[GreenKeys] Extel TTY printer

[George B. Hutchison]

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