(It is easier to view pictures in this post in computer or tablet screen.)
I don't see many Etena pianos in Hong Kong. These piano seems odd for having Yamaha actions. Even the hammers bear the Yamaha stamp. The most recent one I encountered had black tuning pins and serial numbers that looks exactly like a Yamaha uprights from the 70's or 80's.
These information doesn't seem quite right. So I have now gathered information in other languages to form a coherent introduction.
Tenryu Gakki Seizo K.K.
Eterna（エテルナ）was originally a brand under "Tenryu Gakki Seizo K.K." (天竜楽器製造株式会社), means "Tenryu Musical Instrument Manufacturing Company". Originally an airplane part manufacturer, the company was found in 1941, in Wadacho Higashi-Ku, Hamamatsu.
In 1951, the founder Hachiro Hirano (平野八郎) transformed his company into a piano manufacturer with helps from (former?) Yamaha engineers and established the brand "Eterna". (1) (This early information might not be entirely accurate.)
During its early years, the company was most well known for making guitars, and was already making guitars on behalf of Yamaha. (12) The company went public between November 1962 to October 1969, then merged with Nippon Gakki Seizo K.K. (Yamaha). (2,3)
Concerning the company's merge with Yamaha, a source indicates that the company director Taro Ogura (小倉太郎) aimed to make Tenryu Gakki the second largest piano manufacturer in Japan, and distributed their products through Kokusai Gakki K.K. in Kobe. (4) (At the end of this post, you can find a link to a Japanese teacher's blog post, whose childhood piano was an Eterna upright bought in Kobe.)
The "Japan Piano Atlas 2017" contains Eterna models built between 1981 and 1990. The manufacturing company was Yamaha, while Kokusai Gakki was the brand owner. I was not able to find more information about Kokusai Gakki. It still exists and primarily sells traditional Japanese musical instruments.
There were over 250 piano manufacturing companies in Japan, most were bought by Yamaha. The economy was great and everyone wanted to buy pianos. (5) After the merge, Tenryu Gakki continued to manufacture OEM guitars for Yamaha. Although their piano manufacturing department remained operational, the brand's characteristics slowly faded away. (2)
I found a video of a 64-keys Eterna piano (#8448). I'd think it is more of an original Eterna because it is so strange.
Both videos below show the interior of Tenryu Gakki's pianos. One is by a passionate Japanese piano seller, who kept saying "Eterna is actually a Yamaha". The other one is a foreigner who clearly has no idea about the piano, though the video quality is best among other YouTube videos I found.
Eterna Piano Manufactured by Yamaha
Not sure when, there were Japanese made Eterna pianos made by Yamaha. According to a Japanese piano technician's blog, his client wanted to buy a Yamaha piano in 1972, but she had to wait 6-months. So the seller recommended her to buy an Eterna piano instead, saying that it is essentially a Yamaha piano. This was 3 years after the merge. (5) It is understandable that a big company such as Yamaha would introduce cheaper products under another brand to fulfill the market's need without diminishing its high-end brand.
According to another source, because Tenryu Gakki and Yamaha worked very closely together from the very beginning, Eterna pianos and Yamaha are very similar. For example, the Yamaha U1A and Eterna E1A are very alike. (13)
Introduction found in Mainland China's website says Eterna was a brand born after the merge of Tenryu Gakki and Yamaha. And Eterna pianos share the same blueprints and parts as Yamaha's U1, U2, and U3 models. Therefore, its quality is exactly the same as Yamaha's. (11)
Since I have no way to authenticate these claims, I have gathered many pictures for some visual comparisons.
This Eterna (#1724) is the earliest I have seen. Its plate design and action look different from the Yamaha U1 (#80491) from 1957.
This Yamaha piano from 1932 was before Tenryu Gakki, and has more resemblance with the early Eterna.
A slightly later Etern (#8117) looks quite similar to Yamaha U1 (#186184). See how their arrangements of bass tuning pins match.
