Dating early Japanese radios
One can often determine the approximate build date of a given radio by looking at date stamps on parts inside. When lucky, a more precise build date reveals itself on inspection stamps, serial numbers, etc. And, even without date stamps, a more general build date can be calculated by examining the type and revisions of transistors and other parts used.
The following document begins a series in which I intend to identify simple methods for dating individual radios. I'm starting with the "elec caps" (which we call electrolytic condensors in Japan) because these alone can already narrow the build date down to within what I'd reckon is about a month or two of the actual date of production, and the caps that are date stamped are quite easy to read.
Part I - Reading the date stamps on electrolytic capacitors
Only a couple of the Japanese "cap" makers had the foresight to include consistent series of date stamps with which to tantalize collectors today. Fortunately, though, the two brands of caps that are thus stamped predominate in early Japanese transistor radios. I've chosen to use samples in the photos below from radios that already appear on the site in the hopes that they help add some context. I won't bother mentioning capacity of the caps because the samples that include the date stamp do so regardless of the component's capacity and carry the same company logos and color schemes; only the size (and capacity of course) differ.
I recommend reading the stamps on as many caps as possible in the radio you are looking at and calculate the build date as falling approximately one to two months after the date on the latest stamp.
A. NIPPON CHEMI-CON (Nippon Chemical Condenser Co. Ltd.)
NIPPON CHEMI-CON was making electrolytic capacitors long before the advent of the transistor and the company's caps were widely used in early Japanese transistor radios. They appear by the autumn of 1955 and were used beyond 1960, so we are well covered for this discussion that focuses on the fifties. You will find them in most early Sony radios and in the first transistors radios from NEC, Standard, Hitachi, Sharp and other makers.
Be forewarned: As often as not the stamp is oriented such that it faces the circuit board, the speaker, or is on some other angle that escapes view. So, if you wish to read the stamp, you must often be prepared to become an extortionist, to master the use of small mirrors, or to save the reading until the day you dismantle the radio.
To begin, here is a rough approximation of the logo you are looking for:
The dating scheme is quite straightforward. The first pair of numbers represent the month, the next pair is the year, the third pair is the day, and your guess is as good as mine when it comes to deciphering the last digit. This gives us the formula "MMYYDDx" to work with; let's apply it to the couple of samples that follow.
(i) Earlier NIPPON CHEMI-CON caps in a silver tubular package covered by a clear sleeve made of plastic or vinyl are often found in radios made between the fall of 1955 and early '57. The photo above shows a sample taken from my light green TR-6 that is stamped 0556241. Using our formula "MMYYDDx", we can read this to mean the part was produced on May 24th, 1956. Note that I haven't spotted a date stamp present on the earliest silver caps from the first couple of months of 1956. Then again, there weren't many radios built during this period, so it won't be a major concern to most. If you have a silver NIPPON CHEMI-CON cap dated earlier than April '56, please write and let me know!
Here's a couple more views of the silver version showing slightly different representations of the company logo, and a third photo on the right for no good reason whatsoever except to show the printing of the capacity on that particular part.
(ii) In this photo we see the large paper cap from NIPPON CHEMI-CON found in a few of the earliest Sony radio models; this includes some TR-72, TR-6 and TR-66 sets. The sample here is from my early production green Sony TR-66. Once again applying the formula "MMYYDDx", the date stamp 0756032 reflects that the part was made on July 3, 1956. As a trivial aside, the earliest stamp I have seen on this type is 0156261 -- January 26, 1956; it's from one of the earliest TR-72 radios.
(iii) Now we get to the most commonly seen variation in black shrink tubing. The cap seen on the left is stamped 0757294, so we know it was made on July 29, 1957. The cap in front of it is positioned such that the stamp can't be read, but the one to the right is stamped 0857031, representing a production date of August 3, 1957. These caps are found inside the Avegon (NEC NT-7H), a model that was first released in March 1957. As recommended above, it's best to read the stamps on as many caps as possible in the radio you are looking at and calculate the build date as being a month or two after the date on the latest stamp found. In this case, the Avegon has seven of these caps on the circuit board. The earliest is dated July 29th and the latest August 3rd, so we can extrapolate from the stamps a build date for this unit of around late September or early October '57.
