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Old 24th November 2011, 11:27 PM   #1
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Default Resistors failing without optical damage?

an amp of mine failed recently because external mains overvoltage. ok, I was reckless enough not to fit a fuse and a MOV.
there were some indicator LEDs on the front plate. they were all burnt and I replaced them only to find that they were still not working. there were 180 ohm resistors in series with all of them and upon measurement I found that ALL the resistors were damaged (circuit interrupted). NONE of the resistors showed any usual sign of physical damage. basically all the resistors that were connected to the unprotected part of the amp were damaged without any optical trace.
this is the very first time something like this happens to me, I've had tons of cooked resistors (physically burnt) in the past. and the large number of them presenting the same type of failure makes me wonder...
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Old 24th November 2011, 11:33 PM   #2
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Flameproof resistors if done right expire with a tiny puff of smoke and a little pin prick in the coating, unlike their less safe siblings who either go up in an impressive ball of flame or glow cheerfully orange and set the place on fire.
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Old 25th November 2011, 05:36 AM   #3
jitter is offline jitter  Netherlands
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I have seen this behaviour of resistors in an overcurrent fault at work once before.
When I was looking up the failed parts in the bill of materials, I found out that the resistors weren't ordinary resistors but fusible resistors.

If you have the service manual of your amp, the bill of materials should list them as fusibles, and often in the schematic an exclamation mark is printed next to them to indicate they MUST be replaced by identical type.
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Old 25th November 2011, 08:34 AM   #4
Elvee is offline Elvee  Belgium
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I have also seen numbers of ordinary, lacquered resistors, failing open without the slightest external sign of damage.

Sometimes, the cause is known, as when it serves as current limiter in capacitive supplies, but sometimes there is no apparent reason.
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Old 25th November 2011, 01:35 PM   #5
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Long term over power burns resistors. High current pulses can blown them open quickly without lots of heat so no burn marks!
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Old 25th November 2011, 01:50 PM   #6
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yeap, the shock was quick. but I found about the existence of flameproof resistors so I guess the topic wasn't for nothing.
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Old 25th November 2011, 01:55 PM   #7
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Flame proof resistors are always made in blue base color so you can tell the difference.
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Old 25th November 2011, 01:58 PM   #8
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the failing resistors were the regular "buttery" color.
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Old 25th November 2011, 02:03 PM   #9
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Then fuses are a very good idea!
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Old 25th November 2011, 02:06 PM   #10
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Quote:
Originally Posted by simon7000 View Post
Then fuses are a very good idea!
clearly.
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