can handle short circuiting. I was really surprised to see my device turning unusable just due to short circuiting.

[thinkfat]

I have the same psu. Since a year or so it's been fine. I hope it stays like this.