Failed PC amps

Discussion in 'General Electronics Chat' started by atferrari, Mar 10, 2012.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    2,648
    764
    After assembling an audio amp (LM386) for bench tests purposes and when looking for a speaker and enclosure I was surprised of how many failed amps for desktop PCs I had around!!

    After opening them:

    I noticed that in ALL, once plugged, the /edit AC from secondary /edit go straight to the rectifier. No switch in between.

    The ON switch at the front just applies that rectified voltage to the filter, feeding the amp.

    Found from discrete amps to TDA and TBA ICs.

    Found one PCB cracked by the pressure exerted on the ON switch and also a transformer heating madly even after desoldering all the components around.

    To complete my performance I opened the pair of this PC and replaced the failed pot with fixed resistors. Intput level software control does the rest.

    Two years ago I had replaced the filter cap when my daughter discarded it because of a severe buzz.

    To disassemble the pot I had to forcibly cut it because the nut inside a deep circular recess did not allow to free the shaft. I wonder how it was assembled but evidently nobody expected this to be repaired.

    They were all made in China.

    Oh God, music is so nice...!
     
    Last edited: Mar 10, 2012
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    As I understand it, computer PCs make a high voltage high current DC section, it is a transformerless power supply at this stage. However, they convert this high voltage DC into a much higher frequency AC wave, which can use much smaller torrid transformers and much smaller capacitors to filter the high frequency from the now high current DC.

    So they aren't so transformerless after all, but it is not obvious.
     
  3. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    2,648
    764
    Sorry for my mistake. Corrected in the original post.

    The six amps I disassembled had transformers fed with 220V and what goes straight to the rectifier is the AC from the secondary.
     
    Last edited: Mar 10, 2012
  4. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
    2,648
    764
    Repaired the last in my list (office desktop).

    To correct an intermitent left channel I had to replace the input jack & cable. I simply used one from those I disassembled yesterday. Not bad after all.

    To my surprise I found several electrolytic caps of .22 uf / 50V. What is the reason to use them?

    Are they cheaper than non polarized polyester or multi layer?

    I ow myself a tasty espresso to celebrate.

    Everybody is invited tomorrow Sunday at 09.00 at Victoria Cream. Ask for Agustín. :)
     
    Last edited: Mar 10, 2012
Loading...