Advice about repairing an IBM 5151

Hi all, I am trying to identify why my IBM 5151 display has no picture when connected to a normal MDA card in a IBM PC 5150. So armed with an oscilloscope, digital multi-meter and the SAMS Computerfacts for it I started to investigate. First, the card does send out signal and that signal does reach the board inside the monitor. I checked the power part of the circuit, all diodes and the transistor check fine. I probed some of the vertical and horizontal transistors and there is a signal there too. Then on the video board both TR19 and TR20 have on their collector and emitter (respectively) a signal (95Khz). What else can possibly be wrong? The SAMS manual talks about "injecting a video signal" at different pins however I have no idea exactly how to do that. So there is current going out of the power part and there is signal on the video board attached to the neck of the CRT. Regards, Vlad.