ASR-33 vital poop

From: gil smith <>
Date: Thu Feb 27 14:48:00 2003

Hi folks:

This is a summary of input from many people, manuals, and prior notes of
mine. The initial question was of the differences between the standard
Teletype 33, and the version provided for DEC systems. But I have also
added some sundy stuff as well.

Let me know of any corrections/additions.

Standard Teletype "Private-wire" ASR-33 (aka M33-asr):

- The right side module is the CCU (Call-Control-Unit). The "private-wire"
    version is for current-loop operation, and has a blank top plate.
    It is normally configured for 20-mA (but can be jumpered for 60-mA).

- The paper tape reader is also called a TD (Transmitter-Distributor),
    a legacy term from early Teletype days.

- The "standard" tape reader has a 3-position lever:

- An optional "auto" tape reader has a 4-position lever:
    In the AUTO position, the reader can be commanded on/off remotely.
    The DC1 (XON) and DC3 (XOFF) control characters are used as start/stop
    commands for the auto tape reader. Since XON/XOFF chars are used today
    as software-handshaking flow-control characters, you want to disable
    software handshaking if you are connecting to a PC.

- There is a little circuit board mounted in the pedestal stand which
    provides power for the tape reader. In later 3300-series machines,
    this was finally built into the CCU.

- from Jack Hart:
    The Model 33ASR with the "Auto" TD feature had a different
    TD control/power board underneath the machine in the back. It
    had a relay that was controlled by either the momentary "AUTO"
    switch or a stuntbox contact on the printer. There was another
    feature controlling the TD when the 33 was configured for TWX.
    When a connection to the the other end was established, the
    distant machine's answerback would trip and then send
    an "X-ON" to start the tape at the calling machine. The X-ON
    was actually programmed onto the end of the answerback drum sequence.
    Sometimes the modem itself would send the contact closure to trip the
    TD based upon seeing the RS232 Carrier Detect and/or
    the RTS/CTS leads change states.

DEC LT33 (DEC-modified ASR-33):

- The CCU is a standard private-wire current-loop (blank plate).

- Has an 8-position connector (mate-n-lock/molex?) with 6 pins populated.
    Two wires for transmit loop, two for receive loop, and two for
    reader control. Can be wired for full-duplex (two loops), or
    half-duplex (rx and tx wired in series to one loop).

- Uses the standard tape-reader (3-position lever: START/STOP/FREE)
- The pedestal has a small circuit board with the control relay .
   (and reader power supply as well?) (and DEC interface cable?)
    There are a couple of cables connecting it to the CCU.

- from Jim Haynes:
    There was a 6-wire cord and they were full duplex; two wires for
    transmit, two for receive, and two for reader control. They didn't
    use the X-on X-off characters for reader control. The CCU was just
    a plain private line blank plate. What I can't remember for sure
    is how the reader worked. I'm pretty sure they let the distributor
    step the reader, and just had a relay to enable/disable it. Seems
    like I remember a reed type relay. I guess there was a power supply
    in the base for the reader step magnet in any kind of ASR. No doubt
    the printer loop current was 20ma - no reason to use 60 when you can
    use 20. I don't know if the keyboard used a 20ma loop or if it just
    used the thing as a dry switch. But you'd probably want 20ma or so
    in it to be sure the contacts are kept clean.

- from Jack Hart:
    The DEC Model 33's has a simple circuit board attached to the
    Call Control which was operated by the computer (I think the
    computer would send a contact closure to the board on two leads
    or maybe it sent a voltage to the relay...I never used it).
    That board had a reed relay which was in series (or was it parallel?)
    with the switch on the (standard) TD. It couldn't use the
    "auto" TD since it wouldn't be able to stop the TD. The computer
    actually turned the TD on and off with the contact closure.
    The Model 33 usually had a six-wire straight Molex-type of connector
    for the DEC interface. I think the keyboard contacts were on one
    pair, the selector magnet driver input on the next pair and then
    the TD control relay on the third pair.

- John Francini provided this:
    It includes:
      - Teletype Model 33 Line Set wiring printouts
      - DEC LT33 Teletype Modifications Print Set

- some scanned 33 manuals are at:

Gil's Random M33 notes:

CCU (Call-Control-Unit)
The right side module is the CCU (Call-Control-Unit). Here is my limited
knowledge on the interfaces available; the 33s all have some version of
CCU, it's just a matter of which one.

