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Please help me identify this resistor

Alex Santos

Alex Santos

New Member
Hello

I have R466 out of circuit.
The value read is OL so it must be bad.
It's expected to have a value of 68

R466 - 1.jpeg

According the service manual (NAD 7240PE) it has an exclamation mark next to it on the parts list. Here is what it shows:
(!) R466, Res. Carb, 68 +/- 5% 1/2W
"Product Safety Note: Products marked with a (!) have special characteristics important to safety. Before replacing any of these components read carefully the product safety notice of this Service Manual. Don't degrade the safety of this product"

I found a page with the title "Service Safety Precautions (UL)"
1. Use exact replacement parts for critical locations, marked (!).

I can't find any more references to the (!).

The original part number is 0921-116-8-04

What makes this resistor so special? Is there an equivalent?

The PCB has a rectangle to identify resistors but inside the rectangle a slash across it. It seems all the resistors that are marked with an (!) in the service manual have this symbol on the PCB.

R466 - 2.jpeg R466 - 1.jpeg

Thank you.
 
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One more photo, here is R466 and R468.
R466 and R468 - 1.jpeg

R468 is identical to R466 in value and tolerance; when I measure resistance for R468 (completely out of circuit), I get 4.86M not 68
 
The flame-resistant resistors usually fail by going open circuit, so this isn't too surprising. They're intended to burn open rather than letting other things roast and cause a potentially much uglier problem.

Now the trick is finding out why. Generally its because whatever it connects to tried to flow a whole lot more current than it ought to. I don't know what exactly you are working on, but wild guess is these are emitter resistors protecting some sort of transistor or mosfet, and that device may be shorted.
 
avionic said:
Carbon film 68Ω 5% tolerance 1/2 watt
Click to expand...
Thank you. An apology, I just realized that whichever online resistor chart I was using reset my white band selection to black. Thanks for the reply. I was asking because I want to be sure the service manual for the NAD 7240PE that I am 'working on' didn't have a documentation issue. Many thanks!
 
avionic said:
What role it plays in the circuit.
Click to expand...

Thanks @avionic. I am trying to learn something here. I downloaded the service manual for the OP's amplifier but I was not able to find a reference to the word "fusible" or "fuse resistor". The OP included everything relevant about this resistor that can be found in the service manual in his first post.

Could you please expand on how, based on its role in the circuit, you knew this resistor is fusible?

Is it because it is on a power rail, but the resistance value (68 ohms) is so low that it is not likely intended for dropping voltage?

What about the PCB marking under the resistor, the rectangular box with a diagonal slash through it? Is that a symbol for a fusible resistor?
 
roger2 said:
Is it because it is on a power rail, but the resistance value (68 ohms) is so low that it is not likely intended for dropping voltage?
Click to expand...
Give the man a Cee - gar..
roger2 said:
What about the PCB marking under the resistor, the rectangular box with a diagonal slash through it?
Click to expand...
Maybe a reminder to check the service manual for any pertinent documentation.
 
Thank you again @avionic. I am glad to have won that cee-gar!

FYI, I think you may be thinking of leesonic, he is the one who has posted a lot of the NAD rebuild threads. Leestereo may well have posted some NAD info or rebuild threads too, I don't know. Both Lee's are very accomplished and knowledgeable.
 
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