Finding a Short Circut/Replacing Components

Discussion in 'General Electronics Chat' started by Buzzfly, Jun 28, 2009.

  1. Buzzfly

    Thread Starter New Member

    Jun 28, 2009
    I've got an old stereo receiver (from the early 80's) that I thought I would try and fire up a week ago, however when powered on the fuse blew, and I had no idea whether it was a slow or fast blow fuse. So, I went to the local shop and talked to a guy, who obviously had no idea what I was talking about, and promised me that a slow blow fuse would work in it's place, so I tried that and now I've got a burnt out resistor. (I knew I should have used a fast blow fuse...)

    From what I've read, it seems like a short somewhere is the most likely cause for this burnt out resistor (correct?) but I have no idea how look for where the short might be. That is the first thing I need help with, how do you find a short in a circuit? And the second thing is how do I replace the resistor once I've found and dealt with the short?

    Any suggestions on how to even begin going about this would be much appreciated. And I haven't had any real experience with circuitry before so keeping it simple is preferred. :)
  2. loosewire

    AAC Fanatic!

    Apr 25, 2008
    Is the burn't resistor in the power supply.
  3. Tobias

    Active Member

    May 19, 2008
    First off, I have figured out that to find these problems usually requires atleast a 6-pack of beer. It helps me pace myself so I don't get frustrated not finding the problem in a couple minutes and also makes the time more enjoyable.

    Keep in mind I am not at the caliber some of these guys are here, I just know the really sharp ones take some time to reply. Their help for my problems have been much appreciated.

    With my knowledge level relative to some of the people on this deal in mind, if I was in your situation I would get out my ohm-meter, besides the 6-pack. Attach one end to the ground on your board and start probing. If your ohm-meter has the audible alert that will make it alot easier. What you are looking for is like you mentioned a short. Power going straight to ground. I am sure you don't have a schematic of your old stereo board, so you might want to make it a 12-pack instead of a 6. This will take some time. Start off by looking at the traces and see if you can find any that have been exposed to heat. This would be my first hint there is a problem. If it is a burnt out resistor...set your ohm-meter from audible to the true resistance reading. Go to both sides of each resistor, if operating correctly you will get a real number, not an Open reading. I would think that it could be really just a burnt trace, power supply burnt up, or many other things.

    I am betting since you mentioned it is old, the board isn't surface mount but through hole style of components. If through hole, replacing will be pretty easy. Fire up your solder iron. Be careful, you will be on about your sixth beer by now and that thing will be damn hot. Get a nice set of tweezers, heat up one pin and remove the pin with the tweezers. Do the same with the other side. Most likely there will still be solder in the hole you just removed the side of the resistor. Thats no problem, for about ten bucks you can get a solder removal tool at your local Radio Shack. Its a nice suction type deal so heat up the solder and use the tool to suck up the solder. A quicker route will be just to heat up the solder pad to liquifey the solder and slide one pin of the new component into the pad. Repeat with the other side.

    Your deal can be pretty quick or a long term search. I hope it works out.

  4. DC_Kid

    Distinguished Member

    Feb 25, 2008
    i wouldnt follow all of Tobias' suggestions.

    beer? this just makes it longer to find the part, the right part.

    whether the part is good or bad, clamping a ohm meter to ends of a resistor (or any part for that matter) that's still in the board has the possibility of killing other parts in the board. doing this could also still give you a reading even if the clamped part is open, etc.

    you at least need a hypothesis as to what the part might be, then you might be able to come up with a test procedure. if its a resistor in question then perhaps a 1M or a 2M shunt resistor across the resistor in question will tell you if a upstream or downstream part is functioning, etc.

    a schematic would help you. a o-scope is a great tool to have, etc.

    look and smell for burnt parts. use a magnifying glass, etc. NOTE: FINDING A BURNT PART DOESNT MEAN YOU FOUND THE PROBLEM. it perhaps means you found the segment that contains the problem.

