I fixed it!!!

[Chemist_Guy]

I just need to gloat about my success.  It took me hours, but eventually I found the problem.

We have an electronic balance (scale) for the chemistry lab.  It had stopped working.  Before taking things apart, test voltages.  The AC adapter (the scale uses 12 V AC) was not putting out any voltage, so thinking that had just failed, I bought a new one.  Got the new one and nope, still won't turn on.  Take it apart which was pretty easy and get an idea how the circuit board is laid out.  Found the full bridge rectifier, the electrolytic capacitor, the voltage regulator and a bunch of other stuff. Time to test voltages.  The LM340 is putting out only 2.4 V.  No wonder it isn't turning on.  I should have 5 V.  But those are pretty hard to kill.  Move backward.  The electrolytic cap is only putting out 4 V.  Huh, no wonder the LM340 isn't putting out enough voltage.  OK track down a 2200 uF 25 V electrolytic cap.  De-solder, and put the new one in.  Nope still not working. 

More thinking and probing.  I look at the DB105 (Full bridge rectifier!!) on the AC side and it has only around 4 V as well.  Wait a minute, why is my AC in dropping so much when I plug it in?  Must be the rectifier.  Track another one down.  De-solder and replace.  Nope still won't turn on and the voltages are still the same.

At this point I'm wondering if I have wasted my life watching Dave's videos. So still looking at the board on the bottom and then it hits me.  A little SMD cap with a little crack in it.  Might it be? Probe the resistance and 0 Ohms.  Check all the other caps and they give non-zero values.  So this must be it.  A cap with a crack, shorting things out, leaking out all the electrons.  De-solder it and Happy Dance.  It works! 

[fzabkar]

Congratulations. Perhaps you could tell us the model number of the scale, and maybe upload a photo of the PCB?

[james_s]

Always nice to successfully fix something. You're on a roll now, find something else that's broken and tear into it. After a few more successes you'll start to have a feel for what goes wrong and how to most effectively troubleshoot it. Sooner or later you'll also find something that has you banging your head against a wall until you give up, it happens, don't let it discourage you, set it aside and fix something else. Sometimes you'll have a similar experience where you just get stuck and you set it aside, then come back to it later with a clear head and the problem is obvious.

[pcwrangler]

  Good job! Now all your co-workers know who they can pester with their dead electronics. 

[james_s]

Or who they will give their dead electronics to. I've never paid money for a TV in my life, lost count of how many I've had. A significant number of items I own are things I got for free and fixed.

[retiredfeline]

 

[HighVoltage]

One of the best things in life is to repair something successfully.
Congratulations!

[CJay]

I just need to gloat about my success.  It took me hours, but eventually I found the problem.

Isn't it a great feeling 

I still get a kick out of it after nearly 40 years.