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Building Preamp from kit but a resistor keeps burning up

BobK

Jan 5, 2010
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It looks to me like there are several possible solder bridges there underneath the capacitor in your picture.

Bob
 

davenn

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Sep 5, 2009
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yep, new caps new resistors. ill measure the resistance tonight, i havent done that with new cap

heres a pic of the solder of the cap, its dead center (the 2 vertical points)

the soldering around that resistor looks pretty bad .... looks as tho there's possibly a short cct solder bridge to the next solder tag below
 

Jumpybuns

Mar 29, 2015
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alright, i know i dont know what im doing but im thankful to have such an active forum to refer to. so should i just order a new PCB and start over? those little lines coming from the points are not excess solder, its actually a tiny bit scratched. it wasnt like that when during my first pass (which still smoked) but upon having to replace the resistor over and over and the board got beat up
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Not to be mean, but your soldering skills are poor. It appears that there is no bridge (which is good) but you are either using a soldering iron which is not hot enough for the solder you are using or you have very poor technique in heating the joint before applying solder (there are other possibilities too).

I also note some marks where you have slipped with the soldering iron. This is indicative of too much pressure being applied.

Poor soldering can (and from the look of your soldering, will) lead to problems down the track.

Please listen to my advice and develop your soldering skills a little further, and then resolder every joint on the board.

  • You need to be using thin (1mm or less) solder with a rosin (or other flux) core,
  • have a soldering iron with a tip between say 1.5mm and 2.5mm wide at the tip (shaped like a screwdriver),
  • have the soldering iron at the correct temperature for the solder (there is a range)
  • Ensure the tip of the soldering iron is clean with a little solder on it.
  • heat the joint by applying the iron to both the lead and the copper pad (adding a little solder to the tip here aids thermal transfer)
  • add solder to the joint so that it flows smoothly over the wire and pad before removing the solder and then the iron. It can help to slide the tip up the wire as you remove it to drag any excess solder away from the pad.
  • finally trim the wire.
When re-soldering joints, follow the same practice, except use a dry iron (no solder on the tip). The existing solder will b plenty to get heat transferred to the joint. Add a very small amount of additional solder to get the joint to flow nicely. If you have excess solder after this, use solder wick to remove some of it and solder the joint again. With some practice you'll figure out which joints need to have the solder removed before you try to solder them. A "solder sucker" can be used to remove the solder quickly.

Using solder wick:
  • unwind about 5cm of wick.
  • Place an unused section of wick against the side of the joint.
  • place the soldering iron onto the wick.
  • When the solder melts, it will start to be absorbed by the wick. The longer you hold the iron and wick there, the more solder will be absorbed. Don't take more than a few seconds after the solder starts to melt though!
  • Once a few cm of the wick has been used, trim it off and discard.
Using a solder sucker:
  • Arm the solder sucker
  • Place the soldering iron on one side of the joint.
  • As soon as the solder has melted, place the solder sucker over or as close to the joint as possible and fire it.
  • Remove both the sucker and the iron
  • Depending on your skill and a few other factors, the solder sucker can remove almost all the solder from the joint. It is not really possible to remove only some.
  • When rearming the solder sucker beware that solder from the last suck will come out of the tip. Do it into a bin or over somewhere that small pieces of solder aren't going to be a problem.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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You do not appear to have damaged the board significantly. I think it would be perfectly OK to just resolder the joints once you have a little more skill.

It really is possible to make some huge and terrible mistakes, but you have done none of these as far as we can see.

If this is your first attempt at soldering then you have actually done surprisingly well. However, given the nature of the project, you need to do much better.

I assure you, once you start to see nice solder joints you will get another real sense of achievement.

Google for good solder joints to see the sort of thing you should aim for.

Have you got anything you can practice on?
 

Jumpybuns

Mar 29, 2015
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Alright. Thanks for the info. I have a bunch of crap I can mess around on
 

Jumpybuns

Mar 29, 2015
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Well, I resolderd the whole board and it went a lot better! And quicker too!
I checked for shorts but everything seems solid. Plugged it in, still smoking. I'm out of ideas except could it be the Lindell 506 phantom power supply?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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I'm out of ideas except could it be the Lindell 506 phantom power supply?

Very unlikely, unless the voltage is way too high -- and that would cause other problems.

Firstly, with the resistor and capacitor in place, and the power off, set your multimeter to the ohms range. The display should read OL (or similar). When you hold the probes together it should read 0.0 (or similar). It's OK of you get something like 0.1 or some other low value too. Note that it may take a few seconds for the reading to stabilize. That is also normal.

So make sure that the power is disconnected from the board, and measure the resistance across the power supply connections on the board. (Place the black lead on the negative terminal and the red on the positive). What is the reading?

Now, leaving the black lead in place, measure the resistance at both ends of the resistor and both ends of the capacitor. Note down these 4 readings.

Then tell me what you got.
 
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