150w modified sinewave inverter issues?

[AmmoJammo]

So, tinkering with something else that's broken.
I acquired a "can" sized modified inverter.

Now, it looks quite well made, and upon powering it up, it buzzes.

Probing inside, revealed that they take the battery input voltage, switch it with 4 fets and a TL494 switchmode controller IC, then rectify it, to get about 340volts dc.

This all works.

Then, the 340volts dc goes through 4 more mosfets, being controlled by another TL494, to get the modified 50Hz sinewave, 230volts ac output.
What I am instead getting, is a 50Hz, modified sinewave, 9volts ac. 

I'll get some pics, but where does one start? could the mosfets doing the 50hz modified switching actually drop this much voltage across them?

[AmmoJammo]

[Bryan]

I always start with the capacitors. Check them for high ESR.

[SeanB]

Are you getting the 300V rail with no ripple on it? If so then the high side drivers or transistors are open circuit or a track is broken to them. Check each side of the output to ground, as you will have either both sides not switching or one side only not switching, or a very narrow pulse only.

Normally the fault on these is shorted mosfets or blown drivers along with the shorted devices, or broken tracks.
Check the drive from the oscillator is correct and from rail to rail on the supply as well, you could have a faulty driver stage or an open pull up or down resistor.

[AmmoJammo]

Interestingly, I put a 75watt globe on the output, and the input current draw increased...

So its doing something!

I'll do more investigation when I can be bothered... thanks for the info!

[NiHaoMike]

Is the DC/DC stage actually working properly?

[AmmoJammo]

Is the DC/DC stage actually working properly?

At this point in time, I believe so, but now that the weekend is over, the inverter is back on the shelf :p

~340volts DC across the biggest capacitor in the inverter (the one to the right of the transformer) seems right to me.

[Jay_Diddy_B]

Hi,
I found this schematic, it might not be exact but it might help.

http://images.elektroda.net/78_1319388238.jpg

Regards,

Jay_Diddy_B

[NiHaoMike]

Remove the output stage and rewire the output directly to the output of the DC/DC converter. Such a small inverter really is only good for operating electronics, which will run just fine from DC. Also, there's less EMI with DC, and even less with the addition of an EMi filter.

[AmmoJammo]

Hi,
I found this schematic, it might not be exact but it might help.

http://images.elektroda.net/78_1319388238.jpg

Regards,

Jay_Diddy_B

Thanks for that, it does look somewhat similar

Remove the output stage and rewire the output directly to the output of the DC/DC converter. Such a small inverter really is only good for operating electronics, which will run just fine from DC. Also, there's less EMI with DC, and even less with the addition of an EMi filter.

I did actually consider that, as phone chargers, laptop power supplies, and pretty much anything with a switchmode power supply should be fine off the dc voltage...
But then, I don't have much use for it even working properly, so I'd probably rather get it right, and give/lend it to friend or family if they need it for something

[NiHaoMike]

Just modify it to DC output. Then label it "DC output".