some help with the repair of a power supply unit

Discussion in 'The Projects Forum' started by Thomas.R, Feb 17, 2008.

  1. Thomas.R

    Thread Starter Member

    Aug 16, 2007
    20
    0
    halllo,

    I've got a PSU of an audiomixer to repair. After measuring it out i found that there was a schottky diode broken. Apparently the type used here is hard to find and it would cost me 20€ (shipping costs!?!) to get me one.
    So i did the following, I placed a regular diode with the same Volt/Amp specs but this one burned out.
    Isn't there a chance that if i put in the right part this one will burn too?
     
  2. S_lannan

    Active Member

    Jun 20, 2007
    247
    2
    ok it sounds like you replaced it with the wrong type of diode.

    always replace a shottky with a shottky in powersupplies. They're used because they have a MUCH faster recovery rate than normal silicon diodes.

    replace it with a shottky with the same or higher specs than the original diode in the supply. Then you should be back in business

    b.t.w. thats what happens when you use a normal diode in place of a shottky or ultrafast, it burns out.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You likely have more problems in the PSU than simply a burned-out diode.

    I would strongly suspect electrolytic capacitors; they degrade over time, and can fail either shorted or open. If your PSU is more than 8 years old, I suggest replacing all of the electrolytic capacitors before trying another diode.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    To add to SgtWookie's post, the electrolytic caps need to be low-ESR rated. Be careful about voltage ratings and size of the case. A larger diameter might not fit, whereas a taller one probably will.
     
  5. Thomas.R

    Thread Starter Member

    Aug 16, 2007
    20
    0
    is there a way to measure the caps on the board? because visually they are all okay. i know a little theory but there is huge diference between the practical stuff.
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You cant measure the value of a capacitor while it is on the board because it is connected with other components and you take a wrong reading. You can measure it if you take it out of the board but if you make the job to take them all out it is better to replace them with new ones if they are very old.

    Some voltmeters measure capacitance if you want to measure it
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    You can measure the ESR with the capacitors in circuit. You can't measure the capacitance in circuit.
     
  8. rherber1

    Member

    Jan 6, 2008
    15
    0
    In MOST cases you can. If the capacitor is connected in parallel with other resistive components, insitu ESR measurements will not be accurate and will generally read lower than they actually are. Insitu ESR readings may also depend to some extent on the type of ESR tester being used.
     
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