Giga-tronics GT 9000, Operation & Maintenance Manual

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5.3.2 The Computer
All functions are controlled by the computer, and a general computer failure will lead to a
general system failure. The computer is based on the Motorola 68000 microprocessor. It uses a
bus architecture which corresponds on a one-to-one basis with the signals from and to the
68000 chip.
Corrective Action
Four circuit boards house the computer:
A1 — This is the Front Panel Interface board, which acts as an interface between the
microprocessor and the front panel.
A2 — This is the IEEE-488/Timer board. It includes the interface between the CPU and
the remote control bus, as well as the timer circuit for the CPU and driver circuits for
the step attenuator.
A3 — This is the Memory board, which holds the ROM devices in which the operating
software is stored, as well as the RAM chips used by the computer for temporary data
storage.
A4 — This is the CPU board, which houses the microprocessor and the address
decoding logic through which it runs the instrument.
When a computer failure is suspected, the best way to troubleshoot the system is to isolate the
problem to board level by substituting known good PC boards. If the problem persists with good
spares, the problem is most likely on the computer bus board (the motherboard) into which the
computer boards are plugged. Many computer problems are caused by dirt on the connections
between these boards.
If the computer seems to have become lost, it can be reset either by pressing the reset button on
the CPU board (A4) or by turning the power switch off and on.
5.3.3 10 dB Step Attenuator (Option 26)
The programmable attenuator is capable of attenuating the RF output by as much as 110 dB,
depending on the frequency range of the instrument; if the output is not at the desired power
level, the cause may be that the attenuator is improperly set. Note that the attenuator is outside
of the leveling loop, and therefore the leveling system cannot detect, or compensate for,
attenuator errors.
Corrective Action
The internal structure of the programmable attenuator includes a series of relays which add or
remove lossy elements (10 dB, 20 dB, 40 dB, etc.) in suitable combinations to achieve the desired
total attenuation. The relay drivers which select these increments are located on the IEEE/Timer
circuit board (A2). Verify that the attenuator is receiving the appropriate drive signals; if not,
investigate the drivers on the IEEE/Timer board and the computer logic lines which control them.
Model GT 9000 Microwave Synthesizer
5-8
Manual No. 120AM00250, Rev C, September 1998