Lift the playfield and make sure the switch contacts are making a connection. Also make sure the contact point isn't dirty. If you don't have a flex stone to clean the contacts, you could use a bit of 400 grit sandpaper to make sure the contacts are shiny on both contacts.

I don't have a schematic, but I would bet the same relay that turns the when lit lites on also shifts that switch from a point award to awarding extra ball. That relay or another associated with the circuit may have an open switch.

Inside the cabinet, right behind the score motor on the left is the extra ball relay. Maybe the make break switch at the top of the diagram is dirty? With the game off, manually move the relay arm and make sure every switch is moving: M&B: make-break - one switch opens the other closes, N.C. normally closed- should open, N.O. normally open- should close. check to see if all the switches are clean

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Hello,
I bought this Mar's Trek machine and the extra ball does not work. The contact was closed and I'm thinking it burned out the relay?
Wires are good.

Also the "Lock relay" was buzzing. What is it for?

Thanks,
This is my first post so pardon my lack of terminology.

The lock relay generally closes at the flick of the power switch on most Sonics. On others like Bally and Williams you generally can hit the left flipper, put a quarter in, or if there are credits hit the reset button which will all generally cause the lock relay to close and pull in. The pinrepair.com guides go over this in good detail but generally that relay will turn your general illumination lights on in both the back box and playfield. These are often a nice toasty brown color on many EM machines because these work hard and are generally "on" whenever the machine is powered up. They buzz and if that annoys you it is always an option to permanently close the relay and de-power it if you wish. That process is also nicely covered on pinrepair.com.

The ipdb.org entry has the guide to Mars Trek and in there you'll see the extra ball relay. Start up a game and see if manually closing that relay gives you an extra ball. There are a couple of switches on that relay, one that lights the lights and the other that will hold in until you score on the subsequent "extra ball". It could be a mechanical issue with the relay or a dirty or misadjusted switch contact there or in that circuit including the score motor. We can help walk you through the diagnosis.

Good luck.

OK Thanks for your help.

I will try that this weekend and let you know. If I need a new relay are they available?
Do you know?