This Eterna E1A (#80762) and Yamaha U1A (#3991581), both from 1985, are quite similar too.
To compare with U3, I was able to find a larger Eterna (#12900), which match with a 1969 Yamaha U3 (#763157) quite well.
It checks out that Eterna and Yamaha pianos share many similarities.
An Secondary Yamaha Brand for Export Sales
After series of fact checking, I believe 1983 was the year Yamaha first export Eterna to western countries, giving people in the west an impression that Eterna only existed since 1983. Most western publications and websites indicate that Eterna was only established in 1983. For example, a dutch website said "Eterna is a new brand released by Yamaha in 1983" with a factory located in China. (6) Yet, "Eterna" and Tenryu Gakki obviously exist prior to 1983. In 1983, Yamaha had not yet form a partnership with Pearl River or established any factory in China (only in the USA). None of Japanese sources mentioned any big events related to 1983.
The early Eterna uprights made for export were model Er-10, ER-20 and ER-30. Even though ER-30 consists of economical parts, they were essentially the same as the Yamaha U1. On the other hand, Eterna pianos' veneer finished was plastic sheets instead of actual wood veneer. This eliminates the discoloration of traditional wood veneer. Their black finish are all glossy. (6) There were also Eterna grands, model G430 is equivalent with C2X. (12) There might be other models, however I won't dig into all of them now.
By comparing a Yamaha U1 and a Eterna ER-30 with similar serial numbers, it seems safe to assume that Eterna pianos was sharing Yamaha's serial number system starting in the 80's. The plate design, tuning pins and plate screws look very similar, except the absence of logo in the Eterna and the presence of "MADE IN JAPAN" stamp.
According to my other post "Distinctive Features of Yamaha Upright Pianos from Different Ages", Yamaha pianos has been using black tuning pins until the 80's, to the late 80's for the U-series. Domestic models (i.e. U10BL) have black tuning pins, while export models (i.e. U1A, U1E) have chrome-color tuning pins and "MADE IN JAPAN" stamps. Although this 1988 Eterna has black tuning pins, the presence of "MADE IN JAPAN" stamps indicates that it is likely built for export sales.
Chinese-made Eterna for Export
In early 90's, Yamaha realized that their major consumer markets in Japan and Western countries were becoming saturated. Therefore, they were interested in expanding their market to Mainland China. However, people in China buy pianos directly from factories, and is impossible for Yamaha to sell their pianos through dealerships as usual. As a result, they had to open a factory themselves.
In 1995, Yamaha began partnership with Pearl River. Pearl River has a lot of experience in manufacturing pianos in China and their Guangzhou location is close enough to a port for exporting piano worldwide. Their plan was to sell 75% of pianos within China, 15% to Europe and 10% to the USA. (13)
Pearl River was responsible to build the cabinet (soundboard, frame, case) and assemble with Japanese made actions and parts provided by Yamaha. The low-cost model would be sold as "Pearl River" brand, and all pianos intended for export sales would be under "Eterna", to be sold through Yamaha USA in the US. (13) These Chinese-made Eterna piano models all have an additional "C" in their model name (i.e. ER-C10). (6) Although we see ER-C10 around, I have never found any ER-C20 or ERC30 models.
It says nothing about Chinese-made.
In Europe and North America, the "Eterna" brand is seen as a low-cost version of Yamaha that has quality similar to Korean pianos. (7) I mean, if all people see is the small ER-C10, it is hard not think of Eterna this way. A US seller says ER-C10 is a copy of the Yamaha M1, but only 100% Yamaha made pianos can bear the Yamaha logo. (10) Other sources have indicated that Eterna pianos in the US and Germany were seen bearing a "Made in China" label. (14)
Here's a comparison of Yamaha M1, Eterna ER-10 (Made in Japan) and ER-C10 (Made in China).
