[Packet] DCD on MFJ-1270B

[Dr. Gerald N. Johnson, electrical engineer]

The demodulator input is sensitive to a few millivolts or less. It may
be that your squelched radio puts out more than that to an open circuit.
You might try a speaker terminating 4 to 8 ohm resistor, or even a 10 or
20 dB attenuator between the radio speaker connector and the TNC. There
is a DCD mod kit that requires coherent data to be activated. It may
have some adjustment possible, but shouldn't detect DCD on anything but
a true data signal.

Part of the DCD mod that is worth while even without the DCD circuit
board is removing the MF-10 from the demodulator. It was added to
compensate for a poor frequency response in some radios but causes more
problems by saturating from excess gain than it solves. Removing the
MF-10 requires added a capacitor to couple the input audio past it. Its
been so long since I last did that, that I don't remember the precise
details, but I could probably find them if I dug deep enough in my
documents.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.