    
    
                 BLACK BOX MODS FROM JACK PLUG   

  THE BLACK BOX CHARGE INHIBIT CIRCUIT CURRENTLY CIRCULATING
 AGAIN (6.1K RESISTOR, LED, CAPACITOR, DIODE,  ETC) SEEMS NOT
  TO WORK ON SEVERAL SYSTEMS. A USEFUL MOD IS TO REPLACE THE
 EXISTING RESISTOR WITH A VARIABLE TYPE. CHOOSE EITHER A 0 TO
    10K OR A 0 TO 1K VALUE TYPE AS MANY INSTRUMENTS CAN BE
 "BALANCED" WITH QUITE LOW ADDITIONAL RESISTANCES. TRIMMERS OR
      PRESETS ARE THE CHEAPEST, BUT FOR EASY AND ACCURATE
READJUSTMENT GO FOR THE MULTI-TURN CERMIT TYPE OR SIMILAR.    
                                 
 TO SET-UP, INSTALL THE BLACK BOX AND SET THE RESISTOR TO ITS
  LOWEST VALUE POSITION (FULLY ANTI-CLOCKWISE). WITH THE BOX
   OFF, LIFT THE HANDSET AND CHECK FOR A DIALLING TONE. NOW
 SWITCH THE BOX "ON" AND CHECK THAT THE TONE REMAINS. THE NEXT
  STEP IS TO GRADUALLY INCREASE THE RESISTANCE, CHECK FOR THE
DIAL TONE, INCREASE THE RESISTANCE, CHECK FOR THE TONE.... AND
   SO ON UNTIL A POINT IS REACHED WHERE THE DIAL TONE EITHER
 FLUTTERS OR DISAPPEARS COMPLETELY. AT THIS POINT DECREASE THE
               RESISTANCE S L I G H T L Y.     
                                 
   WITH A VOLMETER CONNECTED IN PARALLEL ACROSS THE LINE YOU
   SHOULD NOTICE THE USUAL VOLTAGE DROP WHEN THE HANDSET IS
 LIFTED AND THE BLACK BOX OFF. wITH THE BLACK BOX ON, HOWEVER,
  THE OFF-HOOK VOLTAGE DROP SHOULD BE CONSIDERABLY LESS.    
                                 
 IN USE, THE BOX SHOULD BE SWITCHED ON WHEN SOMEONE RINGS YOU
    OR WHEN YOU HEAR THE RINGING TONE OF THE PERSON YOU ARE
 CALLING. IF THERE IS ANY JUSTICE, WHEN THE BOX IS SWITCHED ON
  THE EXCHANGE METER WILL "SEE" THE VOLTAGE SWING ASSOCIATED
   WITH YOU REPLACING THE HANDSET AND STOP COUNTING. ENOUGH
  CURRENT REMAINS, HOWEVER, TO FIRE-UP THE MIC AND FACILITATE
CONTINUANCE OF THE CONVERSATION. SOME ODD THINGS HAVE HAPPENED
 WHILST WATCHING OTHERS USE THIS DEVICE (!). AT ONE POINT, THE
      BOX SWITCH WAS CLICKED OFF AND ON QUICKLY DURING A
 CONVERSATION AND A DIGITISED VOICE ANNOUNCED THAT "RECALL IS
NOT AVAILABLE, YOU WILL BE RECONNECTED TO THE CALLER," AND THE
  CREDIT DISPLAY IN THE PHONE BOX THAT HAD INITIATED THE CALL
              PROMPTLY STOPPED CLOCKING DOWN.    
                                 
  I HOPE THIS IS OF USE TO SOMEONE. REGARDS: JACK PLUG.     
                                 
 (BY THE WAY, I HAVE FOR SALE OR SWAP A "MOULD" SELCALL UNIT.
 FULLY OPERATIONAL WITH INTACT EPROM/CALLSIGN. WANT VERY CHEAP
   TNC OR RTTY INTERFACE. DESPERATELY NEED CIRCUITS/INFO ON
                        "TEMPEST.")    
 