After the restoration of the Nakamichi 620 amplifier last year, it had been working great for 6 plus months. Then one day I turned it on and there was no sound output. Luckily the speaker protection circuit did it’s job and no speakers were damaged. The right channel had it’s output shorted to B+ (53 volts).

After around two months on and off, trying to repair the right channel module I finally gave up. It is getting very difficult trying to find quality vintage replacement components for older equipment now days.
I decided once again to design my own amplifier modules for the 620 using the TDA7293V 100-watt, DMOS class AB amplifier modules from ST Microelectronics that I used in the Nakamichi 420 Salvage. This time though I would use two modules per channel in the modular configuration which is well documented in the data sheet.

The new modules were designed to fit into the limited space and wedge configuration of the 620 chassis while providing a good heat path to the front heatsink. I also machined a new L shaped aluminum connector to replace the original Nakamichi version.

I kept the TDA modules separated as much as possible to improve heat dissipation, and also added additional soldered buss wire to the high power traces.
Instead of a dedicated low voltage supply I opted for a simple Zener supply off the B+ supply for the mute and standby controls.

For the most part other than the new modules and the Zener supply nothing else has changed, other than replacing the white speaker output wires to the output / fuse board which were just a little too short for my liking and two dedicated ground wires. The speaker protection board was remounted and the unit was tested. I was very happy with the finished results as the new modules met or exceeded the original Nakamichi specifications. For me the current system configuration sounds just as good as it did with the original modules, but that is subjective based on my likes and dislikes, and your mileage may very depending on your preferences.

Many thanks again to Bob Piper for the original donation of the Nakamichi System One.
EagleCAD V7.7 Nakamichi 620 amplifier module rev. 0.12 schematic and board ZIP file
EagleCAD V7.7 Nakamichi 620 protect module rev. 0.2 schematic and board ZIP file