From: @noway.com   
      
   Dave Platt wrote:   
   > In article , Jack  <@noway.com> wrote:   
   >   
   >> I tried probing DC voltages across speaker output and ground at each   
   >> channel with a digital meter with the speakers connected and unconnected   
   >> and the readings were essentially the same, under 10 mv, left and right,   
   >> speakers connected and unconnected. I suspect that this is not an   
   >> effective test.   
   >   
   > Well, it tells you that you don't have a serious DC fault in the   
   > output stage, which is good news.  Some sorts of failures can send the   
   > full DC rail voltage (or near to it) to the output, and this can very   
   > rapidly damage the woofers (high current flow, voice coil heats up,   
   > magic smoke leaks out, bad smells occur, fire or burnt-out coil   
   > results).   
   >   
   > You're almost certainly going to need some way of actually looking at   
   > the audio signal.  You *might* be able to use an AC voltmeter, but an   
   > oscilloscope will make the state of the signal a lot easier to   
   > visualize.  In either case, some sort of audio input signal will be   
   > needed... FM tuner tuned to a station, or tuned in between stations   
   > with the muting turned off, or a signal from a CD player, or etc.   
   >   
   > Hint: if you have a CD burner, and a copy of Audacity (or a similar   
   > audio-editing package) you can make your own "signal generator" for   
   > cheap.  Use Audacity to create a stereo 44.1 ksample/second 16-bit   
   > audio file, containing a test tone (e.g. 1000 Hz).  Store this on disk   
   > and burn it as a one-track audio CD.  Put it in your CD player, set   
   > the player on "repeat", hit Play, and you've got a nice clean   
   > consistent audio waveform that you can feed to the receiver and then   
   > trace through the circuitry.   
      
   I downloaded a 30 second 1000HZ sine wave in 44100/16 format and I have   
   been playing that through the computer into the tape monitor playback of   
   the receiver.   
   >   
   > If you use a lower frequency (e.g. 100 Hz) it'll be within the   
   > specification range of almost any simple AC voltmeter, and should read   
   > out properly... you can trace the signal through both channels, and   
   > see where the differences first appear by comparing the voltage   
   > readings.   
      
   I'll download the 100Hz signal now.   
   >   
   > Try to borrow an o'scope, though... you'll see more about what's going   
   > on, than you can deduce from voltmeter readings.   
      
   My main task is to determine where to probe. I decided that the   
   transistors that feed the outputs transistors is where to go next but   
   until I can get hands on an o'scope... and learn to use it, it's been a   
   while, I will have to rely on the volt meter. The smallest scale for AC   
   is 200 volts. I'll see if the 100Hz tone can be read with it.   
      
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