Nakamichi 620 Amplifier Repair

I was kind­ly donat­ed a Nakamichi “System One” rack of audio equip­ment recent­ly. Overall the equip­ment was in good shape, but had been in a build­ing fire and suf­fered from sig­nif­i­cant smoke dam­age. There was not any detectable heat dam­age and only minor water dam­age.
After suc­cess­ful­ly repair­ing the 610 FM Tuner / Preamp I decide to tack­le the mod­el 620 audio pow­er ampli­fi­er next.

Nakamichi 620 Audio pow­er ampli­fi­er installed in rack

The Nakamichi mod­el 620 is capa­ble of dri­ving 100 watts per chan­nel into 8Ω (stereo). The 620 is a class B ampli­fi­er with some very impres­sive per­for­mance spec­i­fi­ca­tions.
Signal-to-noise Ratio bet­ter than 120 dB (IHF‑A)
Frequency response from 5Hz to 100kHz +0 ‑1 dB
Total har­mon­ic dis­tor­tion 0.005% at 1 KHz between 1 to 100 watts 8Ω
Crosstalk bet­ter than ‑70 dB @ 1 KHz
Intermodulation dis­tor­tion less than 0.002% (60 Hz: 7 KHz, 4:1, 8Ω load, 100 Watt)
Power con­sump­tion 50 VA at Idle, 700 VA both chan­nels clip­ping into 8Ω

Initial inspec­tion of the Nakamichi 620 amplifier

After pulling the plas­tic rear cov­er off the ampli­fi­er I was able to see some of the issues that need­ed to be addressed. Probably the most obvi­ous was the leak­ing main fil­ter capac­i­tors. The good news is that none of the caus­tic elec­trolyte from the capac­i­tors leaked onto any­thing oth­er than the plas­tic case.

Bad elec­trolyt­ic capac­i­tor in Nakamichi 620 amplifier

The next item to add to the list was some water dam­age on one of the ampli­fi­er mod­ule cir­cuit boards, along with some light dam­age to the met­al frame.
The final item oth­er than clean­ing was the dam­aged speak­er out­put jacks with bro­ken plas­tic tabs.

Nakamichi 620 frame com­po­nents removed and ready for cleaning

While wait­ing for the new capac­i­tors to arrive, it was time to do a thor­ough clean­ing of every­thing which required com­plete dis­as­sem­bly of the unit. I used my favorite cit­ric based clean­er which worked very well on all the sticky smoke residue. I was amazed at how much black soot had entered the semi-sealed case. I soaked the water dam­aged areas of the frame in Evapo-Rust which did a good job of remov­ing most of the cor­ro­sion.
The hard­est part to clean was the front heatsink fins, because of their close spac­ing and depth which required mul­ti­ple pass­es before I was hap­py with the results.

Disassembled Nakamichi 620 unit ready for cleaning

One of the cir­cuit boards had a lot of crud build-up where it had sat in a col­lec­tion point for dirty water. A lot of the dirt was attached to the orig­i­nal resid­ual flux from man­u­fac­tur­ing. I used Isopropyl alco­hol for remov­ing the orig­i­nal flux and new dirt. Once the boards were cleaned I also replaced the 47 uF bi-polar input capac­i­tor along with the 1000 uF elec­trolyt­ic capac­i­tor on each amp board.

Channel pow­er mod­ule after clean­ing and capac­i­tor replacement

The old heat sink com­pound was removed from the alu­minum mount­ing block and new ther­mal com­pound was applied before reat­tach­ing to the front heatsink plate.
An in-cir­cuit check of the ampli­fi­er mod­ules showed that all tran­sis­tors were in good shape, with no short­ed out­puts or pow­er sup­ply rails.

Lamp lev­el indi­ca­tor board before capac­i­tor replacement

The lamp indi­ca­tor board also received a good clean­ing with IPA and was fit­ted with a new 2200 uF @ 25V elec­trolyt­ic capac­i­tor which was much small­er than the orig­i­nal light blue capac­i­tor.
I was unable to find any exact replace­ment 36,000 uF capac­i­tors and end­ed up pur­chas­ing some TDK 47,000 µF @ 63 VDC B41456 series screw ter­mi­nal capac­i­tors. The new capac­i­tors required some mod­i­fi­ca­tions to the exist­ing buss bar con­nect­ing the capacitors.

Nakamichi 620 ready for instal­la­tion of new capacitors

I picked up a new high cur­rent spring ter­mi­nal speak­er block and replaced the dam­aged block on the back pan­el. I am not a fan of spring ter­mi­nals, but the new one was of much bet­ter qual­i­ty than the orig­i­nal, with plat­ed cop­per con­tacts on both sides of each terminal.

New capac­i­tors and mod­i­fied buss bars installed in Nakamichi 620

Once the unit was reassem­bled I was able to run it through sev­er­al tests which all met or exceed­ed the orig­i­nal spec­i­fi­ca­tions, or were at the lim­its of my test equip­ment.
This unit was one of the ear­li­er sold units which did not include the speak­er pro­tec­tion relay board which was added lat­er to all units. I will be keep­ing my eye out for a parts unit with a good board, or I might build my own based on the Nakamichi design.

Completed Nakamichi “System One” rack with mod­el 420 added

4 Replies to “Nakamichi 620 Amplifier Repair”

  1. I am super excit­ed to see the System One be brought back to life. I did not see how much need­ed to be done to the 630 pre-amp Tuner. I do see it in the pic­ture of the sys­tem with lights on! Great job bring them back from the fire.

  2. Thank you for the great write up. I’m in the process of restor­ing my 620 amp. Do you have a list of replace­ment parts that you can share?

    1. Hi Rubin,
      The only capac­i­tors that I ordered were the EPCOS-TDK Electronics #B41456B8479M000 units at 47,000 uF @ 63 VDC.
      All the oth­er capac­i­tors were sourced from my spares stock and have vary­ing part num­bers even in the same sizes, but typ­i­cal­ly using Nichicon, and Vishay brands.
      The only spe­cial capac­i­tors are the 47 uF bi-polar input capac­i­tors on the ampli­fi­er mod­ules which I used Nichicon UES series bi-polar units.

      Good luck on your restoration.
      Greg(Barbouri)

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