2019/12/08

Nakamichi SR40 STASIS Stereo Reciever

nakamichi_SR40_1.jpeg

Nakamichi SR-40 STASIS Stereo Receiver

An amplifier with a tuner released in 1987. My audiophile friend asked me to this amp if I could repair it.

gSR-40h has a STASIS power amplifier circuit. STASIS is a non-feedback type amplifier developed by Dr. Nelson Pass at Threshold.

Nakamichi received a patent license in 1985, and in the following year it was first commercialized in the power amplifier "PA-70 / 50" series. At that time, Threshold released only super-high-end products and licensed Nakamichi to develop products in the middle range and below.

There was no pre-main amp product at Nakamichi at that time, and the stereo receiver product with STASIS was the SR-40 (SR-4 in overseas), and it was a high-end model with a sales price of 148,000 yen. There seems to be a product called gSR-30 / 20h in the lower model.

Until this "SR-40" was manufactured in Japan, but the models after that seem to have been manufactured overseas, and the product quality seems to have deteriorated considerably. The successor model was renamed gTA-40 / 30 / 20h and was released until around 1990.

SR-4A-brochure.jpeg

At that time, stereo receivers were very popular in North America, and the market size was also large. In the catalog of gSR-40h, gThe Audiophile Receiverh and gRemote Control STASIS Receiverh are displayed and it seems that it was appealing as a stereo receiver for audiophiles.

SR-4A-bro-2.jpeg

The appearance of the gSR-40h is so good that it cannot be seen by an audio product 32 years ago. The owner said that sound was coming out, but sometimes it was also mixed with noise.
However, I was drowned to turn on the power suddenly, so I opened the case and checked it.

nakamichi_SR-40_inner-1.jpeg

The lower board is the main amplifier board, and a fairly large heat sink is placed in the center.
Dust was piled up on the board, but no electronic parts were burned or burned out.
The old famous device Sanken 2SA1491 / 2SC3855 was placed in a 2 parallel configuration in the final stage.

nakamichi_SR40_inner-2.jpeg

Since it is almost composed of discrete parts, the repair is a little easier.
However, this is the first time I have seen an amplifier with so many styrene capacitors. Styrene capacitors are famous parts because of their good sound, but they are very sensitive to heat (heat-resistant temperature is about 85C), so if a damaged part is found, it will be very difficult. Since it is already discontinued as a part, whether there is dead stock somewhere ...

nakamichi_SR40_inner-4.jpeg

Since there is also an AM / FM tuner board on the front panel side, it is a fairly complex and high-density mounting. To improve the sound quality, it may remove the tuner circuit.

nakamichi_SR40_inner-3.jpeg

The power supply has a large toroidal transformer, and the two large electrolytic capacitors were still safe.

I tried to leave it for a while after turning it on, but it did not generate any heat, abnormal vibration, or smoke. Because of the large heat sink, the temperature rise of the transistor is also mild.

When I was fine and put the music signal in the CD input and turned up the volume, the sound came out from the speaker properly. The main volume can be controlled smoothly without any rust, but the volume knob is quite heavy because of the motor control.

I also found that the display brightness of the AM / FM tuner was low, making it very difficult to see. When the FM antenna was connected, the signal meter was also displayed and the FM received sound could be played properly. I was surprised that the reception sensitivity was better than my FM tuner.

After a while, there was a symptom that sometimes the left channel was not output, but I found that the tone control volume seems to be poorly touched.

It seems to be mainly due to poor contact of the signal path, so it seems to be okay to not complete overhaul. For the time being, I'm trying to clean the contacts in questionable parts.

I've never heard of a STASIS amp, but it sounds like a high-end amp rather than a stereo receiver.
It's not particularly powerful, but it's a bit of a beautiful sound!

The reason why it was not popular in Japan was probably due to the product form of the receiver.

According to reviews from overseas users on the Web, the gSR-40 (SR-4)h has the impression that it sounds better than the gPA-50 (PA-5)h power amplifier with a STASIS circuit.

Around 1990, Nakamichi broke down in terms of contract with Threshold, and was unable to sell amplifier products with STASIS circuits.

SR4_ServiceManual-1.jpeg

This gSR-40h is a complex implementation, so it would be troublesome to completely disassemble and clean it.@I got a service manual, so I would like to repair it in the meantime.B

Note (2019/12/11):
In the running test, sometimes the left channel was not output! Not only due to poor contact in the tone control section. I seem to have to overhaul it.
posted by toons at 23:58 | Audio