[Hallicrafters] Piesoelectric effect case in point.

[Peter Bertini]

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.

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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