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  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.)
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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:
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.
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.
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.
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, 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.
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  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.
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.
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.
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.
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