Glad you got your Plus working again. The "Classic Mac Repair Notes" are really helpful aren't they!

My Mac ED does the same whup-whup-whup infinite reset which is referred to as "activated crowbar."

Unfortunately re-soldering many joints and adjusting R56 did nothing for me and mine will need a replacement analogue board

Testing my logic board in a spare 512k Mac brings up a "Sad Mac" too so looks like it's suffered a fairly serious failure all round!