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Old 25th September 2012, 09:49 PM   #1
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Default Nad 712 heat sink HOT

I have NAD 712 and there a 3 head sinks for a voltage regulators the upper one seem to get extremely hot unless the heat gets transferred from a resistor near by that get HOT also . i can provide a picture . i just worry that there might be something wrong unless its ok for a resistor to get really hot to a touch .
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Old 26th September 2012, 12:03 AM   #2
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Ok, you are concerned about about hot parts in your amplifier. Is it otherwise working normally? Are the parts discoloured or the nearby circuit board charred? Has the amplifier had recent misuse and what is the real reason for looking inside?

Pics are interesting but it may help someone who could advise, for you to locate and post a schematic diagram from a service manual and also for you to identify specific part numbers printed on the PCB where the parts attach. Many amplifiers do have parts that are designed to cope with running painfully hot and designers often exploit this for various reasons. Is your discovery from just curiosity or do you have other reasons to suspect a fault?
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Old 26th September 2012, 01:28 AM   #3
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I have just bought the unit and i noticed that there is lot of heat generated in a middle of the main board. once to took the cover off just to inspect it there are 3 little heat sinks and one (top one) is extremely hot. i don't think its the part attached to the head sink that is getting so hot but a resistor right next to it and is heating up everything around it including the heatsink . Yes the board is discolorated a little from the bottom. Im talking Hot i can probably burn my finger if i hold it against the resistor for more them 5 sec. I just want to make sure its ok get hot and why is it getting so hot. The receiver is working 100% dont see any issues except the heat. I usually leave the unit on most the night its my bedroom unit that i sleep with music playing at low levels. just dont want the heat to make any damage the the receiver.
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Old 26th September 2012, 01:29 AM   #4
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Old 26th September 2012, 01:29 AM   #5
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service manual

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Old 26th September 2012, 01:33 AM   #6
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Old 26th September 2012, 01:57 AM   #7
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resistors that im taking about are R516, R518

http://home.comcast.net/~nosferatu2xlc/11.jpg
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Old 26th September 2012, 03:11 AM   #8
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The finger test suggests you are probably talking of around 60 degrees C. That's not out of spec. for many parts and such discolouration around discrete regulator circuits like this one, for the negative side of the VAS/driver stage, would not be unusual in product this old. As they are same value and current path, R521, 522 & 526 should be just as hot as R516, 518. I must say I haven't tackled this circuit and others may wish to comment from their experience.

Meantime, you could test that the regulator is functioning normally by measuring its output voltage at the negative terminal of C523 or C526. It should read ~28V DC. I assume you have a DVM or similar and can use it, as attempting any work or testing without instruments is a waste time.
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Old 26th September 2012, 04:47 AM   #9
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I agree with Ian, that these parts are probably running as hot as they do by design.

Many semiconductors can withstand junction temps up to 150 C without being destroyed, so 60 C may feel hot to the touch, but shouldn't really be a problem.

Consumer products usually don't get much margin for error designed into them, especially if they're built down to a price. If a component can run hot without being destroyed, they usually don't get a bigger heatsink than necessary.

NAD 712 service manual
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Old 26th September 2012, 01:11 PM   #10
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Quote:
Originally Posted by Ian Finch View Post
The finger test suggests you are probably talking of around 60 degrees C. That's not out of spec. for many parts and such discolouration around discrete regulator circuits like this one, for the negative side of the VAS/driver stage, would not be unusual in product this old. As they are same value and current path, R521, 522 & 526 should be just as hot as R516, 518. I must say I haven't tackled this circuit and others may wish to comment from their experience.

Meantime, you could test that the regulator is functioning normally by measuring its output voltage at the negative terminal of C523 or C526. It should read ~28V DC. I assume you have a DVM or similar and can use it, as attempting any work or testing without instruments is a waste time.
Yes i do have DVM , how exactly would i check voltage at 523 526, i don't want to short anything. R516 is probably twice as hot as R518.
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