Teletype Model 15 KSR, Base 213102

Introduction and Identification

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1. Overview

1.1. General

This is a Teletype Model 15 KSR (Keyboard, Send-Receive) Page Printer, acquired 2010-03-21 in Glen Ellyn, Illinois. The previous owner indicated that he had acquired the unit in 1960, and had used it both in his amateur radio hobby and at one time as a printer for his computer. It looks as if it at one time in the past had a tape perforator (at least the case has a cutout for it). It does not have the Line Test key. It does not have Line Relay or a Relay Socket at the rear, nor does it have a Polar Neutral Key. In other words, it looks to have been stripped down from its full commercial service configuration to a simpler radioteletype or computer terminal configuration.

Since this unit originally had a perforator (tape punch), and as a Model 19 was essentially a Model 15 with a perforator, it is possible that originally this unit would have been designated a Model 19. As received, however, its configuration is that of a Model 15.

It was known to be functional when acquired (I typed on it).

1.2. Components and Featurs Present and Absent

The components present as acquired are:

The following optional features are present or absent, as indicated:

Its Selector Magnets are of the "Holding Magnet" (HM) type. (It has "HM" stamped on the Range Finder; see Selector Magnet Operation Switch , below).

It does have its handle. It came with a sound-absorbing pad which does not seem to be fitted for this unit. The glass cover has at some time in the past been replaced by a Plexiglas cover, irregularly cut at the top. The original Copy Holder has at some time in the past been replaced with one from a Model 28.

1.3. Date

The 15Y Typing Unit bears patent dates not later than 1932, so that component, at least, must have been manufactured after 1932.

The Keyboard, which is quite likely not original to this unit, bears patent dates from the early 1920s, but a maker's plate which indicates Teletype Corp. (a name change made in 1929). Its model designation ("BK" for Bell Keyboard) also indicates that it was made after the acquisition of Teletype Corp. by AT&T in 1930. It was stamped as having been treated for fungus in 1944, so it must have been manufactured no later than that year.

If I had to give someone a simple answer (I'm not good at simple answers), I'd oversimplify and just say "it's from the 1930s."

2. Major Component Identification

I'm a bit obsessive about recording the various maker's and identification plates on my machines. This is actually quite sensible, since an unidentified machine is not a working machine but a puzzle. (Working on Linotypes (a little under 20A 240V 3-phase) and Elrods (23A 240V) cures you very quickly of the ill-advised "let's just plug it in and see what happens" mindset.)

That having been said, pictures of the "ON" and "OFF" markings don't really contribute to the identification of the machine. But I wouldn't be doing any of this if the machines weren't beautiful, and these plates are beautiful.

2.1. 15-C Teletypewriter Base: s/n 213102

Without removing the Typing Unit (which I don't feel like doing at this point), It's a little hard to get a clear shot at the maker's nameplate and serial number plate for the Teletypewriter Base unit. The plates are located on the left side of the machine, on the Base (of course). The serial number is easy enough to read:

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Here's the maker's nameplate, (cropped from the photo above and rotated 90 degrees):

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Here's the Teletypewriter Base model number, up close:

In addition to these necessary plates, the Teletypewriter Base 15-C also bears a nice little plate indicating who made the equipment (Teletype Corporation) and who should service it (the Bell System). (BTW, the silver screw/stud to the left of this plate should have a rubber bumper encasing it. It is one of the studs which guide the Cover down alongside the unit.)

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These several plates are all located on the left hand side of the machine, on the 15-C Teletypewriter Base unit:

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On one of the main "crossbars" of the Base, near the motor, the code "R6-60-CH" is stamped in orange. I have no idea what it means.

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The 15-C Teletypewriter Base also carries the main power requirements/limits plate (see below).

2.2. 15-M Motor Unit: s/n 129489

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It has a 15-M Motor Unit. One might think that this is just "M for Motor," but apparently this is not the case. Gil Smith has a Model 15 with a 15-D Motor Unit, for example.

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This Motor Unit is serial number 129489. This of course differs from the 15-C Teletypewriter Base s/n, as befits the modularity of the system.

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The motor itself was made by General Electric for Teletype:

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The motor specifications are legible in the full-sized image (click through to see it), but are hard to read here. The are:

The longer one messes about with old machinery the greater grows one's fondness for nice, clear, complete motor nameplates and one's antipathy towrd illegible ones. This one is just fine.

The motor service plate has a subtle difference from the others on the machine. The Motor Unit is said to be "supplied by" Teletype, not "manufactured" by them. I'm not sure if this simply means that this plate (and therefore the Motor Unit) dates from a different period, or if it is just an indication of modesty on the part of Teletype. (They did not, after all, manufacture most of the Motor Unit; General Electric did.)

