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MFJ-1278B MULTI-MODE TROUBLESHOOTING
Symptom: The MFJ-1278B appears to be signing on but only gibberish is
printed on the terminal
eThis indicates that some combination of the data rate (baud rate), parity option, or number
of start and stop bits are not set the same at the MFJ-1278B and at the terminal. If possible,
set your terminal to 1200 baud. Also verify that the terminal is set for eight data bits, no
parity, and 1 stop bit. These are the default settings stored in EPROM. Perform a hard reset
by the power switch OFF then ON (out then in). The sign on message should appear.
If the MFJ-1278B still prints gibberish, verify that the terminal is set to the MFJ-1278B's
baud rate and do a power off then on cycle on both the MFJ-1278B and terminal. If the
message still fails to appear try troubleshooting with an oscilloscope, looking first at the TXD
pin (pin 26) of U21 (Z8440), then at the x16 baud rate clock (19200 Hz at 1200 baud) on pin
27 of U21.
Symptom: The MFJ-1278B signs on OK but won't accept commands
After the MFJ-1278B signs on, try giving it a command such as MYCALL or any other
command. If the default settings are in effect, it will attempt to echo each character you type
back to the screen. If it does not echo, be sure that U21 pin 23 has a voltage level between 0
and +0.8 volts on it. The voltage on U9 pin 3 should be greater than +3 volts. If these
voltages aren't correct, the fault could be in U9, U21, J1, R20, R22, soldering, or the
interconnecting cable.
If the above checks are OK, use an oscilloscope to verify that data is present on U21 pin 28
and U9 pin 1 when you strike a key on your terminal. If not, the data isn't getting from your
terminal to the MFJ-1278B. Check J1, the cable and U9 again. Finally, be sure that your
terminal actually uses levels less than -3 volts and greater than +3 volts for signal levels. 0
and +5 volts may not work, especially if they are being used direct from a computer.
Symptom: The MFJ-1278B appears to have "lock-up" i.e. not responding to
any commands
This may be due to some invalid parameters having been stored in the memory. Try turning
the MFJ-1278B off and disconnect JMP 5 on the MFJ-1278B board. This will disconnect the
memory back-up circuit and allow the memory to be erased. Re-install JMP 5 after about 2
minutes and try operating again.