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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Nad7240pe
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.
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#2
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Nad7240pe
"Charles Wm. Edwards" wrote in message ... 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 |
#3
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Nad7240pe
Charles Wm. Edwards wrote:
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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