[GreenKeys] Extel TTY printer

[Sheldon Daitch]

George,

catching up on a bunch of older emails, and that is a great write-up on the
Extels.

AP had some Extels, but only if the customer paid for it.

73
Sheldon

On 12/6/2011 2:17 PM, George B. Hutchison wrote:
> GreenKeyers - - -
>
> The Extel Corporation was formed by some EX TELetype employees, thus
> the EXTEL name.
>
> They came in a number of flavors and capabilities, and the ones I am
> (was) familiar with were units used by UPI to replace the model 15s
> which were on their last legs due to a severe shortage of parts such
> as selector cam sleeves, felts, and the like.
>
> The main boards as I recall started out using the Intell 4004
> processor, and shortly thereafter the 8008 was the processor of
> choice.
>
> Baud rate was controlled by a crystal oscillator. I think 153.6 Kcs
> was the 100 speed.
>
> Early machines had capability of only one speed, and a few months
> later with the improved logic boards you would find they had
> two-speed capability, using two different crystals and a selector
> switch.
>
> Input signals were through a number of different input boards.
>
> Initially all UPI machines were 60 mil current loop from the Lenkurt
> 25A data sets. The same input board could also be configured to
> accept RS-232 signalling.
>
> About eight months into the use of Extels an input board that was
> actually a modem designed to operate with the Lenkurt 25A FDM
> frequency channeling plan started showing up on all new installs.
>
> The print heads were actually seven pin arrays driven by tiny
> solenoids.
>
> The early  print heads were flawed. I think the needles were made of
> carbide. They were easily broken.
>
> Each needle had a head that was an iron slug which was quite a bit
> larger than the needle itself, and there was a small spring
> underneath the head of the slug to withdraw it from the coil. The
> coils were loctited and screwed into the print head body, and the
> retainer caps on the coils were also blessed with thread sealant.
> Replacing broken needles was dicey at best, and had to be done after
> applying heat with a heat gun, and a lot of hope that you didn't get
> it so hot that the coil assembly twsted in the body of the print
> head.
>
> The solenoid coils themselves were set up in a carriage array with
> four coils on one side and three on the other. The coils were
> connected to a small piece if circuit board that acted as the
> terminal for a cable made of 10 thin copper traces laminated with
> some manner of white plastic that really stunk when it got hot.
>
> Hundreds of different computer-type cables  were made up of that
> type of cable. It was common and was very cheap.
>
> That cable was also a major Achilles heel in the Extel. As assembled
> at the factory,  the cable plugged into a dual inline connector on
> the logic board, but used only a portion of the connector. The
> carriage spacing motor was plugged into the same connector, as did
> the platen drive motor, ribbon drive motor, and the carriage home
> and right margin switches. All motors were stepper motors.
>
> The Achilles heel mentioned came about from the way the cable was
> affixed to the main deck of the printer and the way it was clamped
> in place.
>
> When the carriage was at the left margin position the cable was
> clamped so as to allow about a 5/8 inch radius loop as it emerged
> from under the clamp. As the carriage traversed to the right the
> radius of the loop would get larger until it was about 1 1/4 inch
> maximum.
>
> The change in radius had the effect of cold-working the copper
> ribbons in the cable such that over a period of a few weeks, running
> 24/7 in radio wire service, the copper got brittle and started to
> crack inside of the lamination. The cracking caused the resistance
> of the cable to slowly increase, choking off the current to the
> needle solenoids until they ultimately were totally open. The effect
> on the printing was the dots started getting lighter and lighter
> until they didn't print at all.
>
> Another problem with the print heads was the issue of "focus".
>
> Properly adjusted the needle guide would allow printing a nice
> vertical line on all characters.
>
> The needle guides on the early Extels were made of some inferior
> material that would wear quickly, as the needles were hard and the
> material was soft. As the holes in the guide wore, the dots of each
> character would become staggered, making it hard to read.
>
> Lubrication was afforded the needles by squirting a small dab of
> lithium grease in the pocket behind the nose of the guide.
>
> Early design focusing was accomplished by shims underneath the
> screws that held the guide to the print head body. Later designs
> used screws and nuts in a jacking arrangement that allowed the tech
> to correctly position the guide so that the needles were printing
> nice, straight vertical lines.
>
> The later production needle guides were also of a much better
> material and would last ten times longer than the original material.
>
> Many of us RCA techs became profocient at replacing the cables. A
> new print head was about 102 dollars, and the RCA bosses would
> scream bloody murder if we didn't at least try to replace the
> cables. A benefit of replacing the cables in the field was that the
> radius problem previously mentioned could be eliminated by adding
> about 2 1/2 inches to the length of each cable, tuis allowing the
> cable loop radius to be pretty much constant.
>
> Extel also produced a line of non-typing reperforators. Tha main
> user of these machines was in newspapers for use in the typesetting
> systems.
>
> They were extremely quiet machines, and were quite durable unless
> for some reason the punch block got screwed up, and for that the
> only answer was to replace the punch block. Teletype corporation
> punch blocks would endure some manner of field repair, but the Extel
> punch blocks were dead meat if they were mistreated in any way. It
> was mandatory to use tape of recent manufacture. If the tape was too
> dry, the grim punch block reaper was sure to visit.
>
> The logic boards were a troubleshooting nightmare, and I cannot
> recall ever seeing any information on that subject, other than the
> schematics, and they were a challenge with which to try and work on
> a board. Logic loops nested within logic loops ad infinitum.
>
> And that is probably more than one would ever care to know about
> Extels.
>
> As I recall I sent all of the Extel parts I had to Tom Kleinschmidt
> several years ago, save for a little bag of spacing pawl assemblies
> I saw recently. I don't recall what I did with all of the manuals.
>
> George - W7TTY
>
>
>
>
>
>
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