The instructions for installing the MA.251 are in the RA.66 manual so I won’t discuss them in detail. This manual can be downloaded free from BAMA and contains a few additions; one being a hand drawn but detailed sketch of how to make your own MA.251 chassis. In addition, there is also a photograph that someone has added of an MA.251 installed in an RA17. Curiously the cabling in the photograph is wrong. The configuration shown will definitely not work with an RA.66. So why it was done that way?, I do not know.
I have an idea which may explain why the BNC and Burndept connectors were swapped in the MA.251 that was fitted in RA17L N3372. The MA.251 is a buffer amplifier designed to isolate the the RA17 from any loading as a result of connection to an RA.66. The EF93 stage feeds the 3rd mixer in the RA17, NOT the RA.66, as I suspect some people have wrongly assumed … and may be the reason behind the afore mentioned anomalies … see the diagram below.
Partial RA17L schematic with MA251 installed