254 lines
15 KiB
Plaintext
254 lines
15 KiB
Plaintext
Bill Cheek
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COMMTRONICS ENGINEERING - Communications & Information
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PO BOX 262478, San Diego, CA 92196
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November 23, 1990; 4:54 pm
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===========================================================================
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NEW DATA & TONE SQUELCH CIRCUIT FOR THE PRO-2004, PRO-2005 & PRO-2006
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===========================================================================
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Here's an easy modification to make PRO-2004/5/6 scanners recognize
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worthless DATA and/or continuous TONE signals and to resume SCANning or
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SEARCHing within a second after locking up on these types of signals. It
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works similar to the SOUND SQUELCH which responds to silent or unmodulated
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carriers. In fact, the DATA SQUELCH works with the SOUND SQUELCH, but is
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independent of it except for the SOUND SQUELCH button on the front panel
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which activates or deactivates both functions. Construction and
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installation are simple and within the ability of most hobbyists.
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The DATA SQUELCH is ideal for use when SCANning or SEARCHing trunked
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channels! No longer do trunked data channels have to be locked out!
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(They change every day, anyway.) The scanner skips over those obnoxious
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signals! It will also discriminate against cellular data and most FBI-type
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continuous tones. My DATA SQUELCH will likewise discriminate against
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continuous tones used on the Improved Mobile Telephone Service (IMTS) and
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other non-voice signals including digital pagers. In other words, the DATA
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SQUELCH accepts voice signals and rejects most others.
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STRONG ADVICE: You should have the Service Manual for your scanner before
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doing this modification. Order it from any Radio Shack store or directly
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from Tandy National Parts Center in Ft. Worth, Tx; (800) 442-2425.
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CONSTRUCTION OF THE DATA SQUELCH CIRCUIT BOARD
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================================================
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Cut a piece of "perf board" about 1" x 1" though smaller is ok if you are
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good at micro circuits. Refer to the Parts List and, if available, the
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Schematic Diagram:
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DIRECTIONS FOR PRO-2004/5/6: Directly to Pin 5 of U-1, solder the (+) leg
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of C-2 and one leg of each of R-1 and R-2. Ground the free ends of C-2 and
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R-2. To the free end of R-1, solder the cathode of one of the diodes, D-2.
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To the anode of D-2, solder the cathode of D-1. Ground the anode of D-1.
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NOTE: the junction of D-1 cathode and D-2 anode will be the INPUT of this
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circuit.
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To Pin 2 of U-1, solder one end of R-3 and the anode of D-3. Solder the
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free end of R-3 to Pin 3 of U-1. Solder a hookup wire several inches long
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to the cathode of D-3 & let hang free. Solder a several inch long hookup
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wire to Pin 3 of U-1 & let hang free. Ground Pin 12 of U-1. Solder one
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end terminal of VR-1 to Pin 3 of U-1; solder the other end terminal of VR-1
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to ground. Solder the middle lug of VR-1 to Pin 4 of U-1. Pins
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1,6,7,8,9,10,11,13 & 14 of U-1 are not used.
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PRO-2004 ONLY: Solder the (+) leg of C-1 directly to IC-5, Pin 14.
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PRO-2005/6 ONLY: Solder the (+) leg of C-1 directly to IC-5, Pin 7.
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PRO-2004/5/6 ALL: Solder a hookup wire to the (-) leg of C-1. Solder the
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other end of this hookup wire to the INPUT of the above circuit at the
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junction of D-1 and D-2. Solder the ground trace of the new circuit board
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to a ground in the scanner. Solder the free end of the hookup wire at Pin
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3 of U-1 to the OUTPUT leg of IC-8, the +5v supply regulator on the main
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chassis of the scanner. IC-8 is the same in all three scanners,
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PRO-2004/5/6.