The most common CCU (I presume) was the simple interface to a 20-milliamp
dc loop (or 60-mA) for "Private Wire Service." This one had a blank panel
on top and a LINE/LOCAL knob sticking out the front of the case. This is
the model that usually got hooked up to computers or to dedicated lines.

Then there was a "Computer-I/O" CCU, which had six square buttons sticking
out the top (at the front), plus a power button above those. It had a
bunch of TTL-logic-level signals as well as a current loop, I believe. One
of my 33 manuals discusses this one (others do not) -- it says it has a
20-pin connector, with a bunch of signals that look to be TTL-level. I
have not heard of anyone owning one of these computer-i/o units.

The "TWX" (Teletypewriter Exchange) machines had a CCU for
"Switched-Network Service," which used an external "dataset" (modem) that
mounted in the stand, and connected to a conventional telephone line, using
FSK tones for communication. These had either a rotary dialer or
touch-tone keypad for dialing the phone number, a speaker at the front, six
round buttons sticking out the top at the front, four more button/lights up
in the middle of the panel, and an optional way-cool auto-dialing card
reader. My M33 is a Western-Union-branded TWX machine that has a UCC-3
CCU, with the touch-tone dialer and a 101D modem in the stand. I think
almost all computer modems can talk to this M33 modem, when they fall back
to the lowest standard (Bell 103?). I think these may have been a
moderately-common machine.

There was also a UCC-39 CCU which had a built-in modem (but still connected
to an external line interface of some sort), and had six square buttons
sticking out the top. I think this was used for TWX service also. I don't
know how common these were.

Then there was a CCU for "Circuit-Switching Service," which apparently
refers to "Telex" service, which used a network of private lines. This CCU
had a rotary dialer and four round buttons. This is found in Model 32
(baudot) sets, not Model 33 (ascii) machines. The dialer is not for
telephone calling -- the unit connects to a special neutral or polarized DC
wire line system, and the dialer pulses the line to call a station. It is
operationally similar to TWX machines (that used a modem on the phone
lines), but it used a special dc line network and central switching system.

Interfacing -- Data Format
Baud: 110
Data bits: 7
Parity: Even, or Mark
Stop bits: 2
Flow-ctrl: None

As I understand it, most 33 keyboards generated even parity (but early ones
used bit-8 always marking). If you have a terminal emulator set for
8-data/no-parity, the chars will have bit-8 high when the parity bit is
marking (for half the chars typed on the 33 keyboard) -- bit-8 high is
non-standard ascii, and the terminal may display ibm-extended chars or
something else.

I believe bit-8 is ignored by the 33 printer, so you should be able to send
chars to the 33 when the terminal is set to 7-data/any-parity, or
8-data/no-parity. However, 1-stop bit may be a problem for the 33 to keep
up with streaming data. Also, you don't want flow control set to xon/xoff
if your 33 has the special option for an auto-tape-reader (it would have
the 4-position lever, not 3-pos).

Interfacing -- Private-Wire current loop
The transmitter loop output is effectively a series connection of the
keyboard and tape reader sections (via the rotating distributor).
Electrically the TX loop looks just like series switch contacts.
The receiver loop input connects to the an electronic circuit, and is
polarity-sensitive. It is usually configured for 20-mA, but can be jumpered
intenally for 60-mA operation. I have used it with a 30V loop supply, and
am told it runs fine down to a 12V loop. The max voltage for the loop is
unclear -- I have seen 45V and 70V listed as the max (I'll stay at 30V to
be safe).

For full-duplex operation, the TX and RX lines connect to two
externally-powered loops. For half-duplex operation, the TX and RX lines
connect in series to one externally-powered loop.

There is an internal loop supply, but it is used in local mode only, so
that the keyboard can drive the receiver.

I have some schematics for rs-232-to-current-loop converters at:

Interfacing -- TWX dataset (modem)
For TWX machines with a dataset (modem), you should be able to connect to
it (at 110-baud) using a 300-baud computer modem. 300 may be the lowest
rated modem standard, but they pass anything up to 300-baud, as they are a
simple fsk (freq-shift-key) design, using two tones for tx (mark/space),
and two tones for rx (mark/space). The definition of which freq-pair is
tx, and which is rx, determines which end is "originate" and which is

Paper Tape Readers
The "standard" tape reader has a 3-position lever, labeled START/STOP/FREE.
An optional "auto" tape reader has a 4-position lever, labeled
START/AUTO/STOP/FREE. In the AUTO position, the reader can be commanded
on/off remotely. The DC1 (XON) and DC3 (XOFF) control characters are used
as start/stop commands for the auto tape reader. Since XON/XOFF chars are
used today as software-handshaking flow-control characters, you want to
disable software handshaking if you are connecting to a PC.

