1. If the "lock" LED beside the Backshort pot on the Gunn and Phase Lock Loop Module is red, the Gunn is not locked.
2. There are multiple locks possible for the Gunn, but only one of them is correct for the frequency you want. Presumably this lock will be acquired at tune and backshort settings close to those supplied by the microcomputer. To read what those requested positions were, set the rotary silver switch to tune (or backshort) and press the Set/Mon push-button. IN displays the requested position, OUT the actual position.
3. Set the pull toggle switch to Local
4. Put the Micrometer Adjust, Osc and Sweep toggles up.
5. With the silver knob at "Tune", slowly rotate the Tune Pot about the requested position until the lock LED turns green. There are several harmonics to which the Gunn can lock - but only one will give you the right frequency. Presumably this is the one closest to the tune value requested by the computer.
6. Rotate the silver knob to Backshort position and adjust the Backshort Pot through the range of values that the Gunn stays locked. Then reset it to the middle of this range. ALternatively, you can peak the Gunn output by monitoring Vdet on the MODULATOR as you adjust the Backshort.
7. Inhibit the Micrometer Adjust toggle.
8. There are two ways to check that the Gunn is locked to the right frequency.
   1. If you have an unmistakable spectral line, look at your source and see if it's visible.
   2. An alternative procedure is to connect the EIP output from the upper right of the RxB3 Receiver rack to the counter on RxC2. (You'll need an EIP cable about 6 feet long). The value read by the counter should match that shown as Foscillator on the PF1 Display terminal screen.

1. On the MODULATOR set the pull toggle switch to Local, the Loop Control to Open Loop and the rotary silver knob to either Ia or Ib so that you can monitor the junction current as an indicator of quadrupler output.
2. On the QUADRUPLER set the pull toggle switch to Local, the Micrometer Adjustment to uninhibited (up) and the rotary silver switch to INPUT.
3. On the QUADRUPLER use the lower left pot to maximize the Ia(b) value shown on the MODULATOR.
4. On the QUADRUPLER set the rotary silver switch to OUTPUT and maximize Ia(b) using the lower right pot.
5. On the QUADRUPLER set the rotary switch to Vb and maximize Ia(b) using the centre pot, but don't let the readout on the QUADRUPLER go below 3.0 or you'll trip the multiplier interlock. (If the interlock light does come on, just take the Vb value to 3.0 and it should reset to normal.)
6. If there were large changes in any of the 3 multiplier parameters, you might want to iterate through the optimization procedures again.
7. Set the QUADRUPLER's Micrometer Adjustment switch down, the MODULATOR's Loop Control to Closed Loop and the pull toggle switches on the MODULATOR and QUADRUPLER to remote.

The Half Wave Plate is controlled from the MODULATOR module. Its function is to adjust the mixer currents to the optimum values.

1. On the MODULATOR set the pull toggle to Local, Loop Control to Open Loop and the Half Wave toggle up to motor drive position.
2. Set the toggles on the MIXER modules to monitor junction currents and rotate the Plate in CW or CCW directions by toggling the direction switch appropriately. As you do so watch the junction currents. This switch does not reset itself, so you have to manually recentre it if you want the plate to stop rotating. The junction currents are quite sensitive to the orientation of the Half Wave Plate, so quick flicks of the drive toggle away from the centre position will permit final adjustments. The Level Control pot can be used during this procedure to adjust the average junction current while the Half Wave Plate balances the relative currents.
3. When finished, the direction drive switch should be in the centre position and the motor drive inhibited.

Note: at the present time, this section is not required as both mixers are tunerless; it is retained in case we ever have to swap in the spare mixer, which is tuneable.

The procedure is the same for each of the two mixers.

1. Set the pull toggle switches on the MODULATOR and the MIXER-CHAN A(B) modules to Local. Set the Loop Control on the MODULATOR to Open Loop.
2. Set the TOTAL POWER - CHAN A(B) module to Local.
3. Set the SKY and LOAD CHOPPER Modules to Local and select the Cold Load. Note that the current load is indicated by the LEDs underneath the elliptical dial on the TOTAL POWER module.
4. On the TOTAL POWER module module check that the black DVM switch beside the elliptical display dial is set to Level. Adjust the attenuation do the Level is about 200.
5. On the MODULATOR set the silver rotary switch to Ia(b) to display the mixer current. Adjust the Level Set pot so the current is around 15 uA.
6. At the MIXER - CHAN A(B) module put the switch under the LED display to the backshort position and use the left hand pot to add about 30 units to the displayed position (to compensate backlash before you seek the optimum position).
7. Adjust the backshort pot CCW (i.e. down) while watching for a simultaneous peak in the Ia(b) (on the MODULATOR) and the TOTAL POWER Level. You can go quickly at first as you're just taking up backlash in the drive. Keep the Ia(b) value below about 30 uA and the Total Power on scale by adjusting the Level Set pot on the MODULATOR and attenuation on the TOTAL POWER module as necessary. The peak position can be quite sharp. Once you pass through the peak you must go back up about 30 units and come down to the peak again.
8. Repeat this procedure for the other mixer if necessary.
9. A useful function available on the TOTAL POWER module for each mixer is a front-end Trx display. It can be used to display real-time receiver temperature while adjusting bias voltage, mixer current and backshort position. On the TOTAL POWER modules ensure that the IF level is mid-range on the elliptical display. This can be adjusted by setting the modules to Local mode and keying in appropriate IF level attenuation. Set the DVM toggles to Trx, put the Sky Chopper on Load and the Load Chopper in Run mode. The receiver should now switch rapidly between hot and cold loads, displaying the Trx values on the elliptical bar graphs.

Once back in the control room, you may wish to issue the rxb_auto or rxb_retune command to set any parameters that may have been changed incidentally from their requested values during the manual adjustments. The rxb_auto command will tune the receiver without calculating any new demand parameter values. The rxb_retune command will recalculate new parameter values and apply them to all modules that have not been left in Local mode.