>there is no tone upon startup.
First of all, check whether the speaker is properly connected.
>Perhaps it's time for the motherboard dishwasher trick.
Before attempting to clean the logic board, it could be a good idea to closely examine it. Look for leaking electrolytic capacitors, other discoloured components, damaged printed circuits, et cetera.
As you probably know already, repair work inside a compact Macintosh computer can be very dangerous due to high voltages, and must be carried out by someone with the necessary expertise only.