Barrett 900 Series, Technical Manual, Page: 36 / 164

BARRETT 900 SERIES TRANSCEIVERS
5.7.4 Transceiver turns on, internal voltages are normal, but no LCD display.
5.7.5 Main microprocessor faults
- In the case of the remote control head check the remote control head microprocessor for
activity or an LCD module fault.
- In the case of the front panel check the front panel microprocessor for activity or an LCD module
fault.
- Check that the transceiver firmware EPROM U4 and option GAL U30 are fitted, without these the
transceiver will not operate.
- Observe the LED's on the Microprocessor PCB. Check that all the yellow LED's are illuminated at
the same intensity. The LED's check the +5V, +5V D5V ANALOG and +10V rails. If any of
these LED's are extinguished there is a problem on the corresponding rail and the rail should be
checked for shorting to ground.
- Check for a 2.7MHz logic level square wave at pin 1 of U13 74HC4040 on Microprocessor PCB. If
the 2.7MHz is present, then the reference oscillator is active, otherwise there is a fault in the
reference oscillator chain.
A fault in the reference oscillator chain can be localised by
examining microprocessor PCB test points TP31, TP30, TP33 and TP32 in turn. By comparing the
waveforms obtained, with those on the schematic diagram table it should be possible to isolate
the problem.
- If with the standard reference oscillator fitted the crystal oscillator comprising Q13, Q14, L15,
L16, X3 etc is not running, then check that the crystal oven is hot and that the DC levels are
appropriate.
- If the high stability reference oscillator is fitted check for output. If no output is present check the
DC level into the TCXO/DCXO is correct. If it is and no output is evident then replace the
TCXO/DCXO. Incorrect DC levels generally indicate a failed transistor. As Q13 and Q14 are not
DC coupled the faulty device should be obvious.
- If the reference chain is working up to pin 1 of U13 then check for 22.7MHz logic clock at TP34
and 11.3MHz logic clocks at pin 33 of the microprocessor U1. Absence of clocks at either point
indicates a problem with U31, 74AC74 or U32, 741C74.
- If both clocks are present the next step is to check address latch enable at TP6 as in the table on
the schematic, and then check for microprocessor bus activity. This can be monitored on a CRO,
and it should be possible to see continuous bus activity on the address and data pins of the
EPROM.
- If no bus activity is evident check that TP38, the reset line is high and that the stop line, pin 68 of
U1, is also high. Also check that TP5 the interrupt line is normally high with brief pulses low.
Erratic bus activity with no sensible display on the LCD indicates a possible bus problem. The
only way in which this can readily be isolated apart from checking for bus shorts, with a
multimeter, is by fitting a CHECK EPROM Barrett P/N BCS90100 which has been programmed
entirely with OOH data at each address. With such an EPROM fitted the microprocessor will
simply step through each address without stopping or jumping and the waveform on each
address bit from A0 to A14 on the EPROM should be a square wave at successively half the
frequency for each successive bit.
- Normal accessing of EPROM and EEPROM can be checked by comparing the waveforms
obtained at TP6 and TP1 against those in the schematic table.
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