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Strange issues with NAD 7240

Drum

Drum

Active Member
Well I enjoy my NAD 7240 so much (after diagnosing problems thanks to members here) that I got another one. Unfortunaely this one has much bigger issues.

Symptoms

Very low sound from any input - about 60 db at full volume.

Did some tests.

Used pre-amp on another amp and nothing changed.
Pushed in Bass EQ button and music got 10 db louder.
Pushed in Loudness and could not tell any difference.
Tried tone controls and they were backwards - adding bass or treble actually reduced it.

Used a good pre-amp on this recievers amp and it put out plenty of power.

My conclusion is that the pre-amp portion is bad, but why do the tone controls act so strangely?

One more oddity. When I hit the power button to turn the reciever off it keeps playing at the same volue for about 10 seconds. My other 7240 shuts right off.

I don't see any loose connections or bad looking joints, but the 4 large capacitors are swollen at the top. Don't the large capacitors just effect the amp portion not the pre-amp?

Any help in diagnosing would be appreciated.

Thanks
 
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Time for a Recap. When the large value caps start bulging it's time to recap the unit before a total failure occurs. Four different power supplies are on the unit. +/- 71Vdc (6800-80Vdc caps), +/- 43Vdc (6800-50Vdc caps), +/- 28Vdc, 5Vdc. How do the rest of the caps look? Are there any other caps with the plastic labeling looking shrunk on them?. Drum Welcome to AK.
 
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Thanks NAD80. The rest of the caps look better no bulging or shrinking plastic. I'll order the 4 larger caps. I didn't realize they could have such an impact on such different areas of the receiver.
 
Drum did you have the service manual for the NAD? If you need it is up on HiFi Engine.com. Just register and download for free. HiFi Manuals.com is good also.
 
Hello Drum!
Looking at the service manual, you preamp stage is based on ic301. So in order for this to work , you need a 15v on pin 8 and -15v on pin 4! Are these voltages present? If not then check r311/312 1k! check fuses f505/506 1a, r507/508, zener diode d502/501? and q501/502 transistor! You need to have 20 volts on c517/518 or on one side of the r311/312 resistor!

Also , refering to the big caps, are they coverd with plastic? If yes then press the plastic with your finger and see if it became flat! If yes then i think that you are ok for now! Just repair your receiver and you van change them later! Maybe a general recap?
 
last 2 NAD i worked on C320 and C352, in both the big filter caps were broken.
when desoldered if i shaked them it sounded like a applecore was in there :)
these were the caps with NAD plastic wrapping saying CD294 and NAD logo

dont know the age of the amps but i guess somewhere around 2000
 
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I would second (or third) a recap. When the small caps go bad they can cause all kinds of weird problems. I had a 4020 tuner with a bad cap that shorted the power supply and blew a resistor. For the cost of the caps, it makes sense to replace them if you have the cover off for any reason.

Lee.
 
So by recap does that mean every cap (there are about 80 of them) or just the dodgy ones? It's gonna take a while to get them all done!
 
the big caps i guess without looking in the schematic would affect pretty much only the poweramp.
have you cleaned the input and muting switches? relays? and so on.
and yes the amp is about 25-30years old? so electrolytic caps starting to show their age. what brand is mounted? (not the big ones) the smaller ones scattered across the boards
 
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Most of the capacitors are YEC. There are some baby blue ones with an M or an A on them but that is all I can see. I have not cleaned the input switches yet or the relays (will need to look at the schematic to find the relays- I see a few candidates, but not sure).
 
then a recap is necessary i would say.
im not familiar with the model but many nads used relays for switching inputs. if you dont see many of them than its probably just the input switch to clean.
 
Fair warning: a recap is extremely unlikely to fix your problem. They very rarely do. Check for bad solder joints instead.
 
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