[R-390] Microphonic Parts?

[MICHAEL TALLENT]

Yes, ceramic capacitors can be microphonic see this for more info--
http://www.edn.com/contents/images/6430345.pdf

Mike T  W6MXV

----- Original Message ----- 
From: "Barry" <n4buq at knology.net>
To: <r-390 at mailman.qth.net>
Sent: Saturday, April 14, 2012 1:25 PM
Subject: [R-390] Microphonic Parts?


>I know that tubes can be microphonic.  Is it likely for passive components 
>to be microphonic as well?
>
> I have an old Marconi LCR bridge that I'm trying to get working correctly 
> again (it doesn't want to stay NULLed in R mode).  Whilst poking around at 
> the input of the first amplifier (an EF86) with a small plastic stick and 
> looking for bad grounds or solder joints, I noticed that a very light 
> tapping on the components connected to the control grid causes quite a lot 
> of noise at the detector (as observed on the scope).
>
> The components that seem to cause the most noise are a 1M resistor and a 
> 470pF cap.  The cap is connected directly to the grid and the resistor is 
> connected on the other end of the cap to ground.
>
> I've loosened and retightened the ground points at the tube base and 
> gently pushed on the solder joints but nothing seems to produce this noise 
> like the tapping does.  Tapping the tube itself produces a small bit of 
> noise, but not nearly as much as these components.
>
> Is this "normal"?  I tend not to think so, but the control grid of the 
> first amp is a pretty sensitive place to go knocking around on so I'm not 
> sure if this might just be expected behavior.
>
> Not directly related to an R390 but I know you guys are a great source of 
> help for things like this.
>
> Thanks,
> Barry - N4BUQ
>
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