*********************************************************** (C) 15 DEC 88 Eric Gustafson 2018 S. Avenida Planeta Tucson, AZ 85710 MODIFICATIONS TO TNC-1 DATA CARRIER DETECTOR (DCD) CIRCUIT These modifications are to allow TNC-1 modem operation with a narrow IF filter on HF. NOTE! For 1200 baud HF work on 10 meters, the normal 2.4 KHz SSB filter constitutes a "narrow" filter for the demodulator. There are three objectives to these modifications: 1. Provide threshold control for the DCD circuit. This allows the operator to adjust the demodulator to compensate for the DCD threshold shift which accompanies bandwidth limiting the audio fed to the demodulator when a narrow filter is used in the radio. This also allows the operator to adjust the DCD threshold for operation at 1200 baud on VHF FM to prevent excessive DCD falsing when using unsquelched audio. 2. Add hysteresis to the Data Carrier Detector. This reduces the DCD's sensitivity to noise. It does this WITHOUT DEGRADING DCD ACQUISITION TIME for a valid data carrier. It also prevents short glitches from appearing at the DCD output on attack and release. 3. Add DCD hang time on release. This is to keep DCD valid through a short multipath hit in the middle of a packet. Without this, a TNC watching the DCD line waiting to transmit will key up before the other station is finished sending its packet. Currently, this happens very frequently on HF packet channels. Again, this benefit is gained WITHOUT DEGRADING DCD ACQUISITION TIME for a valid packet. The MFJ-1278 is currently the only commercially available TNC I know of for HF packet use which has a modem that is truly optimized for the mode in this respect. The modifications given here will upgrade the modem in your TNC-1 to the level of DCD performance of the 1278 modem. Any manufacturers of the TNC-1 can feel free to incorporate this change into their hardware if they wish without incurring any obligation to myself or TAPR. I am available to answer any quetions in this area. STEP BY STEP MODIFICATION 1. Remove the 470 K resistor at R48. Be careful not to damage the circuit board pads or traces as they will be needed later in the modification. 2. Form a parallel network consisting of a 180 K resistor and a 0.01 microfarad cap. Make this network as compact as possible as it will have to fit underneath the circuit board. Solder this network in place under the board. One end goes to pin 3 and the other to pin 6 of the 2211 socket (U18). 3. Under the circuit board, connect a 4.7 K resistor from U18 pin 1 (+12 V) to U18 pin 6. 4. As close to U18 pin 5 as possible, isolate U18 pin 5 from the circuit by cutting traces to this pin. 5. Remove R50 from the circuit board. 6. Mount a 14 pin IC socket in the wire wrap area of the TNC-1 circuit board. This socket will be used to hold a 74HC14 HEX schmitt trigger to be used for the DCD "Hang Time" circuit. This IC will be called UX. 7. Connect UX pin 14 to the +5 volt rail. 8. Connect UX pin 7 to the ground rail. 9. Connect a 10 K resistor from UX pin 1 to the +5 volt rail. 10. Under the board, solder a jumper wire from U18 pin 5 to UX pin 1. 11. Form a parallel network consisting of a 1N4148 or equivalent diode and a 220 K resistor. Connect this network between UX pin 2 and UX pin 3 (cathode end of diode to pin 3). 12. Connect a 2.2 uF electrolytic capacitor from UX pin 3 to the ground rail (negative side of cap to ground). 13. Connect UX pins 11 and 13 to UX pin 1. 14. Connect UX pins 10 and 12 to UX pin 2. 15. Connect UX pin 4 to UX pin 5. 16. Connect UX pin 6 to UX pin 9. 17. Under the board, solder a jumper wire between UX pin 8 and U32 pin 4. 18. In the wire wrap area, mount a 2N3904 or equivalent transistor. This will be called QX. 19. Connect QX emitter to the ground rail. 20. Connect a 10 K resistor from QX base to UX pin 4. 21. Under the board, solder a jumper wire from QX collector to the TNC-1's modem disconnect header pin 18. 22. Solder one end of a 39 K resistor into the R48 pad which is connected to U18 pin 3 (stand resistor body up above the board). 23. Mount a 100 K linear taper potentiometer (pot) on the TNC cabinet front panel. This will be used as the DCD threshold control. 23. Set the pot to its maximum counter clockwise rotation. Measure the resistance to the remaining arms of the pot. Solder a wire between the one which measures zero ohms and the remaining unused pad at R48. Short the remaining arm to the wiper. 26. Check all the solder connections and install the 74HC14 in the socket at UX. When operating a TNC with a DCD threshold control, set the control so that the DCD LED on the front panel flashes occasionally when there is no signal present. The "false DCD" duty cycle should be approximately 20 percent. When operating VHF FM with the radio squelched, the DCD will not false. If you MUST operate with the radio squelched (thus incurring the penalty of the additional delay time of the squelch circuit), set the threshold fully clockwise as described below. The audio bandwidth of some VHF FM radios is so wide that the DCD will not false regardless of the threshold control setting. This will almost always be true when the audio is obtained ahead of the radio's squelch controled stage before deemphasis. For these radios simply turn the control fully clockwise. This sets the DCD to maximum sensitivity. DCD operation will not be impaired. This completes the TNC-1 modem modification. ***********************************************************