Nakamichi 620 Amplifier Repair

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).

Nakamichi 620 ampli­fi­er with new mod­ules at 120 watts out­put power

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.

Nakamichi 620 mod­u­lar amp boards Rev 0.12 front and back

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.

New Nakamichi 620 ampli­fi­er mod­ule com­po­nent side miss­ing pow­er connector

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.

New ampli­fi­er mod­ules installed in the 620 chassis

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.

Nakamichi 620 cus­tom built speak­er pro­tec­tion board Rev 0.2

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

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