http://it.aminet.net/hard/misc/errormessages.txtYellow on boot is noted as a strange error (12 on this list) but I used to get it all the time on my A600 if the Trap Door Chip RAM expansion had come loose.
Do you have any spare 30-pin simms?
Swap out the 30-pin simm that is the chip RAM one.
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If you get yellow on power up, I believe that 90% of the Amiga is working.
The Amiga uses the 68k CPU to run POST. This means your CPU is working.
It uses the kickstart ROM to load the test routine that runs. It also CRC's it and this means your kickstart ROM is intact. (Or you would have a RED screen)
It uses registers in the Lisa chip to change the screen yellow etc.
Check your onboard battery, acid corrosion of the tracks near the SIMM sockets I imagine that can be responsible for yellow screens.