Quad only powers up when connected to USB

APM 2,5 board with trace cut and external GPS added. Has previously worked fine, have just upgraded ESC (Skywalker 4 in 1) from 20A to 25A.

If I connect the LiPo without USB plugged in quad just gives steady beep. If I connect WITH USB plugged in I get normal startup (four short beeps one long).

TX reports “telemetry recovered” even when quad in “steady beep” mode, but quad won’t arm or respond in any way. Plug in USB cable when quad in this state and it immediately powers up. Unplug USB and it immediately goes back into “steady beep” state.

Can anyone help me fix?

That would most likely be a hardware issue, not an APM:Copter software one.
Is your board a genuine 3DR APM?