power board - don't know what to try

This is the power board from a samsung monitor/tv. The thing stopped working completely with a bit of a bang, so I figured no harm to have a go as an enthusiastic newbie. I got a bit stuck now, so I thought I'd ask for advice here before I throw the whole thing in the bin.

On opening the case it was clear that the resistor at A had blown, you can see the residue on the board, so I replaced that. The capacitor at C was slightly bowed outwards at the top, so I replaced that. All the other capacitors look fine. On putting the thing back together and switching on fuse B blows. I have tried replacing also capacitors at D, and the fuse at B blows still.

What should I try now? My assumptions are that it is most likely to be a fault on this board - is that a reasonable working assumption or could something broken in another part of the monitor entirely cause these sort of symptoms?

Is it reasonably safe to test this board in isolation by connecting it to the power and trying a voltmeter on the output cables in the bottom right corner which seem to be marked +15v and ground. What other diagnostic checks are worth doing?

 

ahh when I said 'not working' - the whole monitor was dead, no signs of life at all. Thought it would be helpful to clarify that.

 

Very helpful reminder, thanks. Don't want to be touching that!

I have a Precision Gold N21FR, which does DC/AC Volts, amps, resistance, hFE transistors, Diodes, etc.

The fuse nearest the Power cable is 6.3A 250v, and the continuity tester beeps when checking that. The fuse that blows marked on the diagram is 3.15A, 250V - no beep.

No chunks, looks perfect condition. f1 J19 FQPF 7N65C

Seems to me in good condition, no chunks or burn marks. A couple of what look like surface scratches, but not what you'd call a crack I don't think (image attached). 0820 TS4B05G

I assume I didn't need to connect the board to the mains power for this test. Voltage 0, Resistance 0 and stays 0.

seems to be 1440 Ohms (on the 2000 Ohm setting on the meter)

(image attached)

Many thanks for looking at my query so quickly, and with all this detailed assistance, I am beyond impressed. Please don't laugh too hard at my soldering, I know I'm not very good.

(edit) oh wait I just realised, the big capacitor shows continuity since the resistance is 0 and the meter beeps, so that means it needs to be replaced right? I wonder if that is the source of the problem.

(edit 2) - just a quick follow up question, I guess I'll be ordering a new big capacitor from the internet. Are there any other spare parts that might be worth adding to my cart just in case - it would make sense to only pay for 1 delivery fee if there is anything else that might need swapping out.

 

Not at all, you've exceedingly generous with your time to assist me, and I'm grateful.

This bit is going to be easy - all of the above measurements seemed to show continuity beep and 0.something on the lowest ohm setting which I'll take to be zero. See attached image of the component I measured

(edit) so from your comment above, if nothing else, it definitely looks like I'll need to replace that mosfet - am I right in saying that http://uk.rs-online.com/web/p/mosfet-transistors/6715301/ looks like the right part for that. I realise I'll need some sort of heat transfer pad to stick it to the heat sink - is there something in particular you'd recommend for that?

AS a related aside, I think I will give some thought to upgrading soldering iron too, currently struggling along with a little 18W effort. I would guess that something 50-60W with variable temperature control would be a sort of reasonable minimum to do a job like this cleanly?

 

It has taken me a while to get back to this, sorry about the delay. I have now got a new cheapo solder station and successfully removed the mosfet. I have checked the resistances again with it removed, and can confirm that it seems to be shorted out. My next step is to remove one end of the diodes on the power section of the board to check their resistance. At this point I thought I'd check in for some advice with the soldering before I wreck the board...

I am using a cheap iron, but 60w - http://www.maplin.co.uk/p/60w-professional-lcd-solder-station-with-esd-protection-a55kj

I set it to a temperature that easily melts my lead free solder (340C) and then dab a bit of solder on the iron to create a good connection with the joint on the board. After a few seconds nothing happens, the solder on the board is just not for melting, and I'm scared to hold the iron on there for more than about 8 seconds in case it damages the components. I've tried upping the temperature of the iron to 375C but still no joy. Any advice, is there some coating on the board that makes it hard or is it just solder with a much higher melting temp that they use in manuf. Is it better to hold the iron on for longer, or use a higher temp? what temp should I use?

 

good news and bad news, but mostly bad. I've removed one half of each diode and there doesn't appear to be any shorts.

unfortunately, on looking closer at the board where I've removed the mosfet, I can see that I have (stupid stupid stupidly) damaged the circuit board, so there's going to be no easy repair there that I can see, I think this means trying to source a new board

 

ahh sorry - looks like the surface has been burned off? See arrows

 

i might have got it all wrong, but as far as i can see there is no metal surface left on either the right or left to solder onto, the metal seems to have gone and it is the surface of the board underneath showing through

 

continuity tester shows most of the metal has gone from the left and a fair amount of the right i suppose i could always bridge it in some way to another component on the same track