Vintage Omega Delatatest & Alitest

Hoping someone might be able to help. I have a non working Deltatest ODT3 with an APU2-2 transducer. System powers on but does not register the rate of the, known working and accurate, Calibre 1310 I am testing. The transducer seems to be outputting signal but the Deltatest registers nothing in the display. Would anyone know where a schmatic could be found? or where to start testing the device.