Nad S100 pre-amp woes

timpy · 12th November 2009, 01:51 PM

I have a Nad S100 pre-amp that blows it's internal fuse (transformer primary side). It's been here for a while but now I have a use for it and want to put it right.

Does anyone have any schematics, or know of any common causes for this fault. A quick measurement across the primary shows in the order of 80 ohms, which to my mind is too low. I don't know if any of the capacitors may have be breaking / have broken down, or whether the transformer primary has shorted out some windings.

I signed up to S100 manuals, but no schematics there for the S100, although there are for the S500 and S170 and loads of other NAD stuff. I think the tranny is a Holmgron, but don't know what secondary voltage etc it is supposed to be.

Any help or pointers much appreciated.

Cheers
Tim

timpy · 15th November 2009, 10:46 PM

No one?

cheers

Mooly · 17th November 2009, 01:05 PM

When you measure the resistance of the primary that is the DC resistance and bears no relation to the impedance at 50hz AC. So 80 ohms sounds fine.

If you replace the fuse does it work at all for a limited time ?
Possibilites are a mains filter cap breaking down. This will be connected between live and neutral on the incoming mains side. If in doubt you can remove these to see if it still blows.
Does the fuse "blacken" or just gently part ?
If you suspect the transformer you can disconnect all the secondaries and then power it up and see if it still blows the fuse.

timpy · 20th November 2009, 12:27 AM

Thanks.
Fuse blackens so it's a healthy blow. I'll take the cap out across the live and neutral and work from there, and report back.
Thanks for your help and the ideas

Cheers

12th November 2009, 01:51 PM	#1
timpy diyAudio Member Join Date: Jan 2007 Location: Cardiff, Wales	Nad S100 pre-amp woes I have a Nad S100 pre-amp that blows it's internal fuse (transformer primary side). It's been here for a while but now I have a use for it and want to put it right. Does anyone have any schematics, or know of any common causes for this fault. A quick measurement across the primary shows in the order of 80 ohms, which to my mind is too low. I don't know if any of the capacitors may have be breaking / have broken down, or whether the transformer primary has shorted out some windings. I signed up to S100 manuals, but no schematics there for the S100, although there are for the S500 and S170 and loads of other NAD stuff. I think the tranny is a Holmgron, but don't know what secondary voltage etc it is supposed to be. Any help or pointers much appreciated. Cheers Tim

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15th November 2009, 10:46 PM	#2
timpy diyAudio Member Join Date: Jan 2007 Location: Cardiff, Wales	No one? cheers

17th November 2009, 01:05 PM	#3
Mooly diyAudio Member Join Date: Sep 2007	When you measure the resistance of the primary that is the DC resistance and bears no relation to the impedance at 50hz AC. So 80 ohms sounds fine. If you replace the fuse does it work at all for a limited time ? Possibilites are a mains filter cap breaking down. This will be connected between live and neutral on the incoming mains side. If in doubt you can remove these to see if it still blows. Does the fuse "blacken" or just gently part ? If you suspect the transformer you can disconnect all the secondaries and then power it up and see if it still blows the fuse.

20th November 2009, 12:27 AM	#4
timpy diyAudio Member Join Date: Jan 2007 Location: Cardiff, Wales	Thanks. Fuse blackens so it's a healthy blow. I'll take the cap out across the live and neutral and work from there, and report back. Thanks for your help and the ideas Cheers

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