Help needed to fix a S-40-12 Mean Well SMPS

[JacquesBBB]

Dear All,

I  tested a  non functioning S-40-12  SMPS. Fortunately, with a  40w bulb in series.
There was a strong BANG !  and  the C9 electrolytic  exploded.

Can someone tell me its value ?

Thanks

Edit (21/1/2018): I have changed the original title of the thread

[wraper]

47uF or 100uF 35V+ is around what usually is used with UC3842 and clones. But I would expect that cap exploded because of some other fault.
I would guess that SG3842 failed open on Vcc pin which caused uncontrolled voltage rise on the capacitor.

[Bashstreet]

47uF or 100uF 35V+ is around what usually is used with UC3842 and clones. But I would expect that cap exploded because of some other fault.
I would guess that SG3842 failed open on Vcc pin which caused uncontrolled voltage rise on the capacitor.

beautiful Diagnostics and knowledge.

[coromonadalix]

and your power mosfet on the photo is not properly monted on the heatsink   the screw with metal bar ...

[JacquesBBB]

and your power mosfet on the photo is not properly monted on the heatsink   the screw with metal bar ...

This is just for testing purpose. Will put it back when fiXed.

[JacquesBBB]

Thanks for these comments.

I have tried several things but without success so far. As  wraper said, the C9 cap was not the source of the problem.

Does anybody has a schematic of this SMPS or approaching ?  I could not find something obvious  on the web.

[wraper]

Replace SG3842 (UC3842), IMO it has very slim chances to be working.

[JacquesBBB]

Replace SG3842 (UC3842), IMO it has very slim chances to be working.

Thanks, I will do that. Will need to wait until week end, as I am  out of my house during the week.

Here are the additional observations I made.

- The Ohm meter at  the + and - of the bridge gives 28 ohm.  The value looks very small,
but I could not identify any dead component in the area.
After removing a few components, I had  an open circuit at  +/-, but none of the component I removed appeared to be damaged.

Is the main  mosfet staying ON for some reason ?,  even if the circuit is not  powered ?

- When plugged, with the bulb,  the bulb stays on. Did not wait long.

Are these compatible with your diagnostic ?

[wraper]

Is the main  mosfet saying ON for some reason ?,  even if the circuit is not  powered ?

If mosfet has low resistance, it should be dead. If it failed, PWM controller IC will be dead in vast majority cases as well. Now thinking about it, probably exploded cap was back powered through the gate of faulty mosfet and then through SG3842. In such case you also need to check any diodes, resistors, zeners in gate circuit as they might fail as well.

[JacquesBBB]

But when I test the mosfet alone with the ohm meter, it has not low resistance. It is open circuit.
With the transistor tester, it looks ok, and recognised as a mosfet.

[wraper]

did you check rectifier?

[JacquesBBB]

Yes, I desoldered the rectifier, and it was all OK.

[JacquesBBB]

The components  I checked for possible short cut ( checked by desoldering them first)
all in the right part of the SMPS board:

Bridge retifier : BD1
Big cap : C5
blown cap : C9 ( replaced)
power  resistors  R3 and the one nearby
The blue cap near J2
The green resistor  R4 ( 1 ohm)
power mosfet Q1
The diode below the green resistor R4.
The C2 and C3 grey caps.

All these components were desoldered and OK.
 
After I removed them, then  the  +/- at the bridge rectifier  vias  show : open circuit

Before : 28 ohm

I have no memory of all the intermediary tests.

Is the main  mosfet staying ON for some reason ?,  even if the circuit is not  powered ? 

Can the ohm meter power  the mosfet to make it open ?

[JacquesBBB]

This past days, I had not much time.

I changed the main mosfet although it did not look faulty, Changed the UC3842 (which was not good), and there is still problems.

I  desoldered the TL431, but had no time to check it.

Next step will be to make a full schematics. Does anybody has already one that is close ?

[Armadillo]

I don't know if he will bestow with a response;
Anyway, do share with the community once you are done with the full schematic, yeah!.