-september.org> 8f6e2952   
   From: @noway.com   
      
   gregz wrote:   
   > Jack <@noway.com> 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.   
   >> When the left channel drops out I can bring it back by pushing down from   
   >> the top with a chopstick on the circuit board that contains the input   
   >> selector buttons. I can then make it go out again by pushing up from the   
   >> bottom on the same circuit board. After doing this several times, the   
   >> channel comes back to stay... until it goes out again, which could be in   
   >> the next ten minutes or ten hours. Also, pressing the input selector   
   >> buttons at random also brings back the left channel. The puzzling   
   >> behavior comes in when I switch channels at the PRE OUT/POWER IN jacks.   
   >> If the problem is in the push button switches, the dropped channel   
   >> should change from left to right, since the switches are upstream from   
   >> the aforementioned jacks, but this does not happen. The dropped channel   
   >> continues to be the left channel. I just don't understand how this could be.   
   >   
   > I hope you got all switches sprayed by now. Vibration, pushing, heat, cold.   
   > You use all and try to narrow it down.   
   >   
   > Greg   
      
   I have to run out and buy a can of deoxit before I can clean the   
   switches, which I'll do tomorrow, but I can't get past the fact that   
   switching channels at the PRE OUT/POWER IN jacks has no effect on which   
   channel drops out. It is ALWAYS the left channel. If the problem is   
   anywhere in the circuit upstream of the PRE OUT/POWER IN jacks,   
   switching channels at those jacks should cause the problem to switch   
   from the left channel to the right channel, but that doesn't happen. It   
   is ALWAYS the left channel that drops out.   
      
   Also, when the left channel goes out while the FM tuner is selected,   
   flipping a tape monitor switch that has a signal being input to it does   
   not bring back the channel. Flipping either tape monitor switch   
   effectively takes the push button switches out of the picture. They are   
   no longer a part of the signal path. However, the tape monitor switches   
   are always in the signal path, but they are mounted on a different   
   circuit board that remains immobile when I wiggle the board with the   
   push button switches that DOES have an effect on the dropped channel.   
      
   From the obvious reaction of the dropped channel to the wiggling of the   
   board with the push button switches on it, I can't escape the conclusion   
   that there must be something going on there, but it just doesn't make   
   sense, at least to me, that the problem does not switch channels when   
   the channels are switched at the PRE OUT/POWER IN jacks.   
      
   Oh, damn, the light bulb just went on. I was expecting the balance pot   
   to reflect a switch at the PRE OUT/MAIN IN jacks. In other words I   
   expected that, when I switched channels at the PRE OUT/MAIN IN, the   
   balance pot would reflect that change by playing the good channel when   
   it was turned left. When the channels are not switched at the PRE   
   OUT/MAIN IN jacks, the good channel plays when the balance pot is turned   
   right. What was happening was the bad channel was switching from left to   
   right at the SPEAKERS. But being deaf in one ear and oblivious to   
   stereo, I couldn't tell that this was happening. After pouring over the   
   schematics, I realized that switching channels at the PRE OUT/MAIN IN   
   jacks has no bearing on the balance pot.   
      
   Not bad for a carpenter.   
      
   Also, just for the record, the left channel does not always drop out   
   completely, but at times merely drops precipitously in volume, but not   
   stone cold dead. When that happens it responds to the volume pot, but   
   still way way lower than normal.   
      
   All indications point to the circuit board with the push button   
   switches, but when they are taken out of the signal path by switching to   
   TAPE MONITOR, the problem persists. The board with the push button   
   switches and the board with the tape monitor flip switches are   
   physically separated.   
      
   Anyway, I can definitely see that the problem is either in the push   
   button switches and/or the tape monitor flip switches and I will have a   
   go at them tomorrow   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)