Nakamichi BlackBox Series Refresh

I was gen­er­ous­ly donat­ed three Nakamichi BlackBox series units, by the same per­son who donat­ed the “System One” rack from a pre­vi­ous post.
The units were in great con­di­tion, and more than like­ly could have been used as-is. They also sur­pris­ing­ly came with their orig­i­nal own­ers man­u­als, which usu­al­ly get mis­placed or lost over the years.

Nakamichi BlackBox mod­ules PS-100 BA-150 EC-100

The Nakamichi BlackBox series con­sist­ed of sev­en add-on mod­ules for adding fea­tures to an audio sys­tem. A required mod­ule is the PS-100 pow­er sup­ply which pow­ers the oth­er mod­ules. It sup­plies a reg­u­lat­ed plus and minus 10 volts DC at 200 mA to the oth­er mod­ules using a 4 pin DIN cable. The PS-100 also sup­plies a mute sig­nal for a short peri­od of time after pow­er up and pow­er down thru the cable to mod­ules that uti­lize the sig­nal. The PS-100 only has one out­put con­nec­tor, but the oth­er mod­ules have a IN and OUT con­nec­tor allow­ing addi­tion­al mod­ules to be daisy-chain con­nect­ed to each other.

Nakamichi PS-100 pow­er sup­ply with orig­i­nal capacitors
Nakamichi PS-100 new capac­i­tors, load testing

The sup­ply uses a 48 volt cen­ter tapped trans­former with rec­ti­fied and fil­tered plus & minus 24 volts sup­plied to the reg­u­la­tor sec­tion. Zener diode ZD401 is the volt­age ref­er­ence for the reg­u­la­tors with Q402 & Q406 pass tran­sis­tors which are attached to a heatsink reg­u­lat­ing the out­put volt­ages.
All the orig­i­nal elec­trolyt­ic capac­i­tors were replaced using Nichicon low ESR units. After clean­ing and load test­ing the sup­ply was reassem­bled and ready for service.

Nakamichi BA-150 Bridging Adaptor back panel

The BA-150 bridg­ing adap­tor mod­ule allows bridg­ing a pair of stereo pow­er ampli­fiers into a more pow­er­ful mono ampli­fi­er pair. When used with the Nakamichi 420 and 620 pow­er ampli­fiers it allows an out­put of 120 or 350 watts per ampli­fi­er, respec­tive­ly. This dou­bles the avail­able volt­age swing at the speak­er load com­pared with the same ampli­fi­er used with­out bridg­ing. The bridg­ing does­n’t increase the cur­rent capa­bil­i­ty of the ampli­fi­er though, typ­i­cal­ly requir­ing a min­i­mum speak­er load of 8 Ohms.

Nakamichi BA-150 bridg­ing adap­tor with new capacitors

The Nakamichi BA-150 is iden­ti­cal to the BA-100 with the excep­tion of an inter­nal pow­er sup­ply, and added DIN con­nec­tors. Once again all the pow­er rail elec­trolyt­ic capac­i­tors were replaced with Nichicon capac­i­tors. The audio sig­nal cir­cuit elec­trolyt­ic capac­i­tors were replaced with upgrad­ed BP (Bi-Polar / non-polar) Nichicon capac­i­tors of equiv­a­lent capacity.

Nakamichi EC-100 Electronic Crossover orig­i­nal capacitors

The final mod­ule is the EC-100 elec­tron­ic crossover BlackBox.
The EC-100 allows adjustable sep­a­ra­tion of low-fre­quen­cy and high-fre­quen­cy sig­nals from an input which when rout­ed to an ampli­fi­er allows dri­ving a 2‑way (bi-amp) speak­er sys­tem. The mod­ule includes a high-pass fil­ter, a phase shifter, and an adder to accom­plish the separation.

Nakamichi EC-100 elec­tron­ic crossover with new capacitors

Just like the BA-150 all the elec­trolyt­ic capac­i­tors were replaced and I also used a pair of Polymer elec­trolyt­ic capac­i­tors to fur­ther reduce any noise and rip­ple at the pow­er input section.

Nakamichi EC-100 PS-100 and BA-150 own­ers manuals

All of the mod­ules were test­ed on the work­bench using my new­ly refur­bished HP 3314A func­tion gen­er­a­tor. I have ordered but am still wait­ing on the DIN inter­con­nect cables to be able to put the BlackBox mod­ules to use at the time of this post.

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