Today we had the pleasure to bring this beautiful Acoustic Research – AR Model R back to life. This is a 60W per channel unit that was built in the early 1970’s and was one of the first direct coupled solid state amplifiers.
Our customer stated the main AC fuse would blow as soon as you turn the unit on. Upon our initial diagnostic, we discovered the main power rectifier diodes in the power supply were damaged. One was split it half, one was blown to who knows where, and the other two were dead shorted. These are 3A 200V diodes so this unit must have been hit with a fairly significant surge, or something in the power supply section was shorted.
We replaced the four rectifier diodes and thoroughly tested all other components in the power supply section. To our surprise, all other diodes, resistors and capacitors in the power supply section tested OK. At this point we examined the rest of the unit and touched up several solder connections. The amplifier protection fuses were still good so it was our assumption that the amplifier section had no major issues, but we tested it anyways. Everything was well within specs.
Now it was time to power up! We replaced the blown main AC fuse with a 4A fuse and then slowly brought the unit up to 120V AC using a Variac. We hooked up an input source, some speakers, and had sound output out of both channels! The controls were extremely scratchy so they all got a nice bath in DeOxit. The quality of sound that this unit produced was amazing.
The unit now works 100% and our customer is happy to have it back in a working condition. For more information on the Acoustic Research model R, click here.