Nakamichi 420 Amplifier Salvage

I have had my Nakamichi 420 audio ampli­fi­er since the late 70’s, and have used it with no issues since it’s pur­chase. Recently I was lis­ten­ing to an album, when it quit pro­duc­ing any sound, and short­ly after pro­duced that burnt elec­tron­ics smell we know so well.

Nakamichi 420 and Hafler DH-110 preamplifier

After many weeks of trou­bleshoot­ing and search­ing for replace­ment parts, I final­ly decid­ed to give up on repair­ing the orig­i­nal ampli­fi­er mod­ules. It was not an easy deci­sion to make.
Both chan­nels had failed with com­plete­ly dif­fer­ent faults, with over 10 bad tran­sis­tors and sev­er­al oth­er com­po­nents. The right chan­nel board had some dam­age to the cop­per traces and areas of burnt cir­cuit board. The very expen­sive tran­sis­tors that I did order end­ed up being fakes and caused some addi­tion­al dam­age to the left chan­nel board. I did final­ly end up get­ting my mon­ey back on the tran­sis­tors, after wast­ing many hours of my time. 

Nakamichi 420 right chan­nel mod­ule before disassembly

The 420 ampli­fi­er is rat­ed at 50 watts per chan­nel into 8Ω (stereo) and a total har­mon­ic dis­tor­tion: of 0.02%. My unit had the speak­er pro­tec­tion revi­sion installed, which prob­a­bly saved my speak­ers from damage.

Nakamichi 420 ampli­fi­er dis­as­sem­bled with bag of bad parts

About the only parts sal­vage­able were the trans­former, bridge rec­ti­fi­er, speak­er pro­tec­tion board, case, and heat sink.
After quite a bit of research, I end­ed up decid­ing to design a board that uses the ST TDA7293V DMOS audio ampli­fi­er mod­ule. The spec­i­fi­ca­tions were slight­ly bet­ter than the orig­i­nal dis­crete com­po­nent mod­ules that they would replace.
While this ampli­fi­er is no longer now an orig­i­nal Nakamichi 420, the upgrade is keep­ing it from becom­ing e‑waste and now pro­vid­ing a use­ful ampli­fi­er again.

New Nichicon capac­i­tors for the main pow­er supply

The orig­i­nal capac­i­tors were not leak­ing, but were way out of spec­i­fi­ca­tions and may have con­tributed to the fail­ure of the unit.
I could­n’t find any new capac­i­tors with a sim­i­lar phys­i­cal size and capac­i­tance rat­ing. So I end­ed up with some screw ter­mi­nal Nichicon capac­i­tors. I also need­ed a new way of mount­ing them as they were incom­pat­i­ble with the orig­i­nal mounts.

Nichicon capac­i­tors mount­ed with cop­per buss bar

The orig­i­nal capac­i­tors were top mount­ed, so I need­ed a way to phys­i­cal­ly attach the new capac­i­tors to the frame. What I end­ed up with was some L shaped alu­minum to mount the clamps and then used some flat head screws to attach the alu­minum brack­et to the frame.
The orig­i­nal 0 volt capac­i­tor con­nec­tion was part of the orig­i­nal attach­ment brack­et that was no longer being used, so I made a cop­per buss bar for the new con­nec­tion point.

Back of cir­cuit board for the TDA7293V module

For the ampli­fi­er mod­ules I stayed with the typ­i­cal ST cir­cuit design, but sig­nif­i­cant­ly changed the ST typ­i­cal lay­out to suit my needs and com­po­nent choices.

I uti­lized the excess capac­i­tor leads fold­ed across the back traces and sol­dered to reduce resis­tance in high cur­rent areas of the boards. High tem­per­a­ture and rip­ple cur­rent rat­ed Nichicon capac­i­tors were used in the boot­strap­ping and pow­er sec­tions of the board. Bi-Polar/Non-Polar Muse ES series audio elec­trolyt­ic capac­i­tors were used in the sig­nal sec­tions of the boards. I also includ­ed a head­er for mute and stand­by log­ic con­trols, along with a clip­ping indi­ca­tor output.

