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Have you ever gotten "lucky?" (when repairing electronics)

Discussion in 'General Electronics Discussion' started by kpatz, Mar 23, 2015.

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  1. kpatz

    kpatz

    329
    88
    Feb 24, 2014
    I know, get your minds out of the gutter... :)

    When troubleshooting or repairing electronics, have you ever "lucked out" and found the faulty component out of sheer luck (or intuition) relatively quickly?

    I did last weekend.

    I have a Zenith Pro895x CRT projector which I'm fixing up for a friend to sell. Originally I thought I was just going to need to change out the glycol coolant in the CRTs due to contamination. I was able to do this no problem, but when I put everything back together and plugged it in (moment of truth, will it work, or will it blow up??), and tried the remote as well as the power button I got nothing. No power, no LEDs light up, no relays clicked, no fans spun.

    Long story short, the standby power supply wasn't supplying (as confirmed by the unlit LED on the standby power supply board). After checking the fuses, and confirming that the mains power was being rectified and filtered fine (resulting in 345V DC in the right places), I pulled the board out of the projector and gave it a good look-over. Nothing looked burnt or overheated. All the caps looked fine.

    Being a typical switching power supply, I suspected the driver FET at first. I removed it and tested it and it was good. Ok, must be something else. But what? There are probably 50 components on the board. I figured I'd start near the standby power output (and the aforementioned LED). There was an innocent-enough looking resistor there, and on a whim I took my ohmmeter and measured it (in-circuit). I got about a megohm one way and several megohms in the opposite direction. Seemed on the high side for a resistor on a power supply. I'm a bit rusty on decoding color bands, so I used a website as a guide, and found it was supposed to be a 0.1 ohm resistor. I removed it from the board and it was open. I don't have one on hand, but as a test I put a 150 ohm resistor in its place just to see if the standby LED would light up now. It did, and I have a replacement resistor on order.

    So, it was sheer luck, or intuition that I happened to check THAT resistor and it was the bad one. My initial suspicion was that some semiconductor blew out due to a power surge, which is why I checked the FET first. Resistors rarely fail open unless they burn up, so how did I know to check THAT resistor out of all of them?

    Anyone else with a lucky break while repairing something?
     
    BobK likes this.
  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    I opened up my guitar amp when it stopped working, no output. There was a socketed TL082. I happened to have one on hand, so I replaced it and bingo, I had a guitar amp again!

    Bob
     
  3. chopnhack

    chopnhack

    1,573
    354
    Apr 28, 2014
    Most of my fixes are lucky breaks ;-) Bad caps are usually my winning streak.
     
  4. shrtrnd

    shrtrnd

    3,827
    520
    Jan 15, 2010
    Shhhhh.
    I tell everybody that I'm good at what I do. Don't spoil it by letting on that we get lucky sometimes.
     
    KJ6EAD and Supercap2F like this.
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