Burnt out resistors

[jheatac]

Hello,

I am verrry much an amateur at electronics.

I am working on the power side of a car audio amplifier. Power transistors are smoked/blown apart.

It also cooked the resistors directly in front of the power transistors. According to the schematics they are all carbon film, but they are blue 5% carbon film.

Being that I can't read the bands, I am removing them and making notes of the resistance I removed. 

My question - Is there a typical fail mode of a resistor? I have several that the schematic says 100 ohm and they tested and 40-50 ohm  and I have several listed as 2 ohm that tested 60 to 90 ohm. None were open circuit.

I just don't want to blindly follow the schematic if this is indicative of anything.

Thanks!

[james_s]

Typically resistors fail open or go up in value, it's very rare for one to drop in resistance. It's not uncommon for schematics to be wrong, sometimes it's an error, sometimes the schematic is for a slightly different model, sometimes the circuit was revised to improve some aspect.

[WattsThat]

If you cannot read the color bands because they’re smoked, use the schematic values as the actual resistance once they’re baked is no longer correct..

As mentioned, resistors (typically) don’t go down in value when cooked, they go up. The resistors fail after the transistors fail since transistors almost always fail shorted, increasing the voltage/current through the resistors.

[SpecialK]

Check out "The Active Region" youtube channel.  Kevin makes pretty good videos on the subject.

[jheatac]

Thank you!