Repairing a switch mode charger

[Andy Twose]

Hi,

I want to repair a charger part number is 3P10-N0508. It is rated 6.4V DC 0.8A

It looks like this http://wlqabt888.en.ecplaza.net/7.jpg

I have identified an o/c resistor in series with the live mains in, it is marked as 18 ohms.
Suspecting that something has taken it out I metered the suspect power transistor. It is s/c all around.
It is marked E1303 which I make to be an NPN silicon transistor. 20W 1.5A, 700V

So far so good, now the problem.

The emitter of the transistor is connected to 0v via a small SMD resistor which is burned and cracked and also o/c.
I do not have any idea of the value of this resistor, but am guessing 1 ohm or something in that region.

Anyone any suggestions please ?

 

[Andy Twose]

Here is the resistor

https://www.dropbox.com/s/xqom1omtwxieh5p/20161213_173050.jpg?dl=0

 

[Andy Twose]

I think I may be OK. I cleaned the crusty resistor with a fibre brush and some writing just became visible.
Initially it looks like 204 which I think would be 200kΩ so almost certainly not what I believe is correct.
As only the top of the '0' is visible then I think it is actually 2R4 which would be 2.4Ω. I have gone for that and ordered some parts. Fingers crossed.

 

[73's de Edd]

Since I see it being used as an emitter resistor, that 2.4 ohm would be right in agreement.
Also your other blown 18 ohm current inrush resistor for the AC to DC power rectifier for the unit
seems like you probably have a power rectifier problem or that power transistor that you have pulled
may be shorted Collector to emiter,
OR your electrolytic capacitors assoiated with the secondary outputs DC power filtering may now be
down in value of a mere fraction of their new state values.

 

[Andy Twose]

Thanks Edd. Yes the power transistor is short C to E. I have ordered parts now. Feeling reasonably happy 2 resistors and a transistor will sort it out. I've always considered switch mode power supplies are just waiting for a reason to blow up.

 

[73's de Edd]

.

Sir Andy Twose . . . . .

Since you are going to have to rustle up some parts, when you do an initial power up, here is no way that I personally, would not have a 60w incandescant lamp in series with the AC line power input.
Faint and decreasing filamant glow after initial turn on . . .probably all is well.
Bright or intermediate light staying on . . . . .OVERLOADED condition.
I can see your HOT part of the board on the left half of the board and the switchmode power transformer stradling the gap at the top of the board.
That then leaves the AC line isolated DC power output of the supply being on the right.
Not having a full closeup of the component side of the board, its impossible to see the maner of derivation of the RAW DC power on the left portion. . . . . .1/2 Wave or FWB ?
Likewise, the power rectifiers and filters on the right half, and its associated electrolytics are certainly fully suspect of being deficient, with capacitance decline, and of having developed high ESR with time and use.

73s de Edd

 

[Andy Twose]

Hi Edd,

Thanks for the tip but I had already swapped the components and powered it up yesterday. Nothing catastrophic happened but it didn't work. I then found that the small NPN transistor 2N3904 was faulty. Part is on order. I did try another NPN transistor in there but to no avail. The initial rectification is bridge and gives me 300v DC.

Good top picture here https://www.dropbox.com/s/q5e70j7hxu1xn28/20161215_090418.jpg?dl=0

Unfortunately there are lots of SMD parts underneath.

I am inclined to believe that only components on the hot side are at fault, as the charger side would be protected by the transformer and the opto coupler.

Andy