Nakamichi Amplifier Repairs
Nakamichi 420 Power Amplifier
This came into the workshop with the right channel not working. I discovered the fuse had blown. I checked the output transistors which are the small MT100 type and one had blown. Checked the operation of the amp by wiring a TIP transistor temporarily in circuit, didn't work correctly, so there were more faulty components. This made it questionable if it was economic to repair as the original transistor would cost £20-25 each! I looked to find a more economical route and ended up removing the old transistor moulding from the metal tab and glueing a TIP42 transistor on it with thermal adhesive, there was also a transistor open circuit in the driver section A915. It sounded okay after repair.
Nakamichi 410 Preamplifier
This was as dead as a dead thingy! Turned out the mains transformer was cooked and needed replacing. I could tell by the burnt smell when the case was opened. The supply voltages to the circuit were +/- 5V where they should be around 10V. The Zener was also cooked at 3.6V and just for good measure the power lamp needed replacing too.
Nakamichi Amplifier 2
This had blown the output transistors, drivers, temp sense transistor and a few resistors 100 ohm, 330 ohm, 1 ohm, and the 1k bias pre-set.