[Packet] DCD on MFJ-1270B

[Dr. Gerald N. Johnson, electrical engineer]

With the input sensitive down to millivolts or less, ground loops are a
problem. The original DCD circuit only shows the PLL detector is locked,
not to what or that its really data. Some versions of the firmware do
look at DCD and won't transmit with it asserted. Some say that the power
supply for the TNC should be separate from that of the radio to cure the
ground loop. I prefer that the ground loop not be a problem, (and I've
not had that problem here with several 1270B, that I run sometimes
directly from the discriminator voltage, though I have used speaker
level too) because I like to have the big station supply running the TNC
to tide over short power glitches better than the wall wart. On our bbs
there is a definite improvement in length of time between TNC hangs on
the big supply compared to the wall wart supplying the TNC.

Removing the MF-10 should reduce the sensitivity to ground loop. The
input connection of the TNC has a capacitor so DC from the speaker line
isn't a problem. The gain of the MF-10 when present is a problem. Makes
the range of voltages acceptable very small.

73, Jerry, K0CQ

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