MFJ MFJ-1270C, Системное руководство

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MFJ-1270C/1274C MANUAL TROUBLESHOOTING
Page 219
While the steps described above may seem obvious, careful inspection often will
point to the problem or give significant clues as to the probable area of the MFJ-
TNC most suspect. After the above inspection has been completed and
apparent problems are dealt with, it is time to proceed to more specific analysis.
Symptom: TNC appears dead
If the MFJ-TNC powers up with the PWR, STA, and CON LEDs lit, followed
by STA and CON extinguishing a second or so later, the processor is working
and the software is probably working correctly. You should suspect the terminal
port at this point. Check all connections and verify the logic levels according to
the terminal interface troubleshooting section in this chapter.
Oscillator and Reset Circuits
If no LEDs cycle ON then OFF during the reset cycle the problem may be more
serious. Check to see that the crystal oscillator is working and that an "M1"
signal (154 kHz square wave, 0 to +5 volts) is coming from U22 pin 27. The
crystal oscillator input to the processor (U22) is pin 6. The input clock should
be a (possibly distorted) square wave signal. Verify that the clock input at pin 6
of U22 is running at the correct frequency (near 4.9152 MHz).
Verify that the battery backed-up RAM protection circuit, composed of Q4, U14
and associated devices, is going to +5 volts at U14 pin 6 after input power is
applied. This signal enables normal operation of U24 and U25. There should be
a logical low on the output of U7 pin 12 coincident with the application of
power and lasting for a few hundred milliseconds. Without this RESET signal,
the Z80 probably won't start up properly.
Digital Logic Lines
Remember that all of the logic circuits operate at standard TTL levels (a "low" is
less than +0.8 V and a "high" is greater than +2.4 volts), and all digital inputs
and outputs switch between these two levels. Thus, if you see logic signals
switching between 0 and, say, 1 volt, you can be sure there is a problem (usually
a short). On the other hand, do not mistake switching transients on digital logic
lines for improper operation -- these show up as ringing and other distortions.
Verify that there is activity on the control bus READ and WRITE lines, the 3
CHIP-ENABLE lines on the memories (U23- 25, pins 20), the IOREQ line on