[Hallicrafters] Piesoelectric effect case in point.

[Walt Cates]

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.