There have been three types of startup powering-on arrangement for Macs.
1) Nothing but a rear mains switch, as in Compact AIOs and some LCs, designated by Apple as 'passive power control'.
2) A so-called soft power key on the keyboard, but designated by Apple as 'active power control' mediated by an inbuilt power management unit (PMU), and no rear mains switch.
3) A limited group, beginning(?) with and exemplified by the Colour Classic, having both a soft power key on the keyboard and a rear mains switch, and characterized by Apple as pseudo-soft power Macs.
If your LC 520 has provision for startup from the keyboard, leave the rear mains switch on and allow the Mac to soak without startup for at least 24hr. This first switch-on after an interval without power will produce the klunk that you reported. When the will to start up reliably has been re-impressed on the Mac it is possible to switch it off at the rear for maintenance or for work on the logic board without getting the klunk after switch-on.
Whenever you try to start up the Mac, use only the power key. The rear switch should remain on for as long as the Mac is attached to the mains, both for reliable startup and for the sake of the PRAM battery. If all you get from the power key is a series of rapid clicks, still let the Mac stand with the rear switch on, and try at intervals to get startup. You will probably be successful, in distinct contrast to the state of affairs in the IIci, IIcx and Quadra 700 where a failure of the TRKL supply is terminal until the PSU has been repaired.
If the keyboard does not have a power key, believe that you need one. This pseudo-soft power characteristic has been rehearsed here and at 'Fritter, both/either of which will go into it in greater depth than I wish to repeat.
de
1) Nothing but a rear mains switch, as in Compact AIOs and some LCs, designated by Apple as 'passive power control'.
2) A so-called soft power key on the keyboard, but designated by Apple as 'active power control' mediated by an inbuilt power management unit (PMU), and no rear mains switch.
3) A limited group, beginning(?) with and exemplified by the Colour Classic, having both a soft power key on the keyboard and a rear mains switch, and characterized by Apple as pseudo-soft power Macs.
If your LC 520 has provision for startup from the keyboard, leave the rear mains switch on and allow the Mac to soak without startup for at least 24hr. This first switch-on after an interval without power will produce the klunk that you reported. When the will to start up reliably has been re-impressed on the Mac it is possible to switch it off at the rear for maintenance or for work on the logic board without getting the klunk after switch-on.
Whenever you try to start up the Mac, use only the power key. The rear switch should remain on for as long as the Mac is attached to the mains, both for reliable startup and for the sake of the PRAM battery. If all you get from the power key is a series of rapid clicks, still let the Mac stand with the rear switch on, and try at intervals to get startup. You will probably be successful, in distinct contrast to the state of affairs in the IIci, IIcx and Quadra 700 where a failure of the TRKL supply is terminal until the PSU has been repaired.
If the keyboard does not have a power key, believe that you need one. This pseudo-soft power characteristic has been rehearsed here and at 'Fritter, both/either of which will go into it in greater depth than I wish to repeat.
de