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PRO-2004 ONLY: Locate CN-504 on the Logic/CPU Board, PC-3, and follow its
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wire bundle back to the main receiver board. Locate the sky blue (light
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blue) wire that connects to the chassis at the right end of the row of
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wires and remove that wire from the chassis. (This wire comes from Pin 15
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of CN-504.) Solder the anode of D-4 to the spot where the blue wire was
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removed. Solder the now loose blue wire to the cathode of D-4. Solder the
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free end of the hookup wire at the cathode of D-3 to the cathode of D-4.
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PRO-2005/6 ONLY: Locate CN-3 on the main receiver Board and follow its wire
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bundle up to the Logic/CPU board. Locate the sky blue (light blue) wire
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that connects to Pin 4 of CN-3. Clip that blue wire halfway between CN-3
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and the Logic/CPU Board. Solder the anode of D-4 to the loose end of the
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blue wire that goes down to CN-3. Solder the cathode of D-4 to the loose
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end of the blue wire that goes up to the Logic/CPU Board. Solder the free
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end of the hookup wire from D-3 to the cathode of D-4.
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PRO-2004/5/6 ALL: ADJUSTMENT OF VR-1: Push the SOUND SQUELCH button ON and
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attach a voltmeter (-) to ground and (+) to Pin 5 of U-1. Tune the scanner
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to a strong, noisy data channel or to a loud, single tone carrier. Measure
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the DC voltage at Pin 5 of U-1, (2.5v to 4.5v, typically). Calculate 80%
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of that measurement, and adjust VR-1 for the 80% level of the above
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measurement. Typically, about 2 to 3.8v. The exact adjustment isn't too
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critical, but if set too low, then voice signals will trigger the
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SCAN/SEARCH RESUME. If set too high, then data & tone signals won't
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trigger the SCAN/SEARCH RESUME. Another way to find the optimum setting is
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to put a voltmeter (+) on Pin 2 of U-1 and (-) to ground and tune the
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scanner to a cellular or trunked data channel. Adjust VR-1, first one way
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and then the other and then to a point so that the voltage on Pin 2 of U-1
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just becomes stable with a nice and steady +5 volts. It takes a steady
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5-volts for about one second to trigger the SCAN/SEARCH RESUME function,
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but don't adjust VR-1 any further than necessary to stabilize the DATA/TONE
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voltage at Pin 2.
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OPERATION & NOTES: The description for the above circuit does not discuss
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the DPDT switch shown in the schematic diagram, and which can be wired as
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shown to select SOUND SQUELCH only or both SOUND and DATA SQUELCH,
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combined. More sophisticated switching schemes can be devised to select
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one or the other or both. As it is, the above described basic circuit runs
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BOTH SOUND & DATA SQUELCH at the same time. That is, your scanner will
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resume SCANning or SEARCHing almost immediately after it locks up on either
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a silent signal or a data/continuous tone signal! Voice signals will cause
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the scanner to stay locked as normal until the signal goes away. Minor
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adjustment of VR-1 may be necessary for optimum results, but the final
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setting will produce a voltage on Pin 4 of U-1 of about 80% of the peak
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voltage on Pin 5 of U-1. The DC input signal at Pin 5 of U-1 will be
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nearly zero on silent or quiet signals and about 2.8 to 4.5v with data &
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continuous tone signals. Pin 5 will show a very erratic and rapidly
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changing voltage from nearly zero to 4 volts or so for voice signals. The
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DC output voltage at Pin 2 of U-1 will be nearly zero on silent or quiet
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signals; and a steady +5v with data & continuous tone signals. Voice
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signals will cause a rapid fluctuation of the signal between 0-5 volts at
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Pin 2 of U-1. When the SOUND SQUELCH button is off, neither SOUND nor DATA
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SQUELCH are operable and scanner operation will be completely normal.
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IN CASE OF DIFFICULTY: The most critical part of this mod is the rectifier
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circuit consisting of D-1, D-2, R-1, R-2, C-1 and C-2. Make sure the diode
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polarities are correct (banded end is the cathode). Make sure polarity of
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the capacitors is correct. Tune the scanner to a strong cellular (879-881
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MHz) or trunked data channel (851-866 MHz), and measure the DC voltage at
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Pin 5 of U-1. There should be between 2.5 and 4.5 volts. You won't
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measure "too much" but not enough is possible. If so, check the wiring and
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components mentioned just above. Next most critical is the polarity and
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wiring of the two isolation diodes, D-3 and D-4. Last but not least is the
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wiring of U-1. The circuit is so simple and affirmative in its action that
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you're not likely to encounter trouble if you follow these instructions.
