Electronics > Repair
Mastech HY5020E 50v 20a Bench Supply: Economical to repair?
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CJay:

--- Quote from: Shock on April 20, 2016, 11:18:49 pm ---
--- Quote from: CJay on April 20, 2016, 12:48:16 pm ---With the dire warning that if it's got auto input voltage sensing then using it on 220V with a series light bulb could just trigger the rapid, violent release of more smoke.

--- End quote ---

I've not read much about this occurring, have you got any instances or references where this has happened and how a good SMPS failed from it? I can only seem to find it mentioned on the repairfaq.org site. Would the same occur using a variac?

--- End quote ---

Personal experience from a couple of years of my life when I worked in a PC repair company, some power supplies aren't 'wide input range' but have a switch to alter the smoothing capacitor configuration (turns it into a doubler configuration and yes, they used to be expensive enough to be worth repairing).

That switch can either be a physical panel mounted one or an electronic one. The electronic ones can get latched into 120V operation and winding the input voltage up higher can have dire consequences for the PSU.

Which is quite frustrating when you've spent an hour drilling out rivets, cleaning up soot and replacing a couple of dozen components.

Some PSUs were (may still be) too sensitive to dips and spikes on the mains supply and latched to 120V in normal operation, we used to call them one shot brownout detectors.

A supply that has a single smoothing capacitor and is rated for 100-264V input is a wide range supply, a switched supply will have series connected smoothing capacitors and the 'centre' connection between them is where the input voltage switch is connected.
richfiles:
Confirming, this supply has a manual 120/240 switch.

Hearing of the issues in the design instills me with such "Damn it, China!".
MrSlack:
It's the Chinese design ethic. Steal someone else's well thought out design, go on a parts minimalism trip removing every bit of protection and reliability so it works long enough to sell it, slap times new Roman on it and whip an army of cheap labour to knock it out.

Like Sinclair but with less black and red and a different typeface.
CJay:

--- Quote from: richfiles on April 22, 2016, 12:30:00 pm ---Confirming, this supply has a manual 120/240 switch.

Hearing of the issues in the design instills me with such "Damn it, China!".

--- End quote ---

A proper switch, excellent, one thing less to worry about.

Looks to me that you should be able to test the various boards in there one at a time and build it up as you verify each one.

I think you can ignore Q401 for the moment, leave it out of circuit, it's being used as a linear pass regulator.

HY023 is an independent power supply, used for soft switching the main supply, powering the PWM controller, linear regulator drive and logic/display so that should power up if you can feed it rectified and smoothed line voltage or a suitable alternative.

HY012 will produce dual, complementary PWM modulated signals on V01 and V02 if you give it, I suspect, 12V or thereabouts on VCC, you should be able to vary the pulsewidth by varying the voltage on V1 (VI?).

Make sure the PWM drive is making it through the drive transformer to the gates of Q202, 203 A and B

Definitely add some 15V zeners to the gates of the primary MOSFETS to ground as someone else suggested, they won't save anything if it goes bang again but they'll help stop it going bang.

Once you get to this point you should be able to power up the main supply and get a non variable output on the drain of Q401. I'd guess somewhere around 60V.

You'll need to test HY013 and HY014 as one unit but again, you should be able to do that without the rest of the supply running. I'm guessing they're not going to be faulty but you don't need much more than 12V to do that.

Front panel meters are powered by HY023, they're real simple.


richfiles:
Thanks CJay for that finely detailed troubleshooting information!

I won't be able to get to it immediately. I had the other day off, so I was unusually active with this, and I also just got an order for some cables that I build (the cable provides power and commutation for a brushless motor in a surgical tool. The cable is built to be able to withstand being autoclaved repeatedly to sterilize it). I'll have to do those first. Hopefully, by the time I finish those, my dual tracking power supply that I bought will have shown up, and I can use that for the testing...

Heck... maybe i should open THAT ONE up and have a look to see if it needs any safety updates. It's a used unit, from a liquidation sale. All I could afford at the moment. Just got SO TIRED of current limiting my Tektronix 500 series supplies. 400 mA is NOT reasonable for a bench supply to max out at. Fine for breadboarding a few logic chips or an op-amp on a breadboard, but not anything more!

I think I can follow most of what you said. I tend to work visually, hence why I do stuff like annotate pictures of the PC board and stuff. You broke this down into a nice bullet point list of tasks, and that's really awesome!  :-+

My boss had a Sorensen DCS 300-3.5 power supply on hand that he uses in place of this unit. He has another Sorensen supply that's 3 or 4 times thicker. I don't recall what that model was It's a Sorensen Elgar SGA400/12C-0AAA (Input 187-242V 3 phase, Output 0-400 Volts DC @ 0-12 Amps). It's a beast of a machine, probably similar to the size of four of these units. Both are rack mount units. As far as I know, he's not even using the big one yet (He's not, he wants to sell it). Needless to say, with those beastly power supplies, I don't think he has a need for this sad little Mastech unit, especially in a production setting, if it's gonna just up and pop again someday. I kinda wanna fix it, and I kinda don't even have a clue what to do with it if I succeed!  :-//

Can always do it for the challenge.  ;D

What are thoughts on the 4 big caps? Obviously, they were in circuit at the time that things blew, and the vinyl labels suffered some cosmetic damage... Given that all 4 of them measure within 2 µf of each other and are within the +/- 20% tolerance for their value, can I just leave them be for now? Replacing them will cost $16-28, depending on the grade I buy (and not downgrading to cheap craps... I mean caps). I do not have an ESR meter, but I should be able to do it with a function gen and scope, right? If they are fine, I'd rather leave them be to keep the repair cost down. If they are borderline though, I would replace them. It just seemed interesting to me that they'd all test so consistently.

Another question, regarding the creepage distances of some parts. Obviously, the design is... less than stellar. My question, is will a thorough isopropyl alcohol cleaning, a bake to dry out, and a conformal coating help with this, or is the issue of an arc over something that would be a potential hazard regardless? I only occasionally work with 120 volt stuff, and have never serviced a switching supply before. About all I've done have been linear supplies and voltage doublers (to run nixie tubes). I've always conformal coated my high voltage stuff... But is that actually good to do?
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