Repairing HY5020E

[zer0net]

Hello Everyone,
I have inherited Mastech HY5020E. It powers on but i am unable to adjust voltage or current, nothing displays. I do not measure anything on the output terminal. When I power off the unit, it takes about 5 second before it powers off, and it flickers 3 times before it completely powers off. I popped open the cover, everything looks ok, there is no burnt components or smell or anything like this.
I do not know where to start checking. Any help will be greatly appreciated.

Thank you

[zer0net]

Here is another  picture

[TheSteve]

Can you take a good picture of the left side of the PCB near the front. I want to see which hardware version the supply has.

edit - Based off the pics you already have it is a different rev then the ones I have seen. I would concentrate your efforts on the PCB near the front left - the one you can only see the solder side of.

[zer0net]

Sorry for the late reply. I have uploaded some better quality images on imigur since I cannot upload them here. Here is the link to images:http://imgur.com/a/B5KSi
I made a small video showing its behaviour and here is the link to the video: http://sendvid.com/fhty7ylv

I hope this additional info sheds some light.

Thank you

[bitseeker]

Welcome to the forum, zer0net.

Regarding the weird behavior when turning the power supply off, this appears to be a common problem. I've heard of it before. In addition, my 5020 does the same thing even though it operates just fine, otherwise. I haven't had time to look into it, but it seems like there's a very large capacitor on the power rail for the control board, perhaps to ensure that the pass transistors' gates don't suddenly open wide during power off. Nevertheless, it looks ridiculous (especially the final flashes of life at the end) and something I'd like to correct. However, the good news is that it is unrelated to the other problems you're having.

Nothing appears to be burned from what I could see in your high-res photos. The MOSFET gates on these supplies have been known to get damaged by overvoltage, though. So, that's one thing you might check by making sure the transistors still operate properly.

You might also start from the other end and verify that the potentiometers effect a change in voltage when they're operated. Also, check for cold solder joints between the pot pins and the PCB.

In your original post, you mentioned that there's no display. It appears to be working in your video, albeit showing all zeroes.

[whitebear]

Well, here is a link to the schematics for HY5020E http://kazus.ru/nuke/users_files/01052009/4813318.pdf

The unit has a switching regulator (controlled by HY012 PCB) and the linear regulator (controlled by HY014 PCB) connected in series. So, I'd first check the voltage between the regulators (drain of Q401). If the voltage is ok - you need to check the linear regulator, if not - look at the switching one.