TDA7293V mono ampli­fi­er mod­ule for Nakamichi 420

Testing of the ampli­fi­er mod­ules at 5 watts and 1,000 Hz, beat the spec­i­fi­ca­tions by ST and also the orig­i­nal Nakamichi spec­i­fi­ca­tions. I used my Ultra-Low-THD Sine Wave Generator for part of the test­ing along with my Keithley 2015 THD Multimeter. I test­ed the mod­ules at wattages up to 75 watts, but the Nakamichi heatsink gets hot very quick­ly at high pow­er levels.

Testing the ampli­fi­er mod­ule with a heatsink

I was able to reuse around 90% of the orig­i­nal wiring and coax cables in the new ver­sion of the Nakamichi 420. An addi­tion to accom­mo­date the need of a log­ic lev­el sup­ply for the mute and stand­by cir­cuits, was a small volt­age reg­u­la­tor board using a MCP1792T‑5 sur­face mount reg­u­la­tor now locat­ed near the speak­er pro­tec­tion cir­cuit board.

Nakamichi 420 Control log­ic pow­er reg­u­la­tor board
Assembled ampli­fi­er dur­ing work­bench testing

The speak­er pro­tec­tion cir­cuit was upgrad­ed with a sin­gle Bi-Polar/Non-Polar capac­i­tor that replaced a pair of old elec­trolyt­ic caps. The incan­des­cent pow­er indi­ca­tor bulb was replaced with a LED indi­ca­tor and drop­ping resis­tor.
In my opin­ion the ampli­fi­er sounds fan­tas­tic with the music type and vol­ume lev­els I like, which is what mat­ters most.

EagleCAD V7.7 Nakamichi Amplifier Rev 1.1 board and sch ZIP file

EagleCAD V7.7 5 volt log­ic sup­ply Rev 1.0 board and sch ZIP file

4 Replies to “Nakamichi 420 Amplifier Salvage”

  1. I am look­ing for some­one that can help with some Nakamichi sys­tems. As I was look­ing for help, I found your link. Do you do work for oth­er peo­ple or just your­self? What I have are 2 420 amps where the pow­er light does not come on. However, it seems like they are still work­ing. I also have a 600 sys­tem that I have no idea where to start on it.

    1. Hi Bob,
      The Nakamichi pow­er amps use a small fil­a­ment bulb for pow­er on indi­ca­tion that is prone to burn out over time. I typ­i­cal­ly replace them with an amber LED indicator.
      I real­ly like the qual­i­ty of sound from the 600 tape decks, but they are next to impos­si­ble to adjust and require spe­cial equip­ment to prop­er­ly work on them.

      Unfortunately I don’t work on equip­ment com­mer­cial­ly, but will work on equip­ment for non-prof­it char­i­ta­ble orga­ni­za­tions if I have the time. Too many projects, not enough time.

      Greg (Barbouri)

  2. Hi bar­bouri, im from Argentina.
    I have this 420 unit. In order to give a maintance..replace the bulb (pow­er on) , its 16v 40ma. Its hard to find one here. I found only in 12 and 24v. Can i replace with one of these?
    And anoth­er ques­tion. i replace the ther­mal com­pound between the heatsink and the alu­mini­um block of both chan­nels. Its recomend­ed to replace also the com­pound between the tran­sis­tors and the alu­mini­um block? Because seems dif­fi­cult to dis­sas­em­bly. thanks! great blog and works.

    1. The 24 volt lamp will work, but will be a bit dimmer.
      I would replace the com­pound between the alu­minum blocks and heatsink.
      If the tran­sis­tors have not been disturbed/removed I would leave the orig­i­nal com­pound intact.
      Be sure to check that the alu­minum blocks have a good chas­sis ground as some ther­mal com­pounds act as an elec­tri­cal insulator.

      Greg (Barbouri)

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