An Instructor pilot and his student experienced smoke in the cockpit apparently emanating from the radios. While attempting to determine the source; they continued to their destination airport and landed safely using light signals from the Tower.

Narrative

We were returning from a training flight when we noticed the smell of burning wires and saw smoke floating in the cockpit; originating from the left side of the radio stack. We immediately turned the radio master switch off and shut down all electrical equipment and opened air vents in cockpit beneath our feet to attempt to ventilate the cabin. We conducted the checklist procedure for ELECTRICAL SMOKE OR FIRE. We continued toward the airport and began troubleshooting approximately 7-8 miles north of the field.We attempted to restart each radio one at a time. We started with COMM2; which immediately brought a burning smell to the cockpit; but no smoke. [We] turned the radio master off. We turned the volume of COMM2 to the off position and attempted to restart the GPS/COMM1. Smoke began to appear immediately and both GPS/COMM1 and radio master were switched off. Both circuit breakers were pulled for COMM and GPS. We realized after we were on the ground that we had not pulled the circuit breaker for COMM2. The ammeter was reading approximately 8 amps without the radios turned on. Neither of us checked what the reading was when they were turned on. We turned our transponder to 7600 and turned the battery on to attempt to communicate to Tower that we had experienced a radio failure. Upon switching the radio master switch back on to emit a transponder signal; smoke appeared again and we immediately turned the radio master off.At this point we were just inside of airport's Class D airspace on the north side and began to scan for traffic in the pattern for the runway. I assigned the student to scan for traffic and look for light signals from the Tower. We saw an opening and entered 45 to the downwind behind another aircraft; flew a normal traffic pattern; and eventually received a solid green light signal on one-mile final. I rocked my wings to acknowledge; landed and taxied off on Taxiway Bravo; continuing to communicate with Tower via light signals. We received a solid red to hold short of the next runway; followed by a green to indicate we were cleared to cross. Parked at our normal spot and tied down the aircraft.