R49 smoking, now reads lower resistance...

[keevhren]

I just built 4 N72s (although Ive previously built 2, so now Ive got 6). While I was powering up the 5th unit, R49 started smoking immediately and got bright red/orange even though power was only on for a second. After turning the power off immediately and unplugging, I inspected the whole area, and surprisingly the resistor doesnt appear to be damaged really, theres no burn marks on the board, and it still looks fine. I measured the resistance and its coming in at 6.6 now instead of the 10 that it should be. I need to change it out right? I dunno if its low due to the burn/overheating or if the part was defective to begin with, but i dont want to plug in and retest till i confirm it.

Ive double checked all my work, everything else is in the correct position, polarities are all correct, ive reflooded most every point on the board for good measure, and the bias trim pot was all the way counter clockwise prior to the initial power up.

Should i Pull R49 and put in a new one?

Thanks for the help! Also, if I need to replace, is it a part I can snag at my local radioshack or electronics store or do i need to get it from 7th circle?

[tpryan]

Whatever caused R49 to smoke has nothing to do with R49 itself. If you replace it without finding the problem, you'll just smoke the new resistor.

[keevhren]

I realize that, which is why ive reflooded every point on the board, checked polarity/placement of everything and it all looks correct... i dont know what else to do/check before trying to power up again but Im assuming that i do need to change r49 as its reading lower than it should be. Right?

Is there some other issue that you'd suggest i check prior to powering it back up? Ive done all of the safeguards/checks from the build instructions and they all seem to be fine (no meaurement between molex pin 1 and 2, or 2 and 3 etc....)

Thanks!

[Soundman1402]

Yes, the resistor is damaged and needs to be replaced. You can use any standard 10 ohm resistor from Radio Shack, Fry's, etc.

When a resistor gets hot, it's because too much power is flowing through it. Your most likely cause is a short circuit, given how quickly it went up in smoke. You reflowed all your solder joints, so a solder bridge is unlikely... But check that anyway, with a magnifying glass. Your next culprit to check for is transistors or other parts in the wrong place. I once smoked a resistor in the phantom power circuit by swapping the PNP and NPN transistors. They markings were really smudgy and hard to read, and it took me two or three looks before I realized they were in the wrong spot. Also be sure that you've got the transistors the right way around, as well as all of your diodes. You'll notice in the schematic that R49 is on the power input rail for the whole circuit (minus phantom power), so unfortunately, there are a lot of places to check for things gone wrong.