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Author Topic: Blown Caps in 0-30v 0-10a bench supply repair  (Read 4282 times)
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Heres some pics of the supply, and the failed caps-

That thing isn't worth fixing. The transformer is puny, and the heatsink is punier. I would say it is maybe 3amp capable, no way near its 10amp rating.

Chuck it up as lessons learned: the high cost of low prices.
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Alright. So when you guy are talking about the transistors that may have been shorted/fried, are they the two square white ones on board near the blown cap? Can someone point it out to me in one of the pics? There just a good # of transistors (mostly the small black d shaped ones) on that board, and i don't know what ones are the output ones.

And i definately agree its a pos supply. The only reason i'm attempting to fix it, is because in all likely hood its going to cost a few dollars for the caps, mabey a few more for the transistors. Even if its not capable of high amp output, if its repairable, the parts cost justify doing so relative to how much i paid for the device. At 3 amps it has plenty of uses, and eq charging fla's doesn't need high current (nor is it advisable) anyway. Just to have a cc/cv supply for charging batteries would be nice, even if it cant pump out 10a. I can't really imagine a scenario i would need the amperage its "rated" for anyway.

But a quality switching supply with overload/overvoltage and reverse polarity correction, like one of those EX mastechs, is defnately what i want, eventually. Just don't want to spend ~$175 on one atm. I'd rather buy a few caps, and possible a few transistors/diodes and make do. And if this is a linear supply, then i could have both, considering they both have their advantages/disadvantages when i do get a quality switching supply.

Also, if i got some heavy duty diodes, is there any way i could wire them up to the output terminals to prevent reverse polarity damage in the future? Perhaps some heavy duty schotty diodes? If so, would it just be connnecting them from + to -, one only allowing flow in one direction, and vice versa for the other? Mabey add some fuses there as well, since the fuses on the board and ac input plug diddn't prevent this failure? I'd imagine there some fairly simple way to use a few diodes and a fuse to prevent this from happening again (reverse polarity to a fla starter battery), or any load that can push back into the device the way the battery did.

I ordered some quality caps to replace the blown ones, and will test the transistors with my multi once i know which ones you guys are talking about. As far as the trace damage, i'm thinking a solder joint, and an epoxy coating (After replacing the cap of course since its fairly close to the solder joints) is the best way to fix that prob and prevent future arcing there. Is that correct? Or should i just epoxy the trace gap, find the nearest solder joint on each end of the trace, and solder a 16gauge copper wire to bypass the broken trace entirely?


« Last Edit: December 27, 2012, 12:30:50 am by Suaveman » Logged

Denmark
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You need to get to the power transistors and test those:




The white ones are Emitter-resistors, those are fine and don't need to be checked


See here how to test the Transistors:
http://www.instructables.com/id/How-to-test-Bipolar-Transistors-if-you-have-an-Ana/

// Per.
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With all due respect, since you seem to be somewhat lacking in knowledge on what a power output transistor looks like, then your chances of identifying the problem are pretty low.  Without a circuit dagram, someone who really knows what they are looking for will have to dig deep into their knowledge of circuitry to identify where the faults lie.    As previously stated, your best bet is to simply replace the damaged capacitors, switch ON and if it works, all well and good.  If it doesn't work, and you really want to keep it, then pass it onto a colleague who has the skills to trouble shoot.  Otherwise you could well be throwing good money after bad, with little chance of success.
« Last Edit: December 27, 2012, 04:03:43 am by jackrae » Logged

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Here is the circuit diagram that I have found from Chinese website, it may be not the same as what you have but you got an ideas of how this type of power will look like.
 
And here is the manual explain the working of the power supply almost the same as this circuit diagram, hope it can help.
POWER SUPPLY USER MANUAL


* 0-30V-PowerSupply.jpg (372.05 KB, 900x728 - viewed 70 times.)
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Denmark
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My best guess, replace the caps and see what happens. If it still does not work, replace IC1 & IC2, Standard opamps, and they have been nice to put them in sockets. I do not believe the 2N3055 is broken by that stunt.

// Per.
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Go to radioshack and get a couple of those caps. Solder them in and turn on the power supply.

If it works, great.

If not, throw away the guts and reuse the case.
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With all due respect, since you seem to be somewhat lacking in knowledge on what a power output transistor looks like, then your chances of identifying the problem are pretty low.  Without a circuit dagram, someone who really knows what they are looking for will have to dig deep into their knowledge of circuitry to identify where the faults lie.    As previously stated, your best bet is to simply replace the damaged capacitors, switch ON and if it works, all well and good.  If it doesn't work, and you really want to keep it, then pass it onto a colleague who has the skills to trouble shoot.  Otherwise you could well be throwing good money after bad, with little chance of success.