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2.3. 15-Y Typing Unit: s/n 167037

The nameplate on the 15Y Typing Unit is on a relatively large "tab" built apparently specifically for the purpose of holding the plate. It is located on the upper right of the machine:

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The list of patents is interesting primarily because it starts relatively late (at least in early Morkrum/Teletype history) with 1,719,489 (filed 1928-04-20, issued 1929-07-02), a patent issued to Amos H. Shangle and assigned to Bell Telephone Labs before AT&T's acquisition of Teletype Corp. It ends with 1,937,376 (filed 1932-07-08, issued 1933-11-28 to Walter J. Zenner of the Teletype Corp). While patent dates provide only a terminus post quem (1932 in this case), the relative density of patents cited for the period 1929-1932 and the complete absence of later patents suggests that this unit might date from the earlier 1930s.

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The actual serial number of this 15Y Typing Unit appears closer to the front of the unit:

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Finally, the Typing Unit bears a Bell System service plate identical to the one on the Teletypewriter Base. It is located on the left side of the machine, on the rearward projection which holds that side of the paper roll.

2.4. BK23JX Keyboard: s/n 66805

As noted elsewhere, this Keyboard is probably not original to the rest of this unit (it has no provision for mounting a tape perforator, yet the Base and Cover do). Moreover, its model designation differs from those of the rest of the unit. They are of the form "15x": 15C Teletypewriter Base, 15M Motor Unit, 15Y Typing Unit. Other Model 15 Keyboards bear similar model designations; for example, Gil Smith's Model 15 ( has a "15-E" Keyboard. By way of contrast, this Keyboard appears to be a "BK23JX".

The designation "BK" is explained in Ransom D. Slayton's "History of Telegraphy from the Teletype Museum" (p. 11, online at Gil Smith's, ). He discusses it in terms of the early production of Model 15 units after the acquisition of Teletype Corp. by AT&T. Apparently at that time "The M15 printer was called a "BP" for "Bell Printer" [paragraph 93] and "The associated Keyboard was a 'plug-in' unit that mounted under the printer unit. Called a "BK", it used the conventional 'cushioned' keytops and most parts were from the Model 14." [paragraph 96]. Presumably "BK" stands for "Bell Keyboard," therefore.

Here's an overall view of the top of the BK23JX keyboard, removed from the machine:

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There have obviously been some keycap replacements in the past:

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The key on the lower right is the BLANK key. While it seems appropriately blank here, most photographs I've seen of other Teletypes which have such a key have an actual blank green keycap on it. I think that the keycap is simply missing here.

(Not all units had this key. To take once again the example of Gil Smith's Model 15 ( (because he's documented it so well) - his Model 15 does not have a BLANK key. (You can see the slot where its keybar would have gone, but the key itself is absent).

The nameplate itself is in a style which differs from those of the Base, Typing Unit, and Motor:

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The serial number is also significantly lower. (It is very faintly punched, and is actually easier to read in a photograph than in person.)

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The keyboard also bears a stamp reading "M.F.P." I have no idea what this means. However, Gil Smith's Model 15 (referred to frequently here) has a stamp reading "MBT" (for Michigan Bell Telephone) in a similar location, so perhaps "M.F.P." indicates the original owner of this Keyboard unit?

The patent numbers cited on the keyboard are all earlier than those cited on the 15Y Typing Unit. Arranged by filing date, these are:

In other words, these are solidly from the early 1920s (1919-1925), whereas those cited on the 15Y Typing Unit are solidly from the early 1930s (1928-1932). This would be consistent with the BK type Keyboard having been made primarily of Model 14 parts, as Slayton suggests.

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However, the (apparently original) Maker's Plate on the Keyboard contradicts this deduction, because it gives the maker as Teletype Corporation. Morkrum-Kleinschmidt changed their name to Teletype Corporation in 1929.

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Finally, the Keyboard bears two stamps indicating that it has been treated for fungus. I like to think that this was preventative treatment. This stamp is useful, however, because it is dated; the keyboard itself cannot have been made later than 1944.

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2.5. Cover, s/n 48149

The cover has what might be a serial number (or might not; I'm only assuming that it is) stamped in ink on the inside. This number is located on the back of the machine, in the cutout in the sound-dampening liner which accomodates the motor's fan.

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Here's a full-resolution view of the rather blurry other number:

3. Power

3.1. Overall Power Limits

Every electrical machine ought to have a plate somewhere near its main electrical service entrance which indicates the overall power requirements and/or limitations of the machine. (I say "ought" because this is in my opinion a Good Practice; I have not researched any more official requirements that are or have been in place with regard to this).

For Teletypes, though, things get complicated. First, a Teletype has two or three kinds of electrical input. It will in all cases have the power to the motor and a local "current loop" control circuit. In radioteletype or computer-controlled use, this current loop will in turn be controlled by some kind of adapter (typically external to the Teletype, and often user-made). In once-standard landline telegraphy, this current loop would have been controlled by a relay (such as a Western Electric 255A) switched by some form of a telegraph Line. This relay typically was housed within the Teletype Printer enclosure, and thus the telegraph Line would have had to have been considered a power input to the machine. In the 21st century we're used to thinking of communications power as trivially small, but for a Model 15, the current loop voltage is significant (120 Vdc).

