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Forum index » Electronix » Repair
Nad7240pe
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Charles Wm. Edwards
electronics forum beginner


Joined: 20 Jul 2006
Posts: 1

PostPosted: Thu Jul 20, 2006 8:37 pm    Post subject: Nad7240pe Reply with quote

I was listening to the unit one night when all of a sudden the sound cut
out. The front panel LEDs still work. After opening the unit I found all
four line in fuses blown. (2) 4 amp and (5) amp fuses. Bought new fuses
and installed thinking a surge in the incoming house power could have
been the cause. As soon as I turned the unit on the new fuses instantly
blew. Can anyone give me an idea as to what could be happening. Thanks.




--
Charles Wm. Edwards
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Arfa Daily
electronics forum Guru


Joined: 06 May 2005
Posts: 588

PostPosted: Fri Jul 21, 2006 12:46 am    Post subject: Re: Nad7240pe Reply with quote

"Charles Wm. Edwards" <Charles.Wm.Edwards.2b9m7v@diybanter.com> wrote in
message news:Charles.Wm.Edwards.2b9m7v@diybanter.com...
Quote:

I was listening to the unit one night when all of a sudden the sound cut
out. The front panel LEDs still work. After opening the unit I found all
four line in fuses blown. (2) 4 amp and (5) amp fuses. Bought new fuses
and installed thinking a surge in the incoming house power could have
been the cause. As soon as I turned the unit on the new fuses instantly
blew. Can anyone give me an idea as to what could be happening. Thanks.




--
Charles Wm. Edwards

Depends on how the fuses are blowing. If they are failing hard, with a
bright flash and the fuse wire totally vapourised, then the problem is close
by - possibly something like a failed bridge. If the fuses blow soft, with a
gentle flash, and the centre of the wire just melted away, then the problem
is well downstream of the PSU - probably one or more short circuit output
transistors. Unless you have experience of working on this sort of kit, you
are unlikely to get to the bottom of the problem. Although shorted output
transistors are easy enough to find, this is seldom the end of the story
with DC coupled amps. You may spend quite a bit of money on replacement
transistors, only to have them just fail again, as soon as you switch on.
Don't believe for one minute that the fuses are there to protect the output
transistors ...

Just as an aside, I assume that you are replacing the fuses with like for
like typing for the originals ? Most amplifier fuses are "T" rated types. If
you use "F"s, they likely will blow as soon as you switch on, even with no
fault present.

Arfa
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Tim Schwartz
electronics forum addict


Joined: 07 Jun 2005
Posts: 51

PostPosted: Fri Jul 21, 2006 11:19 am    Post subject: Re: Nad7240pe Reply with quote

Charles Wm. Edwards wrote:
Quote:
I was listening to the unit one night when all of a sudden the sound cut
out. The front panel LEDs still work. After opening the unit I found all
four line in fuses blown. (2) 4 amp and (5) amp fuses. Bought new fuses
and installed thinking a surge in the incoming house power could have
been the cause. As soon as I turned the unit on the new fuses instantly
blew. Can anyone give me an idea as to what could be happening. Thanks.




Hello,



These fuses (all of which are 'T' type) are on the power amp
transformer rails. It is almost certain that either a pair of output
transistors, 'power envelope' transistors, or more have blown. Usually
only one channel fails. If an output is bad, I'd suggest replacing both
outputs, the driver transistors and the bias transistor at the same
time, and use original devices, though a 2SD669 is a good sub for the
bias device.

If the 'power envelope' transistors have failed, use the BD911 and
BD912 transistors that NAD used on later production, NOT the 2SA... and
2SC... ones the early units had. Bias and offset adjustments must be
made after repair, or it will soon fail again.

Regards,
Tim Schwartz
Bristol Electronics
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