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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.")