Posted by ClarKit (A) on January 12, 2008 at 11:41:22

I have DAC2.1 & M3 driving monoblocks all brand new just built. I have serious microphony in one monoblock. After much work interchanging components between RH & LH mono's cause is AN "stand off" in the HT power section i.e. white "ceramic Tag Strip" onto which HT PS resistors and cap leads are soldered. It is being replaced but has anyone had this problem ? The 300b &/or 6SHB are definitely source of microphony but have swapped these with other mono (where they work fine) they are not the cause. Only thoughts are it could be earthing related - does anyone have any experience of this or ideas as to cause ? By the way the IE monos are really special !!!

• RE: Monoblock Microphony - richardcooper2k 13:31:51 01/15/08 (3)
  In Reply to: RE: Monoblock Microphony posted by ClarKit on January 12, 2008 at 11:41:22

  can you describe the symptom ?
  i can't understand how a stand off could be microphonic as i can't imagine it vibrating.
  a short circuit or a bad cap or a dry joint would be the possibilities that spring to my mind
  
  

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  • RE: Monoblock Microphony - ClarKit 13:07:11 01/17/08 (2)
    In Reply to: RE: Monoblock Microphony posted by richardcooper2k on January 15, 2008 at 13:31:51

    No quick way to say all this but tapping LH mono repeats through speaker (sensitive enough for karaoke!!). Had to be 6SH7 &/or 300B picking up vibrations all voltages OK so temporarily mechanically isolated (suspended) front insert panel c/w 6SH7/300B and attached components away from chassis - tapping chassis then had no effect. Clearly valves picking up vibs - so then swapped whole front insert panel c/w 6SH7/300B & attached R's & C's with same from RH mono but problem stayed with LH mono. So valves etc OK. Ended up swapping every component except mains Tx and HT section from the RH to the LH mono & problem stayed with LH mono. So swapped HT section (2 x porcelain stand off's with attached 5 x resistors & 3 x caps)and the problem transferred to the RH mono. Eureka!! - I then swapped all HT R's & C's between RH & LH mono leaving stand offs in place & the problem stayed with the RH mono. Weird!! so to be sure I then swapped the complete HT section of 2 stand off's and attached components between RH & LH mono and the problem transferred back to the LH mono. Must therefore be stand off(s) (??????????) - visual check reveals nothing odd and they measure oc. Have just received 2 x new stand off's in post does not seem possible they will fix problem but watch this space..............
    
    

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    • RE: Monoblock Microphony - richardcooper2k 13:13:20 01/17/08 (1)
      In Reply to: RE: Monoblock Microphony posted by ClarKit on January 17, 2008 at 13:07:11

      very strange !
      
      maybe a disection will reveal the problem with the offending item
      
      

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      • Sorted - but logic fails - ClarKit 12:56:03 02/05/08 (0)
        In Reply to: RE: Monoblock Microphony posted by richardcooper2k on January 17, 2008 at 13:13:20

        replacing both 6SH7's did the trick no more microphony. but this is where it gets weird - i previously swapped all ht ps components (everything on & including white stand offs) as complete units between lh & rh mono which moved the microphony problem from lh to rh mono. simple - had to be a problem with one of the ht components - wrong !!. to find out which component was responsible i then moved each component back separately from rh to lh mono one at a time testing for microphony after each move (painstaking !!). even after all components were moved back to the lh mono the microphony problem stayed with the rh mono. so i ended up with the problem moved but not the components. i was not going to post this - maybe its time to see my shrink ? can only think that the 6SH7's were working on the limit but plate voltage only 186 -190 so should not be a problem.
        
        

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