From: @noway.com   
      
   Jack wrote:   
   > Jack wrote:   
   >> This is a great piece with one little problem. Often, when switching the   
   >> unit on, the left channel is out, no sound. Turning the volume up always   
   >> kicks it in. Often the volume has to be turned up severely to accomplish   
   >> this so I tried doing it with the speakers switched off. Switch off   
   >> speakers, turn volume up to max, turn volume down to min, switch   
   >> speakers on, turn volume up a bit and the sound is back in both   
   >> channels. In addition, after getting both channels to work, after 10 to   
   >> 20 minutes, give or take 5, the left channel goes out again.   
   >>   
   >> This behavior lead me to suspect the volume switch, so I flooded it with   
   >> switch cleaner and worked it, but to no avail. Problem persisted. Next I   
   >> checked all the cold solder joints and found one that was loose. I hot   
   >> soldered ALL the cold solder joints. Problem continued. Today I used   
   >> some cables to switch the PRE OUT channels into the POWER IN channels,   
   >> left to right and right to left. The next time the problem cropped up it   
   >> was still in the left channel. This is telling me that the problem is in   
   >> the left main amp section.   
   >>   
   >> Given the behavior described above I can only guess that a rise in   
   >> voltage in the main amp produced by turning up the volume with the   
   >> speakers turned off somehow corrects the condition causing this problem,   
   >> but at the moment I don't know what this condition might be. I assume   
   >> that with the speakers turned off there is not a lot of current flowing   
   >> in the main amp when the volume is turned up, but that the voltage is   
   >> increased. As I write this, a dawning suspicion presents itself and that   
   >> is: the problem might be with a transistor, as they are voltage regulated.   
   >>   
   >> Next step is to open up the unit and do a visual inspection, hook it up   
   >> to a pair of speakers and turn it on and attempt to compare voltages   
   >> along the signal path of right and left channels of the main amp when   
   >> the problem crops up.   
   >   
   > 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.   
      
   I downloaded a 1000HZ wave file and this is proving useful for probing   
   voltages. What I have done so far is to measure the voltage across the   
   output legs of the paired output transistors. Balance is set to top dead   
   center, equal balance. With no signal present the right channel, the   
   good one, measures 16.3 mv. The left channel, the bad one, measures   
   16.6mv. With the 1000HZ wave file input into the tape monitor, the good   
   right channel measures 61.8 mv. The bad left channel measures 16.6 mv.   
      
   Measuring across the output terminals of each of the 4 output   
   transistors and ground, I get half the value of measuring across the   
   outputs of the paired right transistors on each of the the paired right   
   transistors and the same for the paired left channel output transistor.   
   So this is consistent.   
      
   My next step is to somewhere between the input and the output of the   
   main amp board and compare voltages between right and left. I'll post   
   the results.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)