NAD C300 - blown fuses, possible speaker cable short circuit

[bonapardo]

Hi,

Recently, I was setting up my NAD C300 when it suddenly started distorting its output as I tried to increase volume slightly. I was not increasing the volume by very much at all and had previously run the amp at much higher volume in the past so was shocked when it sounded like I had blown both speakers. On disconnecting everything, I noticed the + and - wires to my right speaker seemed to be touching. I know this is bad and in the panic did not confirm this was the case - I simply wanted nothing connected!

Today I bought replacement 5A fuses for the two that had blown on the power board. I was hoping this was the answer - the blown fuses were clearly having an effect. However, on powering up, there was an immediate flash from the power board and the fuses had blown again. I tried unsuccessfuly one more time before giving up and saving my fuses.

I was hoping someone may have an idea of another component which may be affected by a speaker cable short circuit to help me narrow down the problem. My assumption is that a damaged component is causing the fuses to blow on power up. I am convinced there was a short circuit, but it is of course possible I'm barking up the wrong tree...

The NAD has not been used very much at all and both speakers are fine having tested them with another amp.

Please help!

Thanks,

Ben.

 

[rexter]

First things first: DO NOT POWER THE AMP OR TURN IT ON AGAIN, until you check the following first.

Visual check for any burnt components or wires/board trace of your amplifier and the speaker crossover.

Simple short test - you'll need a VOM for this one, check the impedance of your speakers it should be close to the specs of your spekers. This will allow you if your speakers are affected. Remember that the speaker is connected to a crossover so you need to open up the box itself. Also press the center of the speaker around the edge of the cone, if you hear any scratching your speaker is dome. Or if you have a mini system, connect the speakers to it and if it works ok

If this task is too much for you then I strongly suggest that you take your equipment to electronic repair store.

If the Amp is powered back on before this post is read, then do one test by un-hooking any connected speakers and try to use a headphone is there’s sound coming out. If there’s sound then your Power Transistor/IC may be damage. If no sounds then have it repaired.

Ask around if anybody has a better idea.

 

[bonapardo]

Thanks Rexter. As I mentioned, I have played my speakers on another amp and they are fine. When I power up, nothing is connected yet the fuses still blow. There are no obvious burnt-out components.

Unfortunately, I have no idea what a power transisitor/IC looks like - though it would seem this could be the cause as a soon as power is on, the fuses are overloaded. While I am good at using a soldering iron, my knowledge of cicuit boards is limited. Would it help if I uploaded an image of the inside of the C300? I do not have a schematic. If I know what component to change, I can easily replace it. I will not take the amp to a repair store until I have no options left.

I will test headphones tonight...

Thanks again, any further help from anyone is appreciated!

Ben

 

[rexter]

Power transistors are the one hook-up to your big heat sink. It's a transistor putting out those big sounds to the speakers.

Although when the fuse keeps blowing first thing we all check is/are shorts which you are right.

What you need to do then is you may want to open up the top and bottom of the Amp then vacuum it. This is to make sure you get rid of any lose wire on the circuit board.

And if you know how to trace the short then it's better. You start from the wires.

One other thing to check is the power supply. This will be most likely being a Center Tap Transformer, so google around and find out how Center Tap or the Transformers works. Some transformers also being use as a choke, although choke may not really that important when finding a short it would help if you know something about it.

I think my best advice is to get you a basic or fundamental electronics book.

I manage to came across this site: http://www.electronics-tutorials.com/basics/basic-electronics.htm