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On one PRO-2005, I noticed a chirping, morse code type of sound on quiet
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channels. If yours exhibits this, change capacitor C-1 from 1.0-uF to
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0.1-uF, #272-1432. If the "tweet" is still there, then solder a 220-uF
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(or larger) capacitor directly to Pins 4 and 11 of IC-5 in the scanner.
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Pin 4 should get the (+) lug of the capacitor while Pin 11 will be (-).
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Radio Shack part number for the capacitor is #272-1029.
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If you can't resolve a problem, send me a SASE and one loose extra stamp
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with a complete description of the problem and its symptoms and I'll
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respond with written suggestions and advice. Sorry, no phone calls,
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please.
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THEORY OF OPERATION OF THE DATA/TONE SQUELCH
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=============================================
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To understand the simple operation of my DATA SQUELCH, it is first
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necessary to understand the PRO-2004/5/6's SOUND SQUELCH (SSQ) circuit on
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which we will "piggy back" the new DATA SQUELCH circuit. The circuits are
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identical among the PRO-2004/5/6 scanners but circuit symbols differ. Bear
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with me here while I use a simple scheme for this discussion. P4 means
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PRO-2004; P5/6 means PRO-2005 and PRO-2006 and P4/5/6 means "all".
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SOUND SQUELCH THEORY OF OPERATION: A weak portion of the receiver's audio
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is sampled at the detector and amplified through IC-5 (P4/5/6). The highly
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amplified audio is fed from IC5, (P4, Pin 14 or P5/6, Pin 7) to a rectifier
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network (P4, D-41 & D-42; P5/6, D-43 & D-44). This rectifier network
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converts the audio signal to a DC signal that is proportional in level to
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the level of the audio signal, and it is used simply as a bias to turn on
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or off a transistor, (P4, Q-21; P5/6, Q-19). Most audio signals are strong
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enough to turn the transistor on while very weak or silent signals keep it
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off. When the transistor is off, 5-volts is on its collector, but when the
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transistor is ON, the collector drops to nearly zero volts. 5 volts and 0
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volts forms the logic required by the CPU for making decisions. The
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collector of the transistor is fed directly to the CPU, (P4, IC-503, Pin
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24; P5/6, IC-501, Pin 18). When the SOUND SQUELCH button on the front
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panel is set to the ON position and when CPU's SSQ pin is at zero volts,
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the scanner scans or searches as normal, locking on any signals which break
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the squelch. Similarly, when the SOUND SQUELCH button is off, a ground is
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placed on the CPU's SSQ pin, which keeps it at zero volts, no matter what.
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When the SOUND SQUELCH button is on, and when the scanner encounters a
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silent or unmodulated carrier, then the transistor discussed above gets
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turned off and a 5-volt level on its collector is fed to the CPU's SSQ pin.
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A 5-volt signal on the CPU's SSQ pin makes the scanner resume scanning
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within a second after locking onto a carrier. Therefore, as long as there
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are voices or other audio signals present, the CPU's SSQ pin will be "0-v
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low" and operation is normal. When that pin goes "+5v high", the CPU is
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programmed to resume scanning or searching.
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DATA SQUELCH THEORY OF OPERATION: Since the CPU's SSQ pin responds only to
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low and high logic and really doesn't know the difference between voice and
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data, we can use this function with a separate circuit to make it
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discriminate against continuous tones and data in the same way it
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discriminates against silent carriers. All we need is a circuit that sends
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a "high" to the CPU's SSQ pin in the presence of strong, sustained audio
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signals such as data or continuous tones. My circuit does this handily
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since voice signals are erratic, varying, and not at all like data or
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continuous tones. C-1 of our circuit samples the amplified audio and
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passes it to a new rectifier circuit, D-1 and D-2, which with R-1, R-2 and
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C-2, becomes a DC signal proportional to the level of the audio signal.
