Greetings to all AMSAT tinkerers.
Modifying the simple Octagon Oslo PLL LNB for external referencing is rather simple.
So far I have successfully modified three units (that use the RDA3560 and 27 MHz reference crystal) to use a 27 MHz external reference that is fed via the L band IF cable. A modified LNB appears externally identical to the original, no mechanical modifications are needed.
The plastic casing may be pried off in one piece if you are careful. Remove the rubber sealant glue with a sharp scalpel etc. and undo the three Torx screws to remove the top cover.
Remove the crystal, solder a series LC bandpass filter of 2.7uH and 12pF directly at the F connector and using a short jumper of single strand wrap wire from the far end to the RF processor chip pin 20 shown in this photograph:
The three I modified will readily lock with reference frequencies between 26 and 28 MHz with an input level of 0dBm and 27 MHz locking is reliable all the way down to -10 dBm, though I would not recommend using such a low level.
At the shack end I have a simple triplexer for separating +9V to the LNB, L band IF and 27 MHz reference.
In my first application I filtered and mixed the L band IF further down to HF (10 - 21 MHz) to feed to a KiwiSDR web radio, but I have also experimented using my Barlow Wadley XCR-30 as a portable battery operated system, which I think is rather cool
For inspiration, here is a block diagram of my current prototype: