Notes on fixing a Rotel RA-971MKII amplifier

10-11-2024

What is it?

An amplifier manufactured by Rotel in the early 2000s, with a set of 4 power transistor pairs over 4 channels. It's a relatively simple design, but after shorting one of the channels, you may find that the amplifier consistenly blows a fuse on power on every time.

Diagnostics

Check all 8 transistors, for they are the most likely to cause this fault. Transistor shorts will cause one of the two fuses next to the transformer to blow, but this may also be caused by the bridge rectifier.

In my case, both a NPN power transistor and the bridge rectifier itself were at fault. If all seems good, it's worth performing a bias check before powering it on again.

Ensure all capacitors are in good condition while at it. Checking all diodes is also good practice, since it doesn't take much more time.

Opening the case

The screws are clearly visible, however, there are two screws that hold the bottom plastic plate in place that are not immediately apparent. These are located under the transformer wiring, and depending on your model, may not be easy to reach.

Replacing transistors

Remove the transistor as usual, make sure you have replacement mica/silicone pads to replace the original ones.