Murphey, the pull out and it worked, quit when you changed frequency implies that the frequency selector contacts are oxidized or dirty.
Pull it out again, pull the covers off, top and bottom, and spray all the segments and wipers on the frequency selectors, spin them a number of turns, and spray again. Repeat several times.
A previous poster described his shop reflowed the solder on some joints of the circuit boards. I have had good success doing this, but a skilled technician should do it for you. In the solid state world, a unit that has worked for 50 years, some form of corrosion is the most likely fault.
Best wishes for the check ride, patience is a winning plan. I did my check ride in a plane with no GPS, no DME, and no ADF. Dual VOR's and ILS were my equipment, and I did VOR approaches with the station on the field, and remote, plus holding patterns defined by intersecting radials. The missed approach was also to a hold defined by radials, so a lot of frequency changes from the hold, to the approach, them hold at the missed destination.
GPS makes all that much easier, but a good pilot can do with the traditional equipment. If you are skilled with that, you are in fine shape if the GPS system has a burp when you are IFR. I had that happen when I was along the coast of Florida when the Navy was testing some jamming equipment.. Not a problem, since I was fully qualified to fly without GPS.