Second, Teletypes of this vintage were very modular. The power service entrance is on the 15-C Teletypewriter Base module, but any number of machines could have been assembled on top of that. Moreover, these machines would be used worldwide in a great variety of power situations (AC or DC, sometimes up to 240V). Teletype addressed this by employing rather broad wording on their plate:

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I think of this as a practical warning (never exceed what the Motor specifies) and an absolute limit (never ever exceed 250V 3A). For reference the motor as installed on this machine is 110Vac 2.2A and the line is supposed to be 120Vdc 60mA.

This plate is located on the front right side of the machine, affixed to the Teletypewriter Base p/n 15-C.

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3.2. Plug

I have no reason to believe that the plug is original to this unit. First of all, it looks much more like a consumer-grade lamp plug, albeit an older one, than an industrial plug of the 1930s or 1940s. Second, the previous owner indicated that the terminal block to which the plug is attached was not present when he acquired the machine.

The plug on the machine as acquired is an Eagle (brand) two-prong plug rated at 10A for 125V (or 5A for 250V). It is not polarized. It is probably correct to call it a "Type A" plug; I might call it an unpolarized NEMA 1-15 plug, save that the distance between the blades seems narrower than a modern NEMA 1-15 plug (hence the splaying as shown in the photograph below, which came from having been forced, slightly, into a modern Type B / NEMA 5-15 receptacle by the previous owner.

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Of course, the plug is the power input for the motor only - not for the "current loop" communications line.

Here is the point of attachment of the power cord to the machine. Quite aside from the fact that the previous owner indicated that he added this terminal block, this just doesn't look factory. The soldering is perfectly functional, but more like soldering that I might be capable of (vs. the finely trained technicians of the Teletype Corp. in the 1930s) There is no strain relief on the cord (or, indeed, any sensible path for the cord outbound through the case; it just sort of runs down the side of the 15-C Teletypewriter Base inside the loose-fitting Case). Finally, of course, the cord itself is plastic-insulated, not cloth.

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3.3. Main Terminal Block

Handwritten labels can be useful.

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However, even though labels are useful, it is always good to check them, as they may be wrong. This one is. An examination of the terminal block shows that Pwr is now (as received, that is) connected to 21 and 23 (not 24 and 25):

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The previous owner of this machine indicated that the terminal block was missing when he acquired it, and that he added one. Since this terminal block looks identical to the one on Gil Smith's Model 15 ( I presume that it is a terminal block in the original style.

3.4. Fuse

The main fuse lives at the back left of the 15-C Teletypewriter Base unit. There is of course no guarantee that this fuse is of the same type as was originally supplied.

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The fuse in the machine as acquired is a Littelfuse (brand) 3AG (physical size; 1/4" x 1 1/4") 312 (Littelfuse series) fuse rated at 6A, 250V. This is a modern, generally available, fast-acting fuse.

Note, however, that such a fuse by design will not blow at exactly its maximum power rating. Its datasheet indicates that at 200% of this rating (which would be 12A here) it will blow in less than 5 seconds. However, at 135% of its rating, it will blow in some otherwise unspecified time less than one hour. At 100% of its maximum rating (6A), it will remain un-blown for at least four hours. This fuse does not, therefore, satisfy the requirements of the main power plate (never exceed 3A).

As an aside, in researching this I just discovered that the "AG" in this fuse type/size designation originally stood for "Automotive, Glass" See Electus Distribution Reference Data Sheet: FUSEPRMR.PDF (I), "Fuses: A Short Primer" at It's always fun to research even small details.

4. Settings/Adjustments As Received

Before messing about with any machine, it's good to record its initial state (even if that initial state is not "factory" and the equipment will be used in conditions other than its last use).

4.1. Selector Magnet Operation Switch (Serial/Parallel Windings)

Gil Smith's Model 15 writeup at has good information on the types of Selctor Magnets in Model 15s, as well as how to identify them. There were two types, "Pulling Magnet" types (which could only be used on 60mA circuits) and "Holding Magnet types (which could be configured either for 20 mA circuits by putting the windings in serial, or 60 mA circuits by putting the windings in parallel. This unit is equipped with "Holding Magnet" style magnets, as shown both by their general appearance (rectangular external shape to the windings) and the presence of an "HM" stamped on the Range Finder.

In some units equippped with HM style Selectors the choice between serial (20 mA) or parallel (60 mA) winding was done by rewiring. In other units, there is a switch to select this. This particular unit has such a switch. It was set, as received, to the Parallel setting. The switch is designed with a sort of a "lock" on it to keep it from being moved accidentally.

Here are the general locations of this switch, and of the "HM" stamping on the Range Finder:

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It's just a switch:

Here's a closer look at the "HM" stamped on the Range Finder.

4.2. Range Finder

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5. Instruction Plates on Cover

I think that these are lovely. I've photographed them in as-received state, without even any cleaning; they've had a hard but honorable life. The versions which come up on clicking are full-resolution PNGs.

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Sadly, the tape perforator is no longer present.

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Turning a machine on may seem important, but the ability to turn it off is imperative.

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