I am not real good with a multimeter so you will have to walk me through where to put the probes (ie black and red) for measurement.
Oh boy. lol Be careful you don't accidentally short anything out. First things first... red is positive, black is ground or negative.
First check the power supply not hooked to the Amiga. On the bottom of the supply, there is a legend that tells you what pins are responsible for what voltage. Be careful not to accidentally short your leads testing that and write down the results.
If you're spot on or extremely close, now present a "load" to the power supply by hooking it to the Amiga. Now you can place the black lead of the DMM To anywhere that's ground on the Amiga (any of the screws) or even the black lead going to the drive. Red to red, black to black. Comparing the with the notes you took, read if the voltage is much different. If so, you may have to re-build or get a new power supply.
Check to see if there is cold solder by "wriggling" the drive's power cable though. While a disk is spinning, apply pressure to the power cable at both ends (computer and drive, one at a time) and see if the light gets brighter or not. When I say "apply pressure", I don't just mean pushing down. Maybe pull up a little bit, twist it or bend the connector left or right. If the drive light gets brighter when you're fooling with the cable, then you have a bad solder joint that needs to be re-soldered.
If you suspect that new Agnus chip is at fault, maybe try removing her and ensuring all her pins are in the correct position. I guess it's possible that if one of her legs is shorting against another, you may experience a low voltage scenario too...
Lot's of various things to check! Welcome to vintage computer repair 101
