John,
You can download the manual here and loads of other details here:
http://bingo.cdyn.com/machines/bally/big_time/
That might help. There is usually a card stapled to the inside of the head describing what to do to change the lamps. Open the back door and have a look for it. I'm not sure if it's the same moving lines on Big Time but the F line on my Sun Valley is a moving line. It is a vertical plastic number strip fixed to a pin at the bottom by a circlip. I have to remove the circlip and slide the strip out from its guide to access the lamps. Then, as they are about 1" deep in the screen, I made a tool from some rubber tube to grab, turn and remove them. I use the same to fit new ones. Don't forget they are special lamps in the screen, not standard 6 volt lamps. You can get them from PBR, number 1464 is what you need.
If you post an image of the screen, we might be able to tell more how you do it.

Edited to remove duplicate post

Mate,
On my game, it's behind the back glass but I don't know if Big Time works the same. I've attached an image of the F line on my game, the only moving line that works that way. All the others are magic squares, even the E line.
If you look at the bottom of the plastic number strip, you can see a slot. A pin pokes through the slot with a circlip to hold the line on. I have to remove the circlip, bend the line forward a little and slide it down. If yours are the same, put a sheet over the playfield glass before removing and especially refitting it. They're real difficult to re fit and easily dropped. The sheet stops the circlip bouncing off, never to be found again.
If you post an image of the screen on yours, we may be able to see how you access the lamps, if it's different to mine.

Sun_Valley_004a.jpg

That photo brings a tear to my eye. Those machines are engineering marvels.

That looks the same as my cypress gardens bulbs were a PITa to change out

WP_20130107_001.jpg

Those lines work completely different to Sun Valley which is clutch driven from a motor via a lock / release coil. Looks to me like you have to change those lamps from the front, not inside the box. I reckon a slim pair of pointed pliers would remove those spring clips. Use a mirror to see how the fit if you can't tell. Then you could slide the lines up or down (whichever it has to be, can't tell from the images) to get at the lamps. I would magnetize the plier tips first, just in case. If it's only a couple of lamps need changing and you can get at them from inside the box, it might be worth unsoldering the lamp holders, undoing the two screws and removing the holder / lamp. Again, magnetize the screw driver first.

Mr Bally, American engineering at its finest. Still working and very little wear after nearly sixty years. The search disc and other parts have been turning constantly all the time that game was switched on and they still show very little wear. Even some of the leather clutch washers are original and still good. They don't build much like that any more.

Way2wyrd, yes mate, same as Cypress Gardens but with an extra F line. Bulbs are a pain to change.

I reckon there will also be screws at the bottom and most likely in the middle, maybe several pairs. They won't be accessible until the number strips are removed. Undoing the top two and bending the alloy guide will probably damage it.

Mate, try a post here. There are a lot of knowlegable members, one of whom, I am sure, will have the answer.
http://ballybingo.fff.yuku.com/

You need to replace lamps on a Miss America series with two cards or a magic screen with OK feature.