Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!munnari.oz.au!goanna!dnl
From: dnl@goanna.cs.rmit.oz.au (David "IGOR" Latter)
Newsgroups: sci.electronics
Subject: Re: Need help damping audio from near-ultrasonic DC-DC converter
Summary: spraying tranformer to reduce noise
Keywords: DC converter
Message-ID: <3625@goanna.cs.rmit.oz.au>
Date: 27 Aug 90 05:40:08 GMT
References: <10355@life.ai.mit.edu>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 37

In article <10355@life.ai.mit.edu>, mbeck@ai.mit.edu (Mark Becker) writes:
 
 I have a disk drive with a very loud DC-DC converter.  The converter
 is based on a TL-497 switching regulator and generates -12V from +12V.
 
 This circuit uses an inductor in a 'flyback' configuration to generate
 -12V.  The inductor whines at about 13 KHz under load and it's loud.
         .......... 
 I've tried two things to eliminate/reduce the oscillator whine:
 
  * Decreased the oscillator capacitor in the TL-497 circuit to
    increase the frequency past, say, 18 KHz.  The output waveform
    degraded immediately (had a nasty spike in it) and the noise had a
    Bad Feel to it.
 
  * Removed the offending inductor, dunked it in silicone rubber, and
    re-installed it on short wires (mechanical isolation from the
    printed-circuit board).  That reduced the noise some.. but it's
    still very loud.
 
 I would appreciate hearing from others who have solved this kind of

----------------
I service a few tv's and often find that some sets have a rather annoying
15.6 Khz whistle, this is normally due to loose ferite transformer cores,
either the eht transformer or the output stage driver transformer.
When you have more than one transformer (as in tv's) press a plastic rod
on the ferite cores untill you find the offending unit. The solution is
simple, spray the core with some sort of polyurethane, getting as much as
possible down past the windings. This, when dry, locks the ferite core down
stopping the ringing. Some transformers also have a mounting bolt which
can be tightened to stop the noise. 
Due to the relatively high frequency, a very large percentage of the adult
population cannot hear any ringing, the rest are driven nuts about it....
Take note if your kids complain about the loud noise in the tv.....
I would have thought that 13 khz was too low, but dont increase the 
oscillator freq. too high, as efficiency drops right off.