inverter repair

I have inherited a 300w 12v DC to 230vAC inverter that has no output. I was told that the internal fuse was the problem but this is entire and fine.I havent stripped it right down but the visible surface of the board has some smoke marks around what I take to be the base of output transistors but I have gone no further than that.I have little understanding of how these work but can work my way round boards and replace parts etc if I am told what needs doing. Now,on this pathetically scant bit of info,does it sound like this is worth pursuing ? What are the usual faults with these things and the causes thereof ? And what does one do about it,bar bin the thing and get another ?

Check the price of the components that "look" bad, or have traces of escaped magic smoke. If those parts are cheap enough to you to replace them, (or take a gamble) then you can pursue removing those parts and checking them to see if they are indeed bad. Typically it is the output side that goes on the inverters so I would say you are on the right track.

Awd,thanks for taking the time to reply to this along with my other query,your effort is much appreciated.    I worked for some years as a sound engineer and had to cope with many large power amps that let the magic smoke out.When dealing with 20k plus (RMS) of sound in excess of 100dB for 12 hour sessions this tends to happen a lot.Normally when you leave the controls for 1 minute for a call of nature.In a most expensive fashion.The smoke,that is.And none times out of ten it was the output tansistors. Hence the statement that I could find my way around boards. Having zero experience of these inverter devices (as a seperate unit at least) it is very useful to hear that it is the output side that goes.The one I am looking at appears to have 4 flat square ouput devices marked SAMWIN 0648 and SW5ONO6, the latter presumably the designation,2 on the side I am looking at show heat damage.    The thing is,what would have caused this? The usual overload situation ? If so it may be worth the fixing. If its another internal fault,probably not worth the time.    Obviously one cannot give definitive answers on an item one has not seen or had access to but anecdotal knowledge and experience is always useful. The board is labeled SEEWIN SI 400w V3, 2005.11.25. The item is case labeled SkyTronic model No.651536

They are probably MOSFETs for each "push=pull" output. They only need to be of a few Amps rating (300W ~360VA/120V = 3A chose 10A or larger. They probably merely broke down. Normally FETS are unlikely to damage control circuitry (though I can recall their breakdown modes) whereas transistors are likely to damage their inputs (control circuitry). But maybe other circuity has gone too (eg, transient absorbing devices, voltage feedback etc). And faulty control signals may also blow output devices, though that is unlikely.    The last inverter I bought was 600W (RMS) for ~$60.

The FETs in an inverter are usually on the primary side of the transformer. Usually 50 to 75 amp 50 volt devices. IRF-Z44 IRF-3205 Etc.

Stupid I know but I thought I "blew" one last week, it was the fuse in the cigarette lighter adapter!

Not surprising considering common inverter loads... IMO, safety-wise, a "mere 150W" is pushing cig sockets. Even half that (7A) can be risky.... I withdraw the preceding sentences from this block should they in any way infer the duty-of-care responsibility to me! Times like that I admire Engel and others that use thermal 10A fuses in their cig-plugs. Sure, it promulgates endless posts about what fuse to replace with, or if thermal is necessary (LOL!), but it has certainly prevented more than a few fires.... {And since that is public knowledge, I am off the hook!)

On the subject of fires.....    the first inverter that I bought was a mini 75w job , from Asda,a uk supermarket owned by Walmart. It was laying in the cubby box for a few months in which time I lost the receipt.When I finally came to use it with a phone charger, as soon as I plugged it in, it began to smoke and rapidly caught fire....NOT what I was expecting at all. When I returned it to the shop,having checked UK purchase law, I was told that without the receipt they would do nothing. I produced a printout of the relevent law to the poor hapless,blameless lad on the customer service desk....at which point he called his supervisor.The super told me the same,etc etc,I told him that if they could tell me the month that these items were on a sale stand, I could produce the bank statement with the card payment listed,which in UK law is enough proof of purchase.He told me he "couldnt be bothered" as he was too busy. Well,I wasnt too bothered as it cost less than a tenner and I had no interest in causing trouble for some poor overworked sap on minimum wage but I DID consider that this was a safety issue and asked for the manufacturers name so I could contact them direct (its not on the label) but he wasnt inclined to help with that one.What the hell, I dont want to contact HIS boss and get him in the doodie but I DO want to stop other people having a fire in their car that they may be less prepared for than I am. Funny thing,it didnt blow the fuse......

Thanks guys. I figured that the FETs were half up,half down like in our old power amps but wasnt sure. I have found a place to buy 10 for under a tenner uk delivered, so that gives me a full new set (4+4) and 2 spares to play with. IF,a big if, I get this working, I was planning to mount it somewhere with a good airflow and possibly use an extra fan from a PC case. Then hard wire it,fused, to the second battery in my 300tdi discovery. Following the old rule of thumb that cheap electrical goods are usually about a quarter as powerful as claimed (certainly the case with PA power amps) I was hoping that it would give a decent 75w supply to run a basic laptop when away from home.Should it work out I will consider another one to go in my caravan trailer. I will update the results as they happen.

Had the same problem once with a 13month old Phillips Razor via Tesco. In the end I got a friend to send me over the (half the price) Norelco from the US, same razor, no problems but coming back to your last post, within reason you get what you pay for, it's a shame that in the UK we don't have the same levels of consumer protection, especially on the safety factor you quoted as the US.