My Q1755 13.8v 23A switch mode PSU just died.

It was doing so well for over 1 year until I did something silly by running 2 ichargers 106B+. Each charger was charging a 6S 2650 Lipo and was set to 9.9A

Usually the chargers would just dumb down to 4.5A but because it was for such a long time I must of stressed the PSU until it went "POP". The PSU was very hot making me think it was a thermal overload and cooked something inside.

After opening the PSU I found no indication of dead components, the slight smell is gone too. When you turn on the PSU the switch will light up to turn off & on, but no output voltage.

So now the journey begins to find out what is wrong & replace to get it working again.

Check the larger power transistors mounted on the aluminum plate. They could have popped. You can check them with a multimeter that has a diode checker built into it. Good luck!

look at the atached image, I would like to clarify and check few things from the beiginning of AC to DC rectification ciccuit.

1. look at transformer circled in yellow. I see only 2 wires. If that one is traditional step down transform, it should have primary side and secondary side, 2 wires for each side. Can you confirm and check if there are other wires.

2. look at capacitor circled in blue. I am not sure is photo quality but seems to me that there are burn marks. Can you check and confirm. If possible, I would like to get the spec on capacitor from outside. It should say xxx uF or mF... XXX V voltage. I want to confirm if the burn mark is from capacitor or from power transistor next to it.

3. the box circle in red. It should be AC to DC rectifier. However, I am not sure whole unit if it is one piece bridge rectifer or 2 pcs power diodes. Also, If you can take out rectifier from chassis mounting, see you could drop the spec from the rectifier, like model # and brand....

After we confirm above, then we may start troubleshooting /w multimeter.

Anyway, be safe and aware of high voltage 110V or 220V while we carry out troublshooting using live circuit. Of course, some components can be checked out without live voltage.

Thanks for taking the time out to comment on my problem

1 - The step-down transformer only has 2 coated wires coming out of it which can been seen in the pics.

2 – No burnt marks on the caps. It’s a shadow from the lighting I used to take the pic. Values of the capacitors are 680uf 200V. I’m 100% sure there are no burnt marks around the caps.

3- The part circled in red is the off/on switch. There is no rectifier that I can see. Could it be that the rectifier is on the same PCB above the caps and next to the AC socket? There are 3x yellow blocks with the following values;

Over active & neutral plug –
.68K275V~X2
CARLI MPX40/100/21

Closest to outside of case
.47K275V~X2
CARLI MPX40/100/21

Closest to Caps
.68K275V~X2
CARLI MPX40/100/21

After writing this all up I decided to take the grey covers over the Mosfet’s (I think that’s what it’s called) that sit on the heat sink and I found an obvious problem.

So what do you think I should do now

Like I mentioned in my post, I thought that may be the problem. They are actually power transistors. Here is the data sheet> http://www.datasheetcatalog.org/data...ec/2SC2625.pdf

Another component may have been wiped out too in the process but I would first try to replace the power transistor (P/N 2SC2625) and see if that does the trick.

I'm not sure who has them but you could try the obvious digikey.com or Newark.com or google the part number... Maybe Radio SHack too but they dont have much these days. You also may have to look for a similar part if they don't make this one anymore. Compare datasheets on the parts and get something as close as you can.

yes, 2sc2625 power transistors need to be replaced. Suggest you get 3 to 5 pcs one time. They are not expensive, about $3.00 each. you may check your local electronic store in Australia first.

s.p.

If you still can't find the 2sc2625, you may find the equivalent transistor from the link

http://alltransistors.com/crsearch.p...nf=&caps=TO218. I have input all critical parameter matching 2s2625, pick the one closest match.

Thank you both for the links. I'll see what our local Radio parts (radio shack) has. Will not be able to buy/install the part until the weekend so for now I'll put this on hold.

The first big transformer is only used to change the "power factor" of the powersupply.
It means power input is smooth out to not overload your AC-mains. Capacitive loads like this PSU tend to draw a lot of amps from the power-line when the AC sine wave has not reached full voltage yet. This inductor makes the PSU draw most of its energy from the net at the peak voltage, thus reducing amp draw at for same watts used by the PSU. Makes your powerlines heat up less.

It does not alter the voltage from the mains to the circuit board. In fact this unit should run fine without it, but regulations make it needed.



By the way, be careful handling an exposed switching supply. Some designs keep a very high voltage on the capacitors for a long time after power is disconnected. Dont get killed out there!

Thanks for the heads up sp00fman. I'll keep that in mind when I get the parts in and start attacking the board with my iron.

Originally Posted by sp00fman

By the way, be careful handling an exposed switching supply. Some designs keep a very high voltage on the capacitors for a long time after power is disconnected. Dont get killed out there!

Good point! I was zapped once by a power supply! Ouch!

Well after failing to find a local supplier that was cheap, I had to resort to Ebay/China.

The model matches what I need, but it does look slightly different from the one I blew up. I hope I haven’t bought the wrong part. If all goes well I have a spare one too.

I guess in 2 weeks time I will find out.

Here is what I bought
http://www.ebay.com/itm/ws/eBayISAPI...m=180855765846

Originally Posted by evltoy

Well after failing to find a local supplier that was cheap, I had to resort to Ebay/China.

The model matches what I need, but it does look slightly different from the one I blew up. I hope I haven’t bought the wrong part. If all goes well I have a spare one too.

I guess in 2 weeks time I will find out.

Here is what I bought
http://www.ebay.com/itm/ws/eBayISAPI...m=180855765846

It should be the one. post your fix and let us know.

Back onto this project.

I ended up receiving my 2sc2625 power transistors today in the mail. Not bad for $3 postage from HK.

So.. I have removed the damaged power transistors and soldered the new one in. Screwing the PCB down and then the cover on, I was getting excited thinking it will all work.

Guess what! Power it all up and no output still.

Now the journey to fixing it continues. So what do you guys recommend I do next?

Originally Posted by evltoy

Back onto this project.

I ended up receiving my 2sc2625 power transistors today in the mail. Not bad for $3 postage from HK.

So.. I have removed the damaged power transistors and soldered the new one in. Screwing the PCB down and then the cover on, I was getting excited thinking it will all work.

Guess what! Power it all up and no output still.

Now the journey to fixing it continues. So what do you guys recommend I do next?

Its really tough fixing something like that w/o a schematic and it becomes a guessing game.. But, I would like to see if one of the smaller transistors has bit the dust. Do you have a diode checker on your multimeter? That can be used to check if the driver transistors are any good. You may have to "lift a leg" (unsolder) on the transistor to get an accurate readout. It looks like the transistors are labeled Q1, Q2, etc. on your board.

Going to be tough... tell me about it

I do have a multimeter that can be used to check if the driver transistors are any good. In fact it has a NPN & PNP port for me to stick the transistors into.

Only problem.. I dont know how to use it as I have never needed to at school (10yrs ago).

If it helps the meter I have is a Jaytech QM1530