[Hallicrafters] Piesoelectric effect case in point.

[Carl]

Walt, That is a bit of a stretch to attempt to prove a point you only tried 
to make yesterday.

I would suspect that putting that cap on a TC-6A or other high end cap 
tester at rated voltage you will discover the actual reason for the failure. 
My bet would be on a internal fracture which are far from an unknown failure 
mode.

Nobody is questioning the piezo effect in ceramic caps, it is how and when 
they manifest themselves that is surrounded by hype that ripples down and 
starts a great panic among the unwashed hordes.

If you really want to do a service then take that radio, hook it to a scope 
and HP Distortion Analyzer and run tests with various construction new 
production capacitors. Run it at audio levels that are within the range of 
the radio.

Carl
KM1H




----- Original Message ----- 
From: "Walt Cates" <cateswa at msn.com>
To: "hallicrafters" <hallicrafters at mailman.qth.net>
Sent: Saturday, January 01, 2011 10:09 PM
Subject: [Hallicrafters] Piesoelectric effect case in point.


> 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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