    perhaps a short, or perhaps just a tad too much current for the problem part to handle, perhaps a land that pulled away from the board (common on old boards), perhaps its the part itself that caused it, or perhaps something else is bad but survived but caused a resistor to burn up. a technician's sleuth dream.

    basic technician skills is to:
    know what it is you're looking at
    have basic knowledge of electronics
    have basic tools to diagnose with

    if you dont have all 3 then many times its futile to fix.

    short story - more than 20+ years ago my mom fried something in a new $1k sony tv. she had her video cam hooked up to the tv inputs with tv volume very high. well, all of a sudden no sound from the tv. damn feedback caused something to fry. video was a-ok. so i had to go digging into the set. sony at that time had modularized construction. 3 main boards, 2 vertical on swing-out wings and one horizontal on the bottom. everything interconnected with cables so i was able to take the boards out and easily inspect them. i knew i had to go probing after audio stuff. had to buy schematic because nothing on the boards gave clues as to where audio stuff was. i only had access to gear in my high school electronics classroom and a fluke meter i still have today. looking at the schematic i found the transistors for amplification but their specs would lead me to believe if they were fried there would have been visual indication of such, and it would have been unlikely that both audio channels were fried. i did what transistor (diode) and other parts tests i could do with the fluke on in-board parts and visually traced through the boards via schematic looking for a fried part. this tv had me trace back way inside a power supply cage on the horizontal board. it was a small glass fuse to the audio circuitry !! cost to repair = $60 for full schematics and 39 cents for fuse. took me about 2 weeks to trace through everything. you might ask why i didnt just check the fuse 1st, i had the schematic right. well, sony decided to simply list the power supply cage as a "module" with no details as to what was inside the cage.
    Last edited: Jun 28, 2009
  5. scythe

    Active Member

    Mar 23, 2009
    If a resistor burnt out, there may be other burnt out components as well. Capacitors, diodes, possibly IC's too.

    I've never been able to successfully repair burnt electronic boards without a schematic for this reason; I lack the patience to locate the problem, much less find a correct replacement component to fix it. It's like trying to reconstruct a document that's been cross-shredded. Sure, you could do it in theory, but is it really possible?

    I side with DC Kid on this one. It may just be futile to try and fix.

    But hey, if you're really serious about fixing it, I would check the power supply (the electronics surrounding the power source). And I would also look for fried capacitors. And for what it's worth, good luck!
  6. flat5

    Active Member

    Nov 13, 2008
    If the resistor is near the output transistors suspect them. Suspect them anyway :) and their drivers. Goggle will help you to learn how to test them. Do some checks on the power supply too. Also look for deformed (leaky or ballooned/bulging) capacitors.
    If none of this makes sense give up.
  7. SgtWookie


    Jul 17, 2007
    My primary suspects would be electrolytic capacitors, particularly in the power supply.

    Look for capacitors that have bulging or burst tops/ends.
  8. Buzzfly

    Thread Starter New Member

    Jun 28, 2009
    Haha, Thanks for the Encouragement. :rolleyes:

    I appreciate the help everyone, to answer some of the questions people have asked, the burnt out resistor is not on the power supply, it on what I'm assuming is the main board (it's the biggest and in the middle of everything) and it's pretty obvious which one is burnt on account of you can actually see the burn damage on it. Unfortunately I don't have a schematic of the system, but I don't expect to fix this in a hurry or anything, it was more like a summer project. :)

    Now to clarify something for myself, when DC_Kid said "perhaps a land that pulled away from the board (common on old boards)" what does that mean?
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    U know pictures can speed up the process

  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    If the fuse is in the power supply, don't forget a shorted diode has caused fuses to blow in the past.

    Rifaa is correct ... a picture is worth a 1000 words and certainly can speed things up.
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
    I seriously doubt it. It could be a whole lot of things, power supply is always the first place to look. Resistors burn up because too much current flows through them, the problem is what caused the high current.
  12. AlexR

    Well-Known Member

    Jan 16, 2008
    Do you people realise that this thread was started almost 3 month ago and in all probability by now the OP has either fixed his problem or else binned the set ages ago.