APPENDIX D MODIFICATIONS TO TNC-2 DATA CARRIER DETECTOR (DCD) CIRCUIT These modifications are to allow TNC-2 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 any TNC-2 or clone (including MFJ-1274) which has a 2211 demodulator to the level of DCD performance of the 1278 modem. Part numbers referred to are for the original TNC-2 as produced (briefly) by TAPR. I have not reviewed schematics of clones produced by all manufacturers so I cannot be sure that these numbers will be correct in all cases. Since I have seen information on the MFJ series, I can say that the numbers are ok for a 1270 and 1274. If you are in doubt about part number correspondence, obtain a schematic of the original TNC-2 as produced by TAPR. Comparison between the TAPR schematic and the schematic of your TNC should resolve any differences. The TNC I used to verify this modification was a 1274 which I use primarily on HF packet. Since the physical layout of the modem area of the 1274 is different that the layout of the original TNC-2 or "pure" unaltered clones, I have avoided giving specific physical location information in the modification procedure. Any manufacturer of the TNC-2 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. NOTE! Do NOT use TNC-2 firmware earlier than version 1.6 with this modification. Firmware prior to V1.6 has no facility for detection of a DCD fault condition and therefore cannot warn you when the threshold control has been improperly set. STEP BY STEP MODIFICATION 1. If you have a TNC-2 or clone (except MFJ-1274 and 1270B), and have not already removed the MF-10 filter and associated header parts, do so at this time. The reason for this is that the operation of this filter circuit is marginal in the TNC-2. This has the effect of drastically reducing the modem dynamic range. Removing this filter is a very easy modification. Simply remove both the MF-10 and the header associated with the MF-10. Then under the board solder a jumper between pins 1 and 8 of the header socket. Removing the MF-10 also unloads the -5 volt supply, improving its regulation and reducing the noise generated by the charge pump circuit. There is ABSOLUTELY NO PERFORMANCE PENALTY for removing this filter. 2. Replace C35 and C42 with 0.01 microfarad caps. This step is not necessary or possible in a MFJ-1270B or 1274. 3. Remove the 470 K resistor at R73. Be careful not to damage the circuit board pads or traces as they will be needed later in the modification. 4. Remove CR13. 5. Replace R70 with a 47 K resistor. 6. Lift the cathode end of CR15 from the circuit board. Install a 47 K resistor in series with CR15. Solder one end of this resistor to the hole vacated by CR15's cathode end. Solder the other end of this resistor to the cathode end of CR15 above the circuit board. 7. Replace R74 with a 4.7 K resistor. 8. 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 (U20). 9. Replace R38 with a 100 K resistor. 10. Above the circuit board, using leads as short as possible, install a 470 microfarad, 10 volt rated electrolytic capacitor between the -5 volt pad on the tuning indicator connector (J3, pin 1) and ground. Connect the positive lead to ground. Remember that this cap will have to clear the cabinet so position it as near the board as possible. 11. Under the circuit board, solder a 2.2 microfarad, 16 volt rated electrolytic capacitor from the junction of R38 and R42 to the -5 volt supply. The negative end of this cap goes to the -5 volt supply. 12. Under the circuit board, solder one end of a 22 ohm resistor to the junction of R38 and R42. The other end goes to pin 5 of the 2211 (U20). 13. On the front panel, Mount a miniature 100 K, linear taper potentiometer (yes, there really IS room for this). This will be the DCD threshold control. It will be used to set the DCD trigger point to the proper value. 14. Solder a wire from the wiper of the 100 K pot to the pad vacated by R73 which connects to C45 and pin 3 of the 2211 (U20). 15. Solder one end of a 27 K resistor into the pad vacated by R73 which connects to the -5 volt supply. Solder a wire from the other end of this resistor to one of the 2 remaining leads from the 100 K pot. Use the lead that is set to zero resistance when the shaft of the potentiometer is turned fully CCW. 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-2 modem modification. 73 Eric Gustafson, N7CL EOF...