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AREC Modification to Reduce Vibrator Hum on AM Transmissions 

 
 
 
 
 
 
 
 
 
 

Background

ZC1's in their original configuration suffer a characteristic vibrator hum on phone transmissions.  It has been found that most of this hum is caused by use of a shared earth return for the microphone capsule and the transmit relay in the microphone cord.  It is easily cured as follows.

The Modifications

  • Remove the binding from each end of the microphone cord exposing a fourth lead in the cord which has been cut off short at each end.  This wire has to be extended at each end by a suitable length of insulated wire.  The easiest and best way to do this is to bind the bare end of the wire in the microphone cord to the bare end of the extension with a thin piece of tinned wire, eg fuse wire, then sweat the two together.  The join can then be covered with a heat-shrink sleeve.  When both ends have been extended the microphone cord should be rebound at each end.

  • On the plug end the extended lead should be connected to the sleeve connection of the plug, this should already be connected to the green wire being the original combined earth.  The green wire should be left connected.

  • On the microphone end;

    • open up the microphone and remove the capsule and cardboard insulator.

    • remove the link between A and B in the diagram below.

    • connect extended lead to A.

    • Reassemble microphone.  Job completed.

This modification will significantly reduce the vibrator hum if not entirely remove it.

     

top                                                                                         Septemberl 2010