[Hallicrafters] microphonics

[Carl]

----- Original Message ----- 
From: "Walt Cates" <cateswa at msn.com>
To: "hallicrafters" <Hallicrafters at mailman.qth.net>
Sent: Sunday, November 20, 2011 10:02 PM
Subject: Re: [Hallicrafters] microphonics


> Chris,
>
> Microphonics is a tough failure to breach, and it is a failure. There are 
> many causes. Miss-alignment can cause it. crystalized carbon resistors can 
> cause it. The piezo effect in disk capacitors and other components can 
> cause it. Bad grounds can cause it. Cold solder joints can cause it. Tubes 
> can be microphonic. dirty and corroded tube sockets and connectors can 
> cause it.

** Ive seen missalignment cause oscillation but not microphonics. Sometimes 
too much gain overloads the audio stages.

Neither have I experienced microphonics in over 50 years from resistors or 
discaps. Noisy resistors of course as well as leakage in some discs. The so 
called disc cap piezo effect is mostly audiophoolery especially at the 
signal levels involved here.
Brands and models using audio stage Couplates can be both microphonic and 
noisy, sometimes both at the same time. I always replace them with discrete 
components.

>
> Gently tapping components can sometimes localize the problem. If all else 
> fails, disable any AVC or AGC then start at the audio output and back up. 
> That is, disable the detector stage and see is the microphonics goes away. 
> If not disable the last I.F. stage and test for microphonics. Keep backing 
> up until you find the offending stage.


** This is where a scope or at least a good VTVM can be a big help.


> In the case of microphonics on only 1 or 2 bands of a multi band receiver, 
> then determine what circuits are common only to those bands.

** Silver micas can create a pseudo microphonic as well as what sounds like 
static crashes. The latter is prevalent in IF transformers using deposited 
silver substrates on mica.


>
> OPINION: To my limited knowledge, none of the Hallicrafters gear made it 
> into production with an inherent microphonic condition. So if it is 
> microphonic then there is a failure.

** Agree

Carl
KM1H

>
> Best Regards, Walt Cates, WD0GOF,