so when its operates correctly, by adding coins, increases the credit wheel, then you push the coin panel start button to start. the game starts and the credit is reduced by one?

I did remove and have a look a the credit unit, when its gets to around 7 credits there is a pin that tries to bend the contacts. the wheel i think goes up to about 16. But there is no way the pin would get past 7 or 8. is this normal?

Correct . Pushing the start button should cause credits to decrease by one . The pin can be removed and put in another hole closer to max number of credits .