If you were able to receive 6-8 channels (digital channels using the converter box) you should be able to receive 6-8 channels on the new digital TV. However, some TV tuners are not as good as others so if they put in a cheap TV it might also have a cheap tuner. Hopefully, you have the Winegard Batwing antenna AND you have added the Wingman add-on necessary for the best reception of UHF channels (where most of the channels are these days). It's possible the shop did not reconnect things properly or may have created a new issue when moving connections around. You will need to start with the 12V check mentioned above and then trace the wiring to the wall plate power supply and then to the back of the TV to ensure all connections are working and they are secure and in the right place.