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Old 10th March 2005, 06:48 AM   #1
dan4 is offline dan4  United States
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Question NAD 2400 problem - need help

I am looking to obtain a schematic and possibly some direction as to fix my NAD 2400 amp.

Here's what happened. I loaned my pre-amp and amp to a friend that was wiring his house for in-wall speakers. We had three sets of speakers (8ohm) hooked up in parallel. We were using impedance mathing volume controls (at x4 resistance setting). My friend disconnected one set of speakers (After the volume control) but did not disconnect the wiring to the volume control. He turned on the system to listen to music on the remaining sets. Music played for about two minutes and then the amp blew the main fuse (5mF 125V 7A). After consulting with an electrical engineer at work, he told me that it was a bad thing to leave the one volume control hooked up (the one without speakers attached). The volume controls were Phoenix Gold VMT100

I replaced the main fuse, plugged it in, and nearly instantly blew the fuse. I tried again and blew the second fuse. Obviously I have a problem.

Does anyone have a hypothesis as what is wrong or what is the next diagnosis step? I am thinking it is the transformer. Can someone provide me with a schematic for a 2400?
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Old 10th March 2005, 11:56 AM   #2
NUTTTR is offline NUTTTR  Australia
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From memory, if you find the fuse is blowing straight away you will most likely find the outputs potentially are blown... Not sure on the volume control causing the problem unless it was presenting a below 4 ohm load (from memory that nad's rated to 4 ohm min? or was it 2?)... Could have caused the amp to heat up a lot and blow the outputs... Of course, make sure everythings unplugged and measure the resistance on the connections of the output transistors (relative to eachother) if any of them are REALLY low like 1ohm, something like that, they are short, blown... Potentially it's more than just the outputs too, check diodes, resistors and small transistors within that circuit, sorry but i do not have a schematic for it
Good luck
Aaron
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Old 8th May 2006, 12:40 PM   #3
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Default nad 2400

I have a schematic if you need it
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Old 8th May 2006, 12:45 PM   #4
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Default nad 2400

schematic
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File Type: jpg nad 2.jpg (84.5 KB, 260 views)
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Old 8th May 2006, 12:47 PM   #5
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Default nad 2400

part 2
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File Type: jpg nad part 2.jpg (84.5 KB, 232 views)
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Old 8th May 2006, 01:28 PM   #6
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The full NAD 2400 Service Manual has just been posted on
www.sharefx.com
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Old 2nd August 2013, 05:48 PM   #7
Jonas83 is offline Jonas83  Sweden
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Using this thread for a question regarding NAD 2400 (thx version). R701,R709,R751 & R759 became "cold joints" due to extreme heating (started to work again when i resoldered these), as the schematic tells this is carbon resistors 1/6w rating. The problem is rating on these are very low! hotspots on pcb maybe there is a reason for it..(?).. If i replaced these with say 2w metalfilm resistors, would that be a good or bad thing??. As for carbon vs metalfilm i donīt think it will affect anyting here, but is this a design issue or made like this to fail here first?
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Old 2nd August 2013, 06:51 PM   #8
Mooly is offline Mooly  United Kingdom
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The posted images are unreadable (if those resistors are even in that part of the circuitry) but to answer your question... it all depends on what the resistors intended function is. If its just a normal part of the circuit then no problems but if the 1/6 watt devices are a special "safety resistor" or "fusible resistor" designed to fail in the event of an overload or fault then obviously replacing with a 2 watt component defeats that.

Its a common occurrence for commercial gear to have hot spots and parts that literally fry... not saying that's what is going on here... but it is commonly seen.
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Old 2nd August 2013, 07:42 PM   #9
Jonas83 is offline Jonas83  Sweden
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Maybe this schematic look better, seems like there is only one channel in the schematic, R751 and R759 is in the exact same position for the other channel.

When i look at this schematic now i see that fusible resistors are marked, so this is not a fusible resistor, then i would be able to use a higher rated resistor to keep things cool......
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File Type: jpg nad_2400_sch.jpg (889.0 KB, 35 views)

Last edited by Jonas83; 2nd August 2013 at 07:58 PM. Reason: Fusible.
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Old 2nd August 2013, 07:50 PM   #10
Jonas83 is offline Jonas83  Sweden
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Location of R701,R709
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File Type: jpg nad_2400_sch_location.jpg (135.8 KB, 33 views)
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