NAD C300 Output is barely audible HELP! - diyAudio
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Old 23rd January 2006, 09:05 AM   #1
Kajman is offline Kajman  United Kingdom
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Angry NAD C300 Output is barely audible HELP!

My NAD C300 Amplifier has just lost the Ghost. Output is barely audible on either channel even when the volume is set to maximum. You can just hear the music when you get close to the speakers.
The question is rather open ended but any suggestions on where to start would really help.

I am no expert but thanks to this forum, I managed to repair my old NAD 304 with just a 3 soldering gun and a 10p resistor. I could have gone round in circles but a similar fault had been diagnosed on here.
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Old 23rd January 2006, 10:33 AM   #2
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Has the C300 got sockets on the back for direct input to the power amp? If so feed a signal in there and see if the power stage is working.
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Old 23rd January 2006, 08:31 PM   #3
Kajman is offline Kajman  United Kingdom
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Default NAD C300 OUTPUT VERY FAINT

Unlike the classic NAD Amps where there is clear seperation from PRE Amp to Power Amp this one does not have the facility so its impossible to use it as preamp to see if its only the output thats faulty.

I will open to see if I can make sense of what the problem is . The only tools I have at the moment are a Digital Multimeter and a soldering gun. All suggestions are welcome
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Old 26th January 2006, 01:07 AM   #4
Kajman is offline Kajman  United Kingdom
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Default I HAVE NOW OPENED THE NAD C300

I opened the NAD C300 tonight and found that there are 2 x 5AT Fuses on the power supply board and these are blown there is another set of 2 x 0.5A Fuse which are intact. I replaced the 2 blown fuses and switched the AMP on. The 5AT fuses blow straight away but the 2 0.5A are intact and the green LED at the front of the unit still comes on. There are seven cables feeding main PCB from the PSU board near the toroid (six output wires from toroid going to PSU board). I assume there are 2 indepedent voltage rails high and low and its the higher side probably to the power amp where there is a current surge.

Any suggestions?
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Old 29th January 2006, 10:28 AM   #5
Kajman is offline Kajman  United Kingdom
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Default I HAVE FOUND THE FAULT

After searching this site for similar problems on other Amps I drew the conclusion that the MOSFETS were blown. Checking the resistance across the mosfets (suggested on one thread on an ARCAM AMP), a pair on one channel showed very low resistance so I soldered them off. Switching on the amp this time did not blow any fuse, so I proceeded to connect some speakers and one channel now works.

I now need to replace 2 Mosfets, P55NE06L made by ST. A search on google draws a blank. I now think these are called STP55NE06L after an extended search and are now discontinued. Does anyone know if STP55NF06L is compatible?
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Old 3rd February 2006, 11:22 AM   #6
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Hi, I think I'm having exactly the same problem with my C300. I rekon I blew it when instaling some new speakers with too long speaker wire protruding from the sheath. I thought "I'll sort that out later when I know its working", but in the meantime I think they crossed, shorting the speakers and blowing something. Sure enough changed the 5A fuses, blew again, so went searching. (The speakers work just fine with my other amps)

Not had a chance to check the Mosfets but pretty sure it will be the same problem and on one chanel like yours Kajman. Hoping so anyway. Won't be able to have a go at this for at least a week but was wondering if you managed to source the ST mosfets? Like you I can only find the STP55NF06L, and they seem to have a lower resistance (0.018 ohm vs 0.022 I believe) I'm not an electrician but willing to give this a go if I can find something compatible. Have you had any joy?
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Old 3rd February 2006, 04:42 PM   #7
Kajman is offline Kajman  United Kingdom
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Default I HAVE FIXED IT

Mine is fixed and running fine now. I measured the resistance across the mosfets as explained. I removed the ones from one channel that appeared to have very little resistance across them. I then replaced both the blown fuses and they did not blow this time. I did this just to make sure there were no other components blown with the mosfets
I ordered the STP55NF06L as I could not find the original STP55NE06L. I got them form Dannel.net for about 1.10 each though they sell them in packs of 5. I have replaced them and they work a treat. You will need an allen key to get them out and a green ceramic will have to be soldered off to get the allen key in (Just remember to solder it back in after all this I almost forgot to do it.). You might need to make a couple of adjustments before attempting to connect any speakers even if all appears to be ok. You will have to check the voltages across the speaker connectors with a meter making sure the reading across each channel is less than 10mV. There are 4 white adjusters in the unit 2 are for the Output offset voltage and the other 2 for idling current (I guessed it should be about 5-7mV for the idling current by looking at a NAD 304 Schematic) I had to play around with the adjusters before satisfying myself that it was safe to connect the speakers.

I might sound like an expert but I had no clue as to what to do myself before all this. The unit has been on for 3 days continously now and its running sweetly.

I hope this helps
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Old 4th February 2006, 12:15 PM   #8
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That is brilliant, thanks very much for your help on this one. Next weekend I should get a chance to check that the mosfets are the problem and I will try to make these modifications as suggested.

Really appreciate the help, will post again to let you know how it goes. Would be good to get the old gal up and running again.
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Old 4th February 2006, 12:32 PM   #9
Byrd is offline Byrd  South Africa
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Well done Kajman, it is always nice when a member let's others here know the progress and solution instead of just dropping the issue when their unit is repaired.
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Old 5th April 2012, 04:02 AM   #10
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Question My C300 has the exact same symptoms ...

Hi there Kajman , my amp seems to have the same problem, at least I hope so.
It occurred after shorting the wires at the binding posts while switching between two pairs of speakers.

I've soldered a few times,but in less delicate scenarios. I'm in no way qualified to attempt a repair but I'm willing to give it a try as I have no other options. Really can't afford to pay for any help.

Could you help me determine the location of the MOSFETS so I can attempt replacing them ?

Some basic instructions written or drawn over the attached photo would go a long way for me.

Thanks in advance , I hope you find the time to help me. Cheers !
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