Nakamichi AV-8 Audio Power Output Assembly

The Nakamichi AV-8 audio power output assembly consists of a pair of circuit boards that sandwich a pair of heatsinks to form a tube with a square cross-section. On the inside end, they attach a fan that is similar to that found in computers for cooling. This interesting and novel approach seems to work well. The plastic ends support and isolate the heatsinks from earth so be very careful as each half of the heatsink carries the split-rail voltage.

I leave nothing to chance, and decided to replace the dried silicon grease that the transistors had by refitting all the power transistors. I spotted that some of them were missing their mica insulators. After a close inspection of the service manual and power output board, I was able to deduce that someone, not very clever, and very likely a Brit, had attempted to replace the power transistors of the centre channel; however, they had forgotten to replace the mica insulators, or probably not understood their importance. This error of course results in the collectors joining the heatsink. In this amplifier design, they do not ground the heatsinks and instead one half carries the negative voltage rail, whilst the other half carries the positive voltage rail. The advantage of this is that you can avoid using mica insulators for majority of the transistors, except for the centre channel. Unfortunately, they had to place the power transistors for the centre channel onto one heatsink block and therefore the manual suggests some transistors requiring mica insulators.

One of the problems with this amplifier design is the heatsink assembly clogging with dust reducing the effectiveness of the cooling resulting in overheating. Although one can regularly vacuum out the dust, if it is clogged deep inside the blades, then taking the assembly apart for cleaning is one major service worth doing. In addition, if there is dried silicone compound, then a total rebuild with fresh compound can go far.

After installing new mica insulators, new transistors, and new resistors, the amplifier came back to life and each channel worked perfectly. Then it was just a simple matter of calibration as set in the service manual, and it was ready for listening.

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