[RJMiranda]

The good VTR maintenance guys at Cuban TV lent me an extender board. As soon as I put the suspected demodulator board on the extender, I noticed the missing RF signal didn´t even got to it. So it was not the DM-board´s fault after all.
At the bottom of all these VTRs there is a motherboard that interconnects the different PCBs. I followed the signal path from the preamplifier board (that was working perfectly up to the output connector´s pin) to the motherboard. The connecting cable was not at fault. The motherboard looked OK even at a close inspection: the soldering points looked well, the board was not bent/cracked/damaged, even the varnish was like new... but it measured open and the ohmeter doesn´t lie. So I soldered a little white jumper in place and presto!
It is the first time in my experience of many years that I have found this kind of trouble in a professional studio VTR. Maybe in a field camera that may get banged accidentally... but this VTR does not show any kind of external damage, and its four legs sit flat against the table... not as if it had been dropped sometime so damaging the circuit.
Let´s hope that this VTR has not additional occult troubles like this. We have a very saline atmosphere in Cuba, but no other Betacam or even UMatics that I know of has had corroded circuits (and as I say, the varnish looks pristine).
Well, it is working perfectly now... thank you very much to all VKs/visitors that looked, and specially to baursam, kf4rca and fsjonsey for their offerings and valuable advice. If coming to Cuba, let me know!