I had EXACTLY the same problem as you with an amiga 500 (rev 6A). I replaced all possbily bad ic's with good ones of another working amiga. I also checked the ohm resistance of all 4 ram chips like you,and found no difference from chip to chip. It seems like a "dead end"...
NOT QUITE!
I finally found what the problem was, by "thermal" testing the ram chips! I simply felt with my finger the temperature of each ic on motherboard. Normally after some time on power, there must be SOME heat on every ic .I take this as a practical way to see if an ic is dead or not. If it's too hot OR no hot at all this probably means there is a problem.
Anyway, what i noticed was that one of the 4 ram chips was slightly warmer than the other 3 (3 was ~30-35c and 1 about ~40-45). This temp difference put me in thoughts: although none of the ram chips was too hot or no hot at all (which normally indocates a problem), this difference seems very suspicious. So i took off a ram chip from a spare A502+ ram expansion i have, place it above the "suspicious" chip and then press it down untill it wedged and couldn't move.
Power up, and... NO MORE BLINKING POWER LED!
Amiga worked like new!
Funny thing is that the ram chip sitted so well and tight above the dead one (i also cut the ground pin of the bad chip, so that will not heat up the good one above it), that i left it as is! To unsolder the bad one and solder back the working chip requires a lot of trouble if you only have a soldering gun and a suction pump. First you must remove the aluminum back plate (which requires to unscrew all the connectors on the back side of amiga) in order to have access to the back side of motherboard, as the 20pins of each ram chip is soldered on both sides of mb. Then you must do 40 unsolderings and 40 solderings (20pins on both sides of mb=40)!
I hope this info will help you too. Power up your amiga, and after 3-4 minutes check how warm are the ram chips. If you notice any temp difference then you might have found the "culprit"!
