The SIS345 receiver is one of three facility receivers found on the left flange. A striking red in color, it is the left most receiver as you face the telescope. On the SIS345 itself, we find the following:
Note: The mixer backshorts and E-planes are mechanical devices. Do not force them if you reach a limit point!
Behind the receiver (close to the flange), there are two other important parts that can be adjusted if needed.
This device is under computer control, but it also can be adjusted manually. It is a small pot (number 3) on the LO chain. Have an FoT or Site Manager show you the location during tuning.
It is adjusted simply by turning the pot by hand. You should be watching the mixer current - if it stops changing then you likely have reached the limit of the pot.
This device is used to balance the LO power into the two channels. It is controlled by a small silver lever that is behind the receiver. The FoT or Site Manager should show you this before you attempt to tweak.
Adjust it by moving it up or down. We generally find that you can get good tunings with it left level (i.e. in the middle of the range). If you find that you are at one limit or the other, find an FoT to help see if we can get a better tuning. Generally, you should avoid being at the LIMIT of any of your tuning parameters-since you effectively lose some control. There is almost always a better option.