This sounds like good advice. I don't have much skills in circitry, but can test componenets and understand the basic principles behind the supply. I taught myself alot about dc electronics, and am just wading into learning how circuits work, so this may be beyond my skill level. I ordered some quality caps, ~5 or so of each (35v, 50v 470uf), i don't much care for radioshack, and would rather give an ebay seller my business, but regardless. They should be here in a few days.

If the caps/shorted lead patch/solder joint fix the problem, then huzzah. If the transistors are bad, i can replace those if necessary as well, so long as the repair costs are under $25 total. I'll have someone with a proper electronics education take a look at it (my father) after i replace the caps, fix the trace, and replace any transistors/diodes that may have been damaged, before powering it up again for saftey reasons. If he says scrap it for parts, thats going to be the gameplan unfortunately, he's fairly well educated in the matter, though has not worked in the field for almost a decade now. Still, he might be able to show me how/what failed, and if its repairable. Busy week with lots of snow in norcal, so it might be a few days before he has some time to take a look at it though.

Might make a nice vlra battery case you could stuff a charger in hooked up to a solar panel if its unrepairable. And i'm sure theres plenty of undamaged components i can scrap for future projects so its not all bad.

Its also relevant to know, it did not pop, and die immediately. The voltage/current display remained lit, but the loud cap pops and smoke made me disconnect everything as quickly as possible. I have not powered it up since then. i'll wait for the caps and trace/fix before plugging it in again.

Live and learn i guess. Should have just spend a  bit more on a mastech. Don't buy these knockoff pos's. Its like 50-75$ more for something quality and protected against stupid mistakes like reverse polarity connection of a fla.
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Should have just spend a  bit more on a mastech. Don't buy these knockoff pos's. Its like 50-75$ more for something quality and protected against stupid mistakes like reverse polarity connection of a fla.

It is not clear why it blew. It could be just due to poor design or poor quality.
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true, true.

The transistors tested ok, so i'm just waiting for the new caps to see if thats going to fix it. The cap's in question, i'e heard nothing good about, so quality issues definately could have been the cause of failure just going off that.

Or the caps could have been from that bad batch made with the stolen seret suace electrolyte formula from a few years back. Nothing is certain, other than the quality of this supply fits in line for what i paid for it. Lesson learned.
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true, true.

The transistors tested ok, so i'm just waiting for the new caps to see if thats going to fix it. The cap's in question, i'e heard nothing good about, so quality issues definately could have been the cause of failure just going off that.
No.

Quote from: Suaveman
Or the caps could have been from that bad batch made with the stolen seret suace electrolyte formula from a few years back. Nothing is certain, other than the quality of this supply fits in line for what i paid for it. Lesson learned.
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Quote from: Suaveman
Those caps do not just blow up out of the blue. You connected your battery backwards, and that blew them. Get over it, it has nothing to do with "cheap" capacitors. Even Nippon Chemicon's or ELNA won't tolerate that kind of abuse!

// Per.
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^^

Well yeah, your probably right. Though i cannot be certain it was connected with reverse polarity. The fact remains the cap quality has been said to be very poor for said brand. In all likely hood it was a reverse polarity stupid mistake, i was just commenting that the caps in this supply have been known to be crap quality. So don't buy the model i did.

I'll  concede it was almost certainly reverse polarity that blew them, but the fact remains you get what you pay for, and when you buy the cheapest possible option, issues like this are common.  Not to say the caps would have blown under normal use, just that they don't have a good rep. Perhaps they would have lasted, assuming i did not make the mistake i did, perhaps they would fail due to poor quality.

The point, is that while it was most likely my mistake that blew them, and any cap would blow in that scenario, they used poor quality components in this supply, and i got what i paid for.

In any case, the replacement caps arrived, and i'll be installing them once i have someone (my father) with a background in electronics take a look at it first. I'll update as to if the repair was sucessful or not when we have some time to take a look at it together. Forgive my rudimentary understanding of electronic circuits, and assumptions it was the low quality components and not user error that caused the failure. The fact remains though, the caps are known to be poor quality.
« Last Edit: December 30, 2012, 10:57:26 pm by Suaveman » Logged

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Caps were replaced and shorts were soldered and epoxy'd over. Checked pos/neg out for a short, and sure enough it beeped at me. I'll tear it apart and start checking diodes, but at this point, i'd considering buying a new one, or looking into building one, with some bad-ass protection circuitry (diodes, fuzes, etc). Too late now to do any more work on the supply.

BTW, it wasn't plugged in when tested for shorts, and tested positive for shorts between the +/- outputs both with the power switch on/off. Does not powering it up have anything to do with it showing a short?
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Checked pos/neg out for a short, and sure enough it beeped at me.

Depending on how you measured it, that can be "caused" by the capacitor.
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It was just a simple diode/short test, what would be the proper way to test it?

Check the resistance between the terminals?
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