Western Union Desk-Fax
Facsimile Machines

B. Hilpert / Nov 2005


For several decades, around the 1940s to 1960s, Western Union operated a facsimile service utilising what were referred to as "Desk-Fax" machines. The main objective of this service was to communicate images of telegrams between client offices and a local Western Union central-office. (See reference [2] for more about this service.)

Some time in the 1990s I received several Desk-Fax units and eventually (2005) got around to attempting to make them function. This article presents a primarily technical description of the Desk-Fax machines. Also presented here is a means of interconnecting two Desk-Faxes.

More specifically, the machines on which this article is based are labeled as type "6500" (plate on the rear), as well as with "6499A" (imprinted on the left side of the chassis). These machines are also labeled with "Telefax". The machines were actually manufactured by Seeburg Co., better known as a manufacturer of juke-box machines.

(Note: "Letter-Fax" was another service provided by Western Union for a different form size and used different equipment.)

Contents (this page):
  1. Desk-Fax Machines in Service
  2. Technical Description
    1. Signalling
    2. Scanning and Transmission
    3. Reception and Printing
    4. Synchronisation
    5. Transmission Control
  3. Operating Desk-Fax Machines Today
  4. References
Sub-pages: Related pages:

1. Desk-Fax Machines in Service


Figure 1 shows my presumption of the original operating scenario, although I have little information about the Western Union central-office or whatever was at the remote end of the network connection to a Desk-Fax.

During operation, I would speculate that, for example, a client wishing to send a message would place a form prepared with the message onto the Desk-Fax drum and then initiate an outgoing call. At the central-office the incoming transmission request might be indicated to an operator on a switchboard. The operator would select an available receiving unit, connect the requesting line to it, and start the receiver. From this point the transmission would be handled automatically until notice was given of completion of the transmission.

A few observations suggest the Desk-Fax machines were not used to communicate with each other, rather that there was a different type of machine at the central-office end:

Around the end of the 1960s or early 1970s these machines were becoming obsolete and they started showing up on the surplus market (for about $20 US in 1972 according to ref[3]). Amateur radio operators picked them up to use in radio facsimile experiments. The units I received came through this lineage and I would guess that is true of most of the units still existant today.

One of the machines I received actually had the "Desk-Fax" label replaced with one from CNCP (Canadian National / Canadian Pacific) Telecommunications. Another one had a label with both "Desk-Fax" and "Canadian National" on it, so it appears Western Union sold or leased the machines to other service providers.

15 Dec 2007: The above presumptions have, in the main, been confirmed by receipt of a page from a Desk-Fax technical manual diagramming the network equipment setup.

An Incoming Message form with the metalised
flip side of another visible underneath.
(See printing section below).

The test message received with one unit.

2. Technical Description

Some Desk-Fax Specifications
Specification Value Units Source
Drum Speed 3 revolutions / S M
Drum Diameter 2 in. M
Chopper Rate / Carrier Freq. 2536 Hz M
Chopper Wheel Slots 45 pulses / rev. M
Image Width 833.3 dots / line (revolution) CM
6.28 in. CM
Horizontal Pitch 0.00754 in. / dot CM
133 dot / in. CM
Image Height 445 lines M~
3.37 in. M~
420 lines R
3.23 in. CR
Vertical Pitch 132 lines / in. CM
0.00757 in. / line CM
130 lines / in. R
0.00769 in. / line CR
Transmission Time 148 S / image CM
140 S / image R
Form Width 6.5 in. M
Form Height 4.5 in. M
Sources: M=Measurement, R=[ref 6], Cx=Calculation from x
Appropriate to the period, the Desk-Fax machines are a mixture of mechanical, relay, and vacuum-tube technology. The image is scanned mechanically on a rotating drum to achieve the horizontal-axis scanning. The drum is moved linearly along it's axis for the vertical-axis scanning. The relays are involved primarily in the transmission and reception control protocol. The vacuum tubes are used for amplification of audio-frequency image data.

The full schematic is available, derived through reverse engineering.

Data for the table of specifications at right was obtained either by direct physical measurement, a reference book which happened to have a few specs, or by calculation from the data from the preceding sources. 'Dot' refers to the linear resolution possible (in principle) from one cycle of the 2500 Hz carrier.

The measured vertical specifications are approximate as they can be affected slightly by mechanical variations between individual Desk-Fax units. With that said, it is interesting that the vertical and horizontal pitch calculations made from these measurements produce nearly identical values.

While most of the Desk-Fax implementation might handle grey-scale levels, a little experimentation and the characteristics of the paper seem to indicate that the machines were intended only for black/white reproduction.

Top view of exposed chassis.

Sensing optics. Note chopper wheel in front of
phototube. See section on scanning below.

2.1 Signalling

The communications line for the Desk-Fax is a 3-wire connection. Two wires are a differential pair, the third is ground. Both AC and DC components are used for signalling over the communications line.

DC component: The DC component is used for a handshaking protocol to control transmission and reception, as well as communicating sync pulses from the Desk-Fax. The DC component flows in ground and in common-mode over the differential pair. Both positive and negative DC current are used, for different aspects of the protocol. The DC current is supplied by the central-office end, not the Desk-Fax. The Desk-Fax indicates to the central-office end by opening and closing the line circuit.

