Marshall as100d 120v plugged 240v

[Relase]

Hi everyone! I have a Marshall AS100D 120v that not power up! The guy who brought it said that he plugged in into 240v socket by mistake! What I managed to test so far is the transformer, that seems alright! When power it up with testing lamb connected on the main fuse the only thing is a deep buzz coming from speaker! Do you have any idea where to start with? Every help appreciate! Thanks

[Enzo]

Pull all the tubes and start with power supply. Are all voltages present?

[Relase]

Thanks Enzo, It’s a solid state amp! I suspect some of the output transistors! But not sure yet!

[Enzo]

Sorry, so no tubes anyway. NO SPEAKER LOAD until you make the amp stable adn no DC on the output.

You said you powered it up with a bulb, but what did the bulb do? Glow bright or dim?

In any case, first thing I'd suspect would be caps over voltaged. Thus wanting to check power supplies.

[Relase]

I powered up with external step down transformer 240-120v with 80w halogen bulb on the fuse! So the bulb is not so bright because of low voltage but definitely there is short! The fuse blowing out straight away!

[Enzo]

Start isolating the problem. Disconnect ALL secondary wires from the power transformer and power it up with its wires in the air, still draws excess current? Check power supply rectifiers, any shorted? Your meter only works at low voltage, but do any filter caps measure as shorted? You have two power amp CIs, one or both could be shorted, try gently removing them.

[g1]

I powered up with external step down transformer 240-120v with 80w halogen bulb on the fuse! So the bulb is not so bright because of low voltage but definitely there is short! The fuse blowing out straight away!

What country are you in and what is the line voltage?

[Relase]

I am in UK! The standart voltage is 240v

[g1]

Ok. I'm not sure why you are blowing fuses but your limiter lamp is dim. It should be lighting up bright I would think. You have the bulb wired there instead of the fuse?

[Relase]

Yes correct! The bulb is wired on the fuse!

[Jon Snell]

If it lights dimly, is the output stage damaged I wonder.
The thing to do is isolate the fault.
Can you put + - DC supply on the amplifier with a power supply and monitor the current?
Or remove the output chips and test again with your bulb limiter.

[Relase]

Thanks! I’ll try all suggestions and let you know how are the things!

[Jazz P Bass]

I have attached the power supply schematic which includes the dual power amp (LM3886 ic)

[Relase]

Hi colleagues, sorry for my late response! All sorted! Found one of the amplifier lm3886 gone and one diode blown! Replaced with new ones and all running now! The only thing what I found after is that both speaker’s tweeters don’t work! I have got 1-2 OMs on the terminals. Any suggestions for any fix or have to replace them? Thanks

[Enzo]

They are probably piezos, are they not? And those will test like capacitors, infinite resistance.