[Hallicrafters] Piesoelectric effect case in point.

[Roger (K8RI)]

On 1/1/2011 11:45 PM, Peter Bertini wrote:
> Hi Walt
>
> That is interesting. I personally was never fond of ceramic disks in audio
> circuits, especially for coupling. But I never expected that the effect
> would be that pronounced. Thanks for the verification.
>
I wonder if this is truly a "piesoelectric effect" of the caps having 
developed a mechanical defect that allows the capacitance to change 
value with vibration, or mechanical shock, thus acting like a capacitor 
microphone.  The results would be the same the mechanics are quite 
different.

73

Roger (K8RI)
> Pete
>
> On Sat, Jan 1, 2011 at 10:09 PM, Walt Cates<cateswa at msn.com>  wrote:
>
>> As luck would have it, today I have a SR-150 in the lab that suffers from
>> "microphonics". It was easy enough to localize the area by lightly tapping
>> the chassis with a plastic tuning wand. I turned the rig over and started
>> tapping components. Sure enough it was C64 a .01uf ceramic disk. This
>> capacitor couples the audio output from the product detector to the first
>> audio amp. When replaced with a modern poly cap the microphonic condition
>> was eliminated.
>>
>> What I have noted over the years is that ceramic disk capacitors used as
>> coupling capacitors seem to be more vulnerable to this condition. This is,
>> as the Kemet report states because of the high difference of potential
>> across the dielectric and termination into a high impedance. In this case
>> with the SR-150 one side is 250v the other side is zero volts. However the
>> low side sees a relatively high impedance of 500k. When there is receive
>> signal present with a strong audio the pieso effect ( that is its
>> susceptibility to vibration ) is dampened. When you remove the signal and
>> there is no audio present the pieso effect into the high Z of the AF GAIN
>> pot (500k) supports the effect and the microphonics gets much more
>> pronounced.
>>
>> Best Regards, Walt Cates, WD0GOF, http://www.myhamshack.com/WD0GOF/<
>> http://www.myhamshack.com/WD0GOF/>
>>
>> I pity the self made man, he is definitely working at a disadvantage. I
>> cherish the help from God, family and mentors.
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