HP82011 (etc) 'classic series' charger
                 --------------------------------------
  
              (Unregulated PSU)
        
-----)||
     )||(----+----->|--+-----+------ +16V
     )||(    |         |     |
     )||(    |  +-->|--+    === 400uF
Mains)||(    |  |           ---
     )||(    +--)--|<--+     |
     )||(       |      |     |
     )||(-------+--|<--+-----+---+
-----)||                         |
                                ---  
                                ///
      
         (Constant voltage regulator)

              c        e
 +16V ----+------\    >---------------------- +4.2V Logic Supply
          |       \  /  (NPN power transistor)
          |      -------
          /         | b
          \         |
          / 470R    |
          \         |
          |         |
          +---------+
          |
         ---,
          ^  4.7V zener
          |
         ---
         ///

 
                (Constant current charger)

                              (PNP power transistor)
               13R            e       c 
+16V -----+----\/\/\---+-------->   /------------- 55mA constant current
          |            | b       \ /                  charger supply
          |          -----      -----  
          |      e    / \  c      | b
          +---------->   \--------+
         (PNP signal transistor)  |
                                  /
                                  \ 4k7 
                                  /  
                                  \
                                  | 
                                 ---  
                                 ///