My coin mech hack of a Sentinel c220b. I attached a micro switch on the coin eject button, so that if you don't have any coins on you, you'll always be able to get a credit. I also removed a bit of metal inside the mechanism where the coin travels and replaced with a micro switch (Had to file some of the plastic down to get a nice fit). So you put and coin in, and it comes straight back as if it were rejected, but you get a credit! Works every time with £1 and £2 coins.
Also attached is a pic of how I attached micro switches to the back of the pedals. Those red plastic things I added to reduce the height of the pedals.