AC component: The AC component on the line contains the actual image data. This is essentially a CW signal with a 2500 Hz carrier, or more accurately described as binary pulse-amplitude modulation. Notice that the black and white levels are reversed for transmission and reception. The AC component is sent in differential mode over the differential pair.

2.2 Scanning and Transmission

The scanning process begins with light from a high-intensity incandescent lamp focussed to a point on the image being scanned. Light reflected from the image is focussed towards a photodetector tube but goes through a slotted 'chopper' wheel on the way. The frequency of the image data may go down to essentially DC (0 Hz) for lengthy black or white sections, but it is necessary to use an AC carrier to transmit it over the communications line. To this end, the chopper wheel interrupts the light from the image at a 2500 Hz rate to effectively generate an audio-frequency carrier for the image data.

The photodetector output is amplified by a couple of stages of tubes before driving a transformer connected to the communications line.

Note: The two lens assemblies can be adjusted for proper focussing according to ref[3], the one assembly to give a fine spot of light on the paper form and the other to give a sharp image at the aperture hole.

Note: When operating a Desk-Fax in Outgoing (transmission) mode with the top cover off, ambient light may interfere with the image as the cover is a light shield for the photodetector tube. At excessive levels this shows up as noisy vertical lines on the image spaced 0.16 inches apart.

2.3 Reception and Printing

The Desk-Fax utilizes a form of electrographic printing, requiring a special paper. The paper appears to be carbon-impregnated fibres with a sputterring of aluminum on the back side and a light-green or light-blue (I have examples of both) powdery opaque coating on the viewed side. The aluminum and carbon provide a conductive medium through to the grounded aluminum drum. High-voltage pulses on the stylus burn away the opaque coating so the black carbon is visible. During the printing process a fairly thick, pungent smoke is produced.

Incoming image data goes through a couple of stages of amplification before being presented at the stylus as pulses of around 100-200 volts.

Note: If the stylus needs replacing, reference [3] states that it should be of carbon-steel wire and suggests wire-brushes as a source.

Note: The Desk-Fax should not be operated in Incoming (printing) mode with no paper on the drum, as the high-voltage printing pulses will be shorted to the drum.

2.4 Synchronisation

To reproduce an image, the rotation of the drums of the sending and receiving machines must be sychronised in frequency (drum speed). This is accomplished inherently via the accuracy of the 60Hz power line standard and the use of a synchronous motor to drive the drum. Binding bearings or dirty surfaces in the drive train for rolling the drum can mess this up. I had to disassemble the drum roll motor in my units to replace the dried-up grease in the bearings.

Ideally, the drums will also be synchronised in phase, so that the received image will be aligned properly relative to the paper form it is being printed on. The Desk-Fax machines do not appear to have the ability to adjust to received sync information, but they do send sync pulses by interrupting the DC line current briefly once every revolution of the drum. As such, it is up to the remote end to synchronise itself to the Desk-Fax in both sending and receiving modes.

Several articles in QST magazine [ref:3,4,5] present methods of adapting Desk-Fax units to synchronise to received sync information.

2.5 Transmission Control

The handshaking protocol for control of transmission and reception uses DC signalling over the communications line.

(This section has not been completed. Following is a partial list of protocol states.)

15 Dec 2007: The protocol for one direction is detailed in a diagram on a page from a Desk-Fax technical manual.

3. Operating Desk-Fax Machines Today

It would be neat to set up a complete period operating scenario, but I have no knowledge of any of the central office equipment still being in existance. It seems that only the Desk-Fax machines made it out to the surplus market, at least in any quantity.

Attempting to get two Desk-Fax units to work together by simply connecting the communications lines together will result in a somewhat poor quality negative of the original, along with the production of copious amounts of smoke from all the black area being printed on the receiving unit from a mostly white original on the transmitting unit.

The image shown here was made in this manner. It was sent in by a reader: "by a couple of original-geek grade 11 students at Handsworth Secondary School in North Vancouver in 1971 who should have been doing their homework. Readers can see the actual machine in the West Coast Railway Museum in Squamish." It's a scan of a page from MAD magazine.

One is left perhaps, with two possible approaches to using the machines today:

Of course, the requirement for the special paper for printing also presents a problem. I'm fortunate to have a small stock of it.

4. References, External Links

  1. Examination and reverse engineering of Desk-Fax units (schematic).
  2. "The changing picture of fax" by Jonathan Coopersmith, a history of facsimile.
  3. QST magazine, May 1972, Vol. 56/5.
  4. QST magazine, Oct 1976, Vol. 60/10 (short article about improved sync method).
  5. QST magazine, Nov 1978, Vol. 62/11 (another short article about improving sync).
  6. Buchsbaum's Complete Handbook of Practical Electronic Reference Data, by Walter H. Buchsbaum, Prentice-Hall, 1973, pg 340,342 (presents a few specifications for Desk-Fax).
Late additions:

  Manual Pages | Peer-to-Peer Interface
Nov 2005