Garbled characters, just like no display, can be caused by bad 7448s. If your tops are working but the credit display is bad then I would swap in another 7448. The same 7448 that controls the tops controls the credit/match display. If you get the same result with two 7448s the 7448 is probably working fine. That means the decoder on the credit/match display is probably bad. I am not sure if it is the same UDN6116 as the 6 digit displays use, if you need to snap some pics of the IC on the credit display and I will ID it for you.