More Tx audio
Need more TX audio to drive a commerical radio or inject past the limiter? The following mod will increase the stock output from a MFJ1270B or 1274 TNC from about 200mv with tone twist to about 500mv with no tone twist. Change R56 from 7.5K to 4.7K Jumper out cap C71 Remove cap C73 Install 1mfd caps at C34 and C35 (caps shown on board but missing) The above changes will make the TNC TX audio the same as the older MFJ1270 and TAPR TNC2 units. I am presently using this mod in order to drive Motorola and GE radios in my node stack.
MFJ 1270/1274 Tone Calibration Procedure
Have you ever wondered to yourself whether or not the Packet TNC you are using may or may not be generating correct mark and space tones in reference to the proper frequency? As the owner of 5 MFJ 1270/1274 TNCs, the above question came to mind. I am not equipped to make adjustments on these TNCs with the use of various testing devices, and turned to the MFJ Owners Manual for a possible remedy. They tell you in the Manual how to adjust the tones if you desire HF operation (300 Bps) using their 1270B, or 1274 Model, but when trying to find out how to check for proper calibration on the VHF/220 Mhz/UHF (1200 Baud) -- it's not there. The TNC 2 has a couple of GREAT features built in to its software. CALSET and CAL are in the list of Parms, but not explained in detail that would enable the average packeteer to do his own calibration.
1. 1200 Hz. - MARK TONE (Low tone )
2. 2200 Hz. - SPACE TONE (High tone)
3. 1685 Hz. - CENTER TONE (Demodulator)
On the CENTER FREQUENCY tone, it states in the TNC 2 Manual that1700 Hz is ideal. Continued experimentation has shown that 1685 Hz is best. If you are experiencing alot of retries, and having a difficult time with connects, chances are that your TNC is not generating compatible Mark and Space tones. To calibrate the 1200 Baud tones on your TNC 2 or Clone. Follow these instructions:
Remove the cover of your TNC. Make sure the HF/VHF button is in the "VHF" position!
Get a screw driver small enough to adjust the trim pots R-76 through R-79 located toward the rear center of the TNC circuit board.
Boot up your computer, and enter Command Mode (CMD:) on your TNC.