Help with NAD 7250PE

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vloft

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Joined 2010
#1
I have a NAD 7250PE which I have had for about 18 years old. Never had a problem with it, but I now notice that the left channel speaker moves in and out significantly with the lower frequency music and mid to high volumes. The right channel is absolutely fine. I cannot turn the volume up much because it will bottom out the speaker. Have any of you seen this before? What could the problem be?

I am ok with a multimeter and a soldering iron, but no expert. So if anyone can point me in the right direction that would be great. Also can someone please email me the Schematics? I would not know what to do with it, but know someone who could help me, but will want one of these first.

Many thanks guys
 
#2
Well you don't mention having tried this so first swap the speakers over left to right. If the speakers are sealed boxes (infinite baffle types) and one has an air leak then that could give that effect as the cone has no "air resistance" to work into. Also old foam surrounds on the speaker itself can dissintegrate giving this effect as well.
 
#4
Almost impossible to diagnose without running some basic tests.

Is it distortion or is the cone physically moving and hitting the end stops ?

It needs a scope and a quick frequency sweep into a dummy load to see what's happening. Low frequency instability is very rare but can happen, usually dried out electroylitic caps, even main supply caps are the cause... but it's all guess work. A 'scope would show the problem in seconds. You at least need to isolate the problem to pre or power amp although it sounds like a power amp/supply issue if volume affects it.
 
#5
There is no distortion, but I dont dare turn the volume up more than a 1/4 but you can clearly see the left channel cone moving significant more than the right also it seems it does not happen as soon as the amp is switched on generally after about 5 min it starts. So i suspect there is a component that is on its way out, but as you say it needs proper diagnostics which I can't do. Anyway is it worth getting this amp fixed or just put the money towards a new one after all it's 18yrs old and i guess every thing has a design life?
 
#6
Very hard as mentioned to diagnose.

Is the cone movement "slow" like a DC shift. Can you see it move in and out fairly slowly, say less than 10 times a second ? Does the sound "change in character" when this happens like full bass boost etc.... just trying to get clues on what's happening. Does it do it on all inputs ? Can you defeat any tone controls etc ?

Probably a cap... semiconductors and most passives don't fail like this. For the price of caps it's worth just replacing them all (electroylitics only). If it's just one channel affected then can't see it being the main reservoir caps, more likely a local decoupler somewhere.

Why not post a new thread asking for a circuit... someone may know or have one. You can change the caps without a manual anyway.
Post some piccys of the inside.
 
#7
Having viewed the circuit this is a class G amp (switched rails) and my first thought is to check around that area... in particular two diodes that supply the power to the output transistors and that might being going open circuit under load.
 
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