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.

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.

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.

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.

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.

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.

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.

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.

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.


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
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.
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)
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.
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)