Seiko 7A38 - by the numbers

Earlier this week, a new visitor arrived at our site, by googling 'What are the Seiko 7A movements?'.

I'm not sure whether he found the answer he was looking for, and he didn't appear to hang around for very long. 

In fact, I'm surprised that he found us at all, because if you type his question into a Google search, our site comes well down the list:

So I think we owe it to him - and to ourselves, to rectify that situation. 

Reserved for basic explanation and general specification.

World's First Quartz Analogue Chronograph - nominally 1/10s, but hand actually moves at 1/20s .

15 Jewels, 4 separate stepper motors, finger tension spring on chronograph centre seconds.

Adjustable in 0.26s increments. All metal gears (apart from plastic 7A38 Day / Date driving wheel). Bla Bla Bla ....

Size 13.25 Lignes 31.1mm x 4.00 - 5.00mm (depending on configuration)

Markings are stamped (differ between 7Axx movements):

SEIKO sometimes also SEIKO TIME CORP. (on the battery positive plate) JAPAN, FIFTEEN 15 JEWELS, UNADJUSTED, <-PUSH

Early production 7A28A movements and some re-branded movements have the coil functions stamped on the anti-magnetic backplate, as per the (annotated) examples (below) posted by David Johnson on WatchUSeek: (clockwise) H.M.S, GG-5/100S, CG-S and CG-M.

See: http://forums.watchuseek.com/f9/seiko-15-jewel-seiko-7t28-movement-etc-116713-post735389.html#post735389

Reserved for 7A07 Quartz 1/10s Stopwatch

An extract from the Seiko 7A07A technical guide: http://service.seiko.com.au/i/seiko/documents/technicalguides/7A07A.pdf

A parts list for the 7A07A movement can be found on Seiko Oceania's database under Calibre: 7A07A or by using Model: U7A0720A

Reserved for 7A28

Superb photo of the 7A28A movement courtesey of Steve G: http://ninanet.net/watches/others13/Mediums/mseiko7a28.html

A parts list for the 7A28A movement can be found on Seiko Oceania's database under Calibre: 7A28A or by using Model: U7A2810A

Reserved for 7A28 'alternatively signed calibers'.

Yema N7 / Shimauchi Ltd. V905(A)

Although both Bridgestone Motorsport and Honda F1 (7A28-7160) branded 7A28's have been seen (on Yahoo Japan and eBay), I'm assuming that these used standard Seiko 7A28A movements.

Reserved for 7A34

A parts list for the 7A34A movement can be found on Seiko Oceania's database under Calibre: 7A34A or by using Model: U7A3430A

Reserved for 7A36 Surf Timer

Unfortunately the Bills of Materials / Parts Listing for the 7A36A Caliber is not found on Seiko Oceania's database.

Reserved for 7A38

A parts list for the 7A38A movement can be found on Seiko Oceania's database under Calibre: 7A38A or by using Model: U7A3820A

Reserved for 7A38 'alternatively signed calibers'.

C.G.H (Yema, Kamatz, JAZ) Shimauchi Ltd. V906(A), Orient J3920, JEPIC Corp. Y19, Ferrari Cal. 531, Junghans V906, etc. ....

Reserved for 7A48

A parts list for the 7A48A movement can be found on Seiko Oceania's database under Calibre: 7A48A or by using Model: U7A4820A

Although more may exist, the only hard evidence I have found, to date, of the Seiko 7A48A caliber having been used by another watch manufacturer, and marked in an alternative manner is in the Cartier Ferrari Formula moon-phase chronographs. These movements are signed (printed, not stamped) Ferrari Cal. 532 (along with the iconic Prancing Horse logo).

Edit: Subsequently stumbled across an Orient J38901-70 '7A48' moon-phase chrono on WUS.

See: http://www.seiko7a38.com/apps/forums/topics/show/7748702-first-orient-j38-not-j39-re-branded-seiko-7a48-spotted-on-wus

7A54

The 7A54A calibre was only used in one model, manufactured by Seiko in small volumes - the Lassale branded 7A54-7009 CAC002J.

It was an unusual 1¾" size quartz pocket watch, combining 1/10s chronograph and moonphase / date pointer in the 12 o'clock subdial.

Unlike the 7A07-0010 stopwatch, the pushers on the 7A54-7009 are arranged radially - acting directly on the movement's actuators.

Effectively the movement is a lightly modified 7A48A, fitted onto a larger diameter base-plate, to support the slightly bigger dial.

A parts list for the 7A54A movement can be found on Seiko Oceania's database under Calibre: 7A54A or by using Model: U7A5420A 

7A74

While never branded as a Seiko 7Axx movement as such, the 7A74A caliber used in the Jean Lassale Thalassa quartz chronographs, clearly belongs to the same family of 7A movements. It differs from them in having a date complication in a window at 4:30. In effect, it appears to be a lightly modified 7A34A movement fitted with a 7A38 date wheel and gold anodized back-plate and a battery positive terminal plate signed Jean Lassale Inc. Or to look at it another way - a 7A38A movement with the day wheel complication omitted.

Unfortunately, there is no reference to the 7A74A calibre on Seiko's database to be able to confirm this.

See this thread in the Non-Seiko 7Axx Discussion Area (Re-branded mvmt's) section for futher details:

http://www.seiko7a38.com/apps/forums/topics/show/7614141-jean-lassale-thalassa-quartz-chronograph-the-missing-7axx