As a side note, I find in most cases that the dates on the caps in any given radio are close to each other. Still, the odd exception exists where the date stamps are a month or two apart, likely because of parts being found on a shelf in inventory or recycled from somewhere around the plant. Again, the obvious thing to do in such cases is to calculate based on the later date.
B. NATIONAL & KADOMAX (Matsushita/Panasonic)
Electrolytic capacitors made by National begin to appear in 1957 and become fairly common by '58. Reading the dates on those made in the spring of '58 and later is easy using the formula "YMMDDx" or with a space as "YMM DDx" to decipher the 6-digit code. All evidence I've seen to date indicates that the ones branded with the KADOMAX names are in fact OEM parts made by National, and these appear the same in all manners other than the brand name.
The National cap pictured here was made on July 11, 1958. I've shown it from two different angles so you could get a good look at the company logo. The logo is a circle with what looks like a bolt of lightning through it and the name "National" written in Japanese phonetic across the center. The sample above is from the National AB-145.
Now for the exception:
Early National/Kadomax caps made before March '58 offer a bit more of a challenge for Westerners. The formula here is "YY.MM." with an inexplicable letter off to the right, or "YY. M." (with a space after the first period) for single-digit months. Now, the trick is that the year is not given according to the western calendar. Rather, it uses the Japanese "nen-go", or era name, which follows the year of the reign of the current emperor. The Showa emperor ruled the country from 1926 until his death in January 1989. Without getting into a long explanation, 1955, the year the first Japanese transistor radio shipped, corresponds to the year "Showa 30". Okay, let's apply this knowledge to the cap shown above from a Standard SR-F23. It reads "32.11. U" meaning the cap was made in November 1957, (or November of the year Showa 32). It's unlikely you will see many caps labelled this way, but it is good to keep the "nen-go" system in mind as it can be found as a dating method on other parts within early Japanese transistor radios and related literature as well.
Okay, we're just about done. Having come this far, I figured it a good idea toss in one sample of a cap that carries the KADOMAX brand logo. And for fun I've made it one that uses the "nen-go" dating scheme. The stamp reads "33. 2. J" which corresponds to a production date of February 1958.
C. Brands of elec caps that don't reveal a date stamp (or require further study)
(Brand name noted first followed by the company name in the late fifties)
NCC (Matsuo Electric Co. Ltd.) Found in a variety of early Sony radios
Elna (Sankosha Mfg. Ltd.)
Fox Chemi-con (Kitsunezaki Electric Co. Ltd.) - trivia: Elna and Fox merged in 1968 to form Elna Co. Ltd.
Sanyo (Sanyo Electric Co. Ltd.)
Sharp (Hayakawa Electric Co. Ltd.) - That's the caps; a number of early Sharp radios do use Nippon Chemi-con caps
Toshiba (Tokyo Shibaura Electric Co. Ltd.) - Fortunately, there is another easy way to date most Toshiba radios
Rubycon (Shinei Telecommunications Co. Ltd.)
While dating elec caps and other internal parts has long been standard practice for my radio buddies in Japan, I'd like to take a moment here to recognize the exceptional work of Fred Mason. Fred published a document on the Internet some years ago in which he identified the two brands of caps above that reveal date stamps. (That's three brands if we include one OEM variation of the National caps.) Fred used sharp observation, perseverance and close examination in evolving his theory unaided, and his article surely helped to make reading date stamps intelligible to collectors in the West. In other words, those who came across Fred's article before already know how to handle most of what I've presented above. Ladies and gents, a big round of applause for Fred, please!