MFJ MFJ-1270C, Системное руководство

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MFJ-1270C/1274C MANUAL TROUBLESHOOTING
Page 221
If no one seems able to decode your packet transmissions, it is often the case that
your transmitter is being overdriven. The solution is to reduce the drive level via
trimpots R157 and . Note that direct connection to typical microphone inputs
requires R157 or to be turned to near the minimum signal position to produce
sufficiently low signal levels.
If you are having problems hearing other stations, the demodulator circuitry
associated with U20 may be at fault. Check the center frequency of the VCO in
U20 using the calibration procedure. Working in the direction of flow of the
input signal from the radio, verify that it is being passed through to pin 2 of U20,
the input pin. The signal there should be above 50 mV and below 3 V peak-to-
peak for proper operation of the demodulator. It should be relatively clean,
although a few tens of millivolts of noise is normal, and the signal amplitude
should not change by more than about 25% between high and low tones.
Note: Make sure that JMP 8 is ON.
Terminal Interface Troubleshooting
If you can't get the MFJ-TNC to sign on and accept data from your terminal or
computer, the problem may be in the RS-232C interface. The troubleshooting
guide below is provided as an aid to help in resolving problems that may be
related to the RS-232C port.
Symptom: MFJ-TNC won't sign on to the terminal.
If you find the MFJ-TNC won't send data to your terminal, one of the first
things to do is to verify that the RTS line at pin 4 of J1 is not being held low. If
the software flow control option is disabled, the MFJ-TNC will not send data to
the terminal unless its RTS is asserted. If the terminal does not implement the
RTS/CTS protocol, the RTS/CTS lines (pins 4 and 5 on J1) should remain
unconnected.
Verify that the voltages on the MFJ-TNC are correct. If the MFJ-TNC is in
otherwise good condition, check the following pins on the SIO, U21 (Z8440).
Pin 23 should be TTL low (between 0 and +0.8 volts). If this voltage is
incorrect, check the voltage at U9 pin 3 and verify that it is greater than +3 volts.
If this voltage is correct, U9 or the traces around it may be bad. If this is not the