We had a unique coincidence in the shop recently where two different amplifiers arrived with the same problem. These units, McIntosh MC312 and MA12000 power amplifiers were both blowing fuses immediately upon power on. After verifying power supply wiring, main rectifier, and filter capacitors were OK, we turned our attention to the heatsinks. The power transistors were measuring bad. After further evaluation, we found the transistors were short circuited to the heatsink / ground. That would certainly blow the fuse.
We had our first clue as to what went on as shown in the pictures, upper left. The slight burn mark on the transistor clip was evidence of some heat in the area. We pulled the heatsink boards and inspected the transistors. It was evident that both units had suffered a short circuit between power transistors and heatsink. The damage is shown in the photos upper right. The photo lower left shows the large holes in the thermal tape caused by the short circuit.
Any kind of metal filings, a solder ball, or other debris are sufficient to cause this kind of issue. Even if not shorting immediately, debris can work their way through the tape over time and eventually short circuit. The high current of the short causes the metal to heat and vaporize. This leads to the pitting in the transistor and heatsink. Bob the Tech smoothed the damaged areas with a file to remove any high spots or sharp edges. We ensured there was no debris prior to installing new thermal tape and transistors. If we didn’t do this, the burrs would immediately damage the new thermal tape and lead to another short. We replaced the thermal tape and failed transistors, and reinstalled the boards.
Once repaired and reinstalled both units were verified to power without blowing fuses, and to perform to specs. If you have issues where your McIntosh MC312 or MA12000 power amplifiers are blowing fuses, contact Bob the Tech Audio.
