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RD Audio RD-1750.1 Powers On To A Dead Short

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A friend of mine gave me an RD Audio RD-1750.1 amp that would blow fuses as soon as it was powered on. I opened it up and replaced 5 broken MOSFETs (the legs were broken off right at the body) but it still doesnt work. Basically, when I switch the amp on (remote goes high), the protection light comes on for a second, then just as the light goes out, the power light comes on, and the amp goes into dead-short mode (Im using a bench power supply with over-current protection so as not to waste fuses).

Where do I start? Ive looked for anything noticeably broken (burnt resistors, diodes, bulging/leaking caps) but I dont see anything.
 

Nigel Goodwin

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Find and replace all faulty components, check everythinjg in the DC chain - usually if the outputs are dead, something caused them to blow - and not curing the cause may have killed the new ones as well.
 
The old ones were physically damaged, the leads were severed from the body (Im not sure how).

"Find and replace all faulty components" is a given ;) I just dont know where to begin looking. Im thinking a capacitor or a diode, since it doesnt go into a dead short right away, it takes a second first.
 

Nigel Goodwin

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Most Helpful Member
The old ones were physically damaged, the leads were severed from the body (Im not sure how).

"Find and replace all faulty components" is a given ;) I just dont know where to begin looking. Im thinking a capacitor or a diode, since it doesnt go into a dead short right away, it takes a second first.
More likely transistors, because they are DC coupled they fail like a chain of dominoes.

First job draw the circuit out, unless you can download one?.
 
Haha, if I had to draw out the entire circuit I would probably just tell my friend to buy a new amp. I searched for the circuit but didnt find anything.

The internals look very similar to this:


As Im sure you realize, having to draw something like that out by hand would take an incredibly long time, and its not worth it to me to go to that kind of work.
 

Nigel Goodwin

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Most Helpful Member
You don't need to draw the entire circuit out - what's duff, the PSU or the amplifier?, just draw out the section you need - and for the amplifier, just a single channel.
 
Not sure whats gone, all I know is that when I power on, it goes to a dead short, and Ive no clue where to start. Ill get my EET teacher to look at it today after class and see what he says.
 
Haha I figured Id open it up and just find a blown cap or something, replace it, and it would all be better ;) When you open it up and aside from some broken MOSFETs it looks pristine, its a bit harder. It's a Class-D amp by the way
 
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Nigel Goodwin

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The picture you posted shows how easy it is to spot which part is which (hopefully your's is similar?), the lefthand half is the inverter, and the righthand half the amplifiers.

So which transistors were duff?.

Hopes of just a faulty capacitor were a serious delusion :D
 
Yes, mine has the same layout (Ill take pictures of it today) - there were three MOSFETs on the bottom row and two on the top row, on the very right, that were blown.
 
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