From: liz@poppyrecords.invalid.invalid   
      
   John Robertson  wrote:   
      
   > On 2024/03/14 1:56 p.m., Liz Tuddenham wrote:   
   > > I was asked to repair a Pioneer QX-949 quadraphonic tuner/amplifier.  It   
   > > is a complex beast and one channel had gone down to almost nothing.   
   > > Luckily the service sheet was available and I traced the fault to the   
   > > board which houses the tone controls and filters.   
   > >   
   > > The gain is in two stages with the tone controls around the second   
   > > stage.  On one of the channels, the voltages on the single transistor   
   > > for that stage were all wrong.  After lifting the transistor off the   
   > > board, it became apparent that the base bias was far too high and the   
   > > finger of suspicion pointed to the inter-stage coupling capacitor   
   > > feeding in leakage current from the preceding stage.   
   > >   
   > > I tested the capacitor, which was marked "4.7".  It read exactly 4.7k in   
   > > both directions on an ohm-meter !!!   
   > >   
   >   
   > Can digital ohm-meters be fooled by ESR?   
      
   Possibly, but not to that extent.  The meter was a Philip PM2518, which   
   I believe uses DC for the ohms measurement.   
      
   A new capacitor solved the problem and saved a rather expensive and   
   special piece of kit from being consigned to the scrap heap.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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