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This DC signal is fed to Pin 5 of U-1, a Voltage Comparator IC. A
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reference voltage is adjusted by VR-1 and fed to Pin 4 of U-1. As long as
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the DC signal at Pin 5 is less than the reference signal at Pin 4, the
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output of U-1 at Pin 2 will be zero volts "low". When the DC signal at Pin
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5 exceeds the reference voltage at Pin 4, then the output of U-1 goes to
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+5v "high" at Pin 2.
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The output of U-1, Pin 2 is coupled to the CPU via isolation diode, D-3. A
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"high" will tell the CPU to make the scanner resume SCANning or SEARCHing
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while a "low" does nothing unusual. When VR-1 is correctly adjusted, the
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output of U-1, Pin 2 will never go "high" long enough to trigger the CPU
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unless data or continuous tones are present. Voice signals may make U-1's
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output go high momentarily, but the interval will not be long enough to
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trigger the CPU, because a duration of about .5 to 1 second is required
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before the CPU will trigger. Therefore, most voice signals of interest
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will not send a "high" to the CPU, but continuous tones and data will!
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Therefore, our DATA SQUELCH works exactly like, though inversely to, the
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SOUND SQUELCH.
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Isolation diodes, D-3 and D-4, allow the SOUND SQUELCH and the DATA SQUELCH
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to work simultaneously and not interfere with each other. Both silent and
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data/tone signals will cause the scanner to resume SCANning or SEARCHing,
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but voice signals will not be affected by the SOUND or DATA SQUELCH!
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WRAPUP: Sometimes, natural pauses of a second or more in speech signals
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will be interpreted by the SOUND SQUELCH as a silent signal. SCAN or
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SEARCH may resume before speech begins after the pause. It is, therefore,
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ideal to route the SOUND SQUELCH and DATA SQUELCH functions through an
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external selector switch to permit selection of one, the other or both
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functions for special monitoring situations. Two toggle switches or a
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cumbersome rotary switch are required to do this. Next to ideal is a
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simple DPDT toggle switch, which when wired as shown in the schematic
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diagram, will select both DATA and SOUND SQUELCH simultaneously, or DATA
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SQUELCH only.
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PARTS LIST FOR DATA/TONE SQUELCH CIRCUIT
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===========================================
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Circuit
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Symbol Quan Description with Radio Shack part numbers
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======= ==== ==========================================
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C-1 1 1-uF/35vdc #272-1434 (See Text)
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C-2 1 2.2-uF/35vdc #272-1435
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D-1,2,3,4 4 1N4148 Switching diodes; #276-1122
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J-1 1 IC Socket, 14-pin DIP, for U-1 below; #276-1999
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R-1 1 390-ohm; #271-018
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R-2 1 12,000-ohm; non-Radio Shack, but a 10-k and a 2.2k can be
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wired in series to make the needed 12-k resistor; use one
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each of RS #271-1335 and 271-1325 if need be.
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R-3 1 3,300-ohm; #271-1328
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S-1 1 DPDT toggle switch, for optional control; #275-626
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U-1 1 LM-339 Comparator; #276-1712
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VR-1 1 10,000-ohm trim pot; #271-282
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Misc Perf board; #276-1395
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Misc Hookup wire; #278-776-Salvage the inner wires for hookups
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SCHEMATIC DIAGRAM FOR DATA/TONE SQUELCH CIRCUIT
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===============================================
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Since you copied this file from a BBS, there is no way a schematic diagram
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can be reliably conveyed. You may not need one, as detailed as this is,
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but if you do, send a #10 Self Addressed Stamped Envelope and four (4)
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loose, extra first class postage stamps for the schematic and a printed
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copy of this manuscript. This offer expires January 31, 1991, after which
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a nominal charge will apply.
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===========================================================================
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COMMTRONICS ENGINEERING PO BOX 262478 SAN DIEG0, CA 92196
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===========================================================================
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