I remember watching it on tv the 2 protagonists went to an arcade and they insert the coins into an arcade machine and this would activate the mech. They also changed the cast 3 times so I think the show has multiple seasons.
NG Knight Lamune & 40 (1990)
Could this be a slightly mixed up memory of NG Knight Lamune & 40?
Tama-Q- (Voiced by Chie Koujiro) A hand sized advisor robot that can summon the guardian knights if Lamune drop coins into the slot on his head.