• Gunn Operating voltage
• DBI Failure

Gunn Operating Voltage

The Gunn operating voltage must be set to 6.0 volts for sensible operation. There are circumstances when this will not happen automatically:

• After cycling the power on the electronics rack, the Gunn operating voltage may occasionally come back set to 0 Volts. Subsequent attempts to tune the receiver are doomed to fail. The symptom is that you can't get any junction currents. The PF1 engineering screen will show V(Opt) = 0. next to the Gunn schematic. The easiest fix is to use the engineering screen to select the Gunn (Oscillator) and V, then enter 6.0.
• After rebooting the micro (and maybe after the ICL rxb_reboot command) the Gunn operating voltage will be set to 9.99 V. This will probably work OK for most frequencies. However, it seems to make locking the Gunn more difficult at some frequencies. It is better to use the above procedure to reset this value to 6.0, which was the value used to define most of the Gunn LUT values. This problem is supposed to have been cured by changes to the software, but keep an eye open for it, anyway.

DBI Failure

Ensure that DBI arm 1 is homed correctly. Two or more successive home commands may be necessary, but there is another way if you are sufficiently ahead of the game:

• After rebooting the micro, the DBI will often fail to home properly (watch it on the PF2 screen). The reason for this is not clear - Russell supposedly fixed the timing-out problem we thought was causing this. Successive home commands issued at the engineering screen will eventually sort this out, but it is more straightforward to prevent the failure by the following trick.
• Before rebooting, configure the engineering screen so you are looking at the PF2 screen (diplexer) and you have selected arm 1 of the DBI on the command menu and are ready to enter a new P(osition) for that arm. After rebooting, the default tuning parameter is for DSB which means that arm 1's position will be about 3.5. Such a small number requires a long slew from the home position (~23) and this is when the failure occurs. To preempt this failure immediately after the reboot, enter a new position of 13.5, which is approx. what is needed for SSB tunings. This should permit the DBI to set itself up without failure, and the FE command will provide the exact position that is needed.