According to this website
http://amiga.serveftp.net/A3000_HardwareGuide/booting-problems.html it's the following
White boot screen, but resets continuously
The power supply generates a 50Hz timing "tick" (or 60Hz for America) which is received by CIA U350 for use as an accurate time reference.
If this signal is not present, the computer will enter a continuous cycle of booting to a white screen then resetting.
This can be the case when a 3rd party power supply is used (e.g. an AT power supply) which does not provide the timing tick, or if the standard A3000 power supply develops a fault.
To get around this problem, move motherboard jumper J350 from TIC to VSYNC. VSYNC is derived from the motherboard crystal oscillators, so is not as stable as the mains power supply frequency. Using VSYNC instead of AC mains synchronisation can lead to the system time running fast or slow when the computer is running. The correct time will be restored when the system is rebooted, as the time is re-read from the battery backed real time clock during startup.
Dave G 8-)
