More Box PlansCorrection- The 39K resistor used for setting the gain of the amplifier in last month's issue is incorrect. It should be betwoen 5 and 6 thousand ohms'in value. Too large a value will cause a buzz in the amplifier from too much gain.
Let's clear up a few details in last month's Blue Box construction plans. For those of you without a lot of test equipment, this might help you a bit.
1- The Twin T oscillator has maximum stability for voltage and temperature changes when it is tuned at least 5% above the initial oscillating frequency of the oscillator. Otherwise, a voltage drop of a volt or temperature drop of 20 degrees might turn it off. For example, tune in your resistor or capacitor values of the 1100 cycle oscillator so that it starts oscillating at about 1000 or 1050 cycles, and tune it up to 1100. The starting point of oscillation is always the purest sine wave, but the oscillator is on the verge of turning off, so a slight distortion of the wave can and must be tolerated in this circuit.
2- Tuning up to a touch tone phone(frequencies given last month) will result in greater than 1% accuracy. A hammond organ should be even better. The MP receivers that process your calls will respond to an error of 30 cycles or greater with a reorder signal. That's a wide margin, and it varies from place to place. So if your tones come closer than 30 cycles to the correct ones. you will have better success and less of "I'm sorry, your call did not go thru". 3- Repeat-Call only from pay phones, never from your home phone! An exception is to call from a phone that will probably never be boxed on again. The phone company has records of 800 calls and 555-1212 calls that are abnormally long, and they have been known to investigate.
4- If your call doesn't go thru, it may be your tuning, an excess of distortion(tones will sound very harsh),too loud or soft a tone, extra noise leaking into the phone, and most important, an overload in Bell's circuits that night from all the other blue-boxers making calls. Also remember that if you initially dial and beep off a 800 or information number in the area code you will be boxing, do not key in the area code. just the 7 digits(7D).
5- The diodes are non-critical, small signal units and don't have to be 1N914. The transistors are small signal NPN type with HFE of 50 or more. The transistors mentioned, and many others also will all work perfectly.
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