From: themend@gmail.com   
      
   Hello all,   
      
      
   Well I've recently finished up the electrical portion of my S-5000   
   restoration.  I've learned quite a few things along the way, and even though   
   I've done several of these before, I made a couple mistakes this time that   
   taught me a few things about how    
   this amp operates.   
      
   First off, let me cover the mistakes I made in posts previous to this.   
      
   Mistake #1   
      
   Changing the chassis capacitor in the way I did it was a bad idea.  I had a   
   fundamental misunderstanding of the way that capacitor was designed to work in   
   this amp, and it wasn't until I noticed increased hum that I realized my   
   blunder.  Simply using a    
   single cap as is so common in radios and other types of amp doesn't pan out   
   well with the S-5000.  The reason is that the original cap is actually two   
   caps connected in series and tied in the center to the chassis.  One end of   
   these caps are on hot, the    
   other neutral.  The result is a cancellation effect.  A single cap simply   
   dirties the chassis currents by connecting it to only one leg.  One option   
   would be to simply remove the cap for safety, but after thinking long and hard   
   about it I began to ponder    
   whether the cap was there to deal with inductive spikes on the power   
   transformer when switched on or off.  Without any way to answer that question,   
   I simply punted and restored the original capacitor back into the circuit.    
   The added hum was gone    
   instantly.  I'll write up that whole AC line redo in a little while...   
      
   Mistake #2  (JRC, I think this might be your answer to phono hum)   
      
   Believing my eyes and the schematic.  The bias supply turns out to be a   
   finicky thing on this amp.  If you read the schematic, either Sherwood OR   
   Sams, you see that there are three capacitor sections in the supply.  In the   
   original amp, these are all in    
   a twistlock can and share a common positive, rather than a common negative   
   like just about every other twistlock can.  If you look at the pictures I   
   posted earlier, you see that there are several things tied to the "ground" on   
   this can, but that is    
   incredibly misleading.  You see, while this cap has a common positive, it is   
   NOT on the can body!!  There is a terminal that has the common positive, and   
   it is NOT connected to the can.  Whoever drew the schematics made the same   
   assumption I did, that    
   the common positive is connected to ground.  It is not.  It is connected only   
   to the center-tap of bias winding, the DC filament return, and the ground bus   
   that for the entire preamp section!!  If you follow the schematic or your eyes   
   when you rebuild    
   the bias section, and you connect the common positive to ground, you will be   
   instantly greeted by a NASTY hum as soon as you connect a magnetic phono   
   cartridge!  JRC, since your bias supply was rebuilt, and kinda sloppily, you   
   may wish to redo that    
   section and be sure that the common positive is not tied to the chassis!     
      
   I got everything dead quiet in this amp until I tried the phono section which   
   was working beautifully before my rebuild.  It now had a horrible hum, and so   
   I knew it must have been something I had done.  I poured over the amp with a   
   fine-tooth comb,    
   uncovering 3 mistakes from the factory in the process including two joints   
   that were never soldered, one was the center-tap of the AC primary at its   
   chassis junction!!  The other was a tone control resistor that was connected   
   but never soldered to the    
   pot.  Obviously, this amp had some pretty green hands making it.  After hours   
   of frustration, I started measuring things.  I compared the bias circuit   
   carefully to my restored version.  After finally getting my meter out and   
   measuring the original cap (   
   still in place but disconnected) I discovered the truth!  The schematics are   
   wrong and that circuit connects to the center tap of the bias secondary ONLY,   
   and is floated from the chassis.  One wire snipped out and voila!  A dead   
   quiet phono pre!   
      
      
   I've got a lot more pictures to take and upload, and I'll probably get to that   
   tomorrow.  But I wanted to go ahead and correct my mistakes for the record   
   now, lest anyone else go down my erroneous path with the bias and AC supplies.   
      
      
   Cheers,   
      
   -forkinthesocket   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)