MFJ-1270C/1274C MANUAL TROUBLESHOOTING
Page 222
problem, disconnect the terminal and check it again. If this doesn't help, U9,
R20 or R22 may be at fault.
If the above checks are ok, observe pin 26 of U21 with an oscilloscope and cycle
the power switch on the MFJ-TNC . Transitions on this pin shortly after reset
indicate that the MFJ-TNC is sending data. Verify that transitions are also
present on U3 pin 1. If these tests fail, the fault could be with U13, R25, R26,
U21, J1, the attached cable or faulty soldering (shorts, cold joints, etc.)
Symptom: The MFJ-TNC appears to be signing on but only gibber-
ish is printed on the terminal.
This 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-TNC 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-TNC still prints gibberish, verify that the terminal is set to the MFJ-
TNC 's baud rate and do a power off then on cycle on both the MFJ-TNC 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-TNC signs on OK but won't accept commands.
After the MFJ-TNC 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 doesn't 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-TNC . 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.
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