Total power lost 1981 FI

[Montreal]

Hi
When we connect the battery, the clock works as the headlight etc.
When i try to start, there is even no cranking and we lose all electric power. No more headlights no more clock nothing.
Wait hours reconnect the battery: same thing. Light in dash are on, try to start, you loose everything.

Battery is good
Ground is clean to body.
Positive connection is good
Ignition key is new and connection are ok
We Replaced the double relay to eliminate that possibility.

We have also a 1978 carburetor, and it is much easier!!

What could we test ?
Thanks to all.