The End of Eterna
In 2000, Chinese-made Eterna pianos ceased to export due to poor quality. (6) According to a former technicians of Yamaha USA, "Yamaha did indeed have a "partnership" with Pearl River up until two years ago but it was a 10 year relationship forced on them by the Chinese government as the price for doing business in China. They did indeed build some Eterna pianos there but were never satisfied enough with the quality to export them...they all went to the Chinese market and their production was discontinued by Yamaha after about two years of production. That would have been about 10 years ago. Pearl River dealers have continued to claim that their pianos are or were built by Yamaha much to Yamaha's chagrin but they're being disingenuous in the extreme." (14)
This scenario sounds similar as I researched W.Hoffmann and looked into Bechstein's brief partnership with Samick. It was after the end of the partnership, Bechstein set their goal to build "all European made" pianos.
The failing partnership was bad for the company's reputation. Even before their contract ends, Yamaha established their own Chinese branch in 2002 and started manufacturing their own pianos in Hangzhou. Since Yamaha was not partnering with any company this time, they had full control on quality including certified machineries and employee training. With these improvements, the Hangzhou factory has been manufacturing quality pianos since 2004. (13,14)
Eterna Pianos are Everywhere
Most reviews by westerners on Eterna are for the smallest ER-10 model. However, there are numerous "gray market" Eterna everywhere, such asthis Eterna ("Tenryu Gakki" on plate) andthat Eterna ("Nippon Gakki") in UK , both are very early Eterna pianos. Aside from major western countries, I have also seen them in Hungary, Slovakia, Spain, Thailand, Vietnam, Indonesia and Sri Lanka.
Since Hong Kong did not have big piano manufacturer, "Gray market" pianos were popular. Therefore, it is not surprising to see Eterna pianos around us.
Serial Numbers and Production Year
From my observation, Eterna pianos with 4 to 5-digit serial numbers and all no-ER models were manufactured before 1984 or 1985. They started to share serial numbers with Yamaha around 1984 (#390xxxx and after). This change indicates that Eterna was being manufactured by Yamaha factory, likely the ER-10, ER-20 and ER-30 for export purpose. The newest Eterna with Yamaha serial number is from 1995 (#543xxxx). From 1996 to 1998, Eterna pianos were being manufactured in China and used another serial numbering system.The "Pierce Piano Atlas" (USA) has records of Tenryu Gakki Seizo K.K.'s serial numbers, but they are very early records. It seems the company was only producing 100 to 200 pianos in their early years.
The "Atlas der Pianonummern" (Germany) has a different set of Eterna serial numbers from 1996 to 1998, which should correspond to the Chinese-made Eterna pianos. (8) If Pearl River indeed only manufactured Eterna pianos for two years, they have only produced around 6000 of them. This is such a small number compared to the production volume of Yamaha's Hangzhou factory.
Other Interesting Stuff
This webpagedocuments the restoration of a pre-Yamaha Eterna piano. The first paragraph says: "It looks older than I imagined. This is a very rare model, which would normally be terminated in Japan."
This Japanese piano teacher's childhood piano was an Eterna piano, which he associated with an impression of "eternality". He only learned that the original company Tenryu Gakki was long gone after he has grown up. This fact is a complete opposite of "eternality". On the other hand, he thinks his piano is exactly a Yamaha U1.
Sources
(1) エテルナピアノ ピアノオーバーホール色の塗り替え～～黒から白に
(2) 天竜楽器(エテルナ)がYAMAHAに吸収されるまでの歴史が知りたいのですが、どなたか知ってますか？
(4) 日本国内と世界各国のピアノメーカー大図鑑（Ａ～Ｚ）約1930種類一覧
(5) 「エテルナ」というピアノについて
(6) Geschiedenis Eterna Piano’s
(7) Yamaha Eterna piano, any advice/thoughts?
(8) Atlas der Pianonummern by Jan Großbach (1997)
(9) Grey Market Yamaha Piano: The True Facts from the Piano Buyers Guide
(10) Yamaha Eterna ER-C10 43″ Console Piano in Polished Ebony
(11) ETERNA钢琴 伊特娜/伊特纳钢琴品牌_雅马哈钢琴旗下品牌
(12) エテルナとは？