There is a little circuit board mounted in the pedestal stand which
provides power for the tape reader. In later 3300-series machines, this
was built into the CCU.

typewheel/keyboard for "latest" ASCII code, variable customer-activated
options (auto/manual tape punch, auto CR/LF function, even-parity
keyboard...), and included, as standard, various features that were
previous options (paper-low/out sense, DC1/DC2/ENQ/EOT function contacts,
end-of-line space suppression...). My manual shows that they have a
one-piece cover, so the tape reader/punch didn't look added-on -- but I
think early 3300 units had original covers (separate covers for the reader
and punch, like the classic asr). The 3300 CCU (Call-Control-Unit) for
"private-wire" service (20-mil loop) also included the power supply for the
tape reader. Earlier 33s had this little board mounted in the stand.

The M33 (and the baudot M32) were designed for light-duty use. They did
not heat-treat parts, adjusted some things by bending parts... The 33/32
just seems to wear out after a while -- someone said about 1500 hours of
runtime will be about it. The elapsed-hour counter in my 33 says 1495, so
I should have a good 5 hours left! Your unit may or may not have an
elapsed-hour meter, since it was an option.

Before you spend big bucks on ebay
A Model 33 has a four-row keyboard (not including the space bar).
A Model 32 has a three-row keyboard (not including the space bar).
The 33 is ascii, and the 32 is baudot.
You cannot (prctically) modify a 32 into a 33.

There is a 2-volume desciption/adjustments manual set, and a third parts
manual -- the three manuals are often on ebay for $20-$40.
Schematics are harder to find. Also check these sites (scroll down):

**DO NOT** use WD-40, since it gums up over time. I use 3-in-1 oil for
light lubing, 30W motor oil for heavier needs, and lithium grease for gears
and cams. Some folks don't even like 3-in-1, and will use a 10W for light

For paper tape, check out ebay or:

Note that the paper-tape for the ASR-33 should be 1" wide, oiled paper.
Non-oiled paper or mylar tape will wear your punch pins (so I'm told). The
7/8" or 11/16" paper tape for baudot machines will not work at all.

For standard roll paper, check out ebay or:

For sprocket-feed paper, you have to special order paper that is 8.5" wide,
with the holes on 8" centers. Sprocket-feed machines are pain in the ass,
since standard teletype roll paper (or letter sheets) will not feed
through, nor will the commonly-available "continuous-form tractor-feed"
computer paper which is 9.5" wide, with tear-off sprocket holes. I have
located a special-order paper at Office-Max, which is indeed 8.5" wide
sprocket-feed, 3500 fan-fold sheets to a box. But this is a "green-bar"
computer paper -- I have not found white or buff. I did find an unusal
ROLL of sprocket-feed paper once.

For platen refinishing, check out:

Teletype used the same ribbon for most of their machines, apparently the same
one used in old Underwood manual typewriters. Newer replacement ribbons
are nylon and only lightly inked so they don't last as long, but they work
fine. I found a decent $3 black ribbon at Staples: Dataproducts #R3300
(replacement for Okidata ML80/82/84/90 printers).

SMD (Selector Magnet Driver) Board
I found un-soldered power resistors on the selector-magnet-driver board in
my 33 -- it's a shi**y pcb design, with no thermal considerations at all.
I have run across another fellow with the same problem. I repaired it with
heavy buss wire and solder. This board amplifies the 20-mA input to
500-mA, for driving the typing mechanism magnet (solenoid).

I took some large gauge non-insulated buss wire, and tack soldered several
sections of it to the traces from the hot resistor pads (to the next
component along the trace). After it was tacked in place, I twisted the
wire around the power resistor leads and soldered it, then soldered all
along the trace to bond the wire -- this left a nice big mound of
solder/wire extending away from each of the power resistor pads. This
beefed up the mechanical strength, and added some thermal mass to help pull
the heat away from the big frickin' resistor leads.

Stuff I need
My 33 is a sprocket-feed machine that uses 8.5" wide paper, with the holes
on 8" centers. I'd like to find fan-fold paper in white, buff, or even
green-bar. I'd also be interested in rolls of sprocket paper.

I could use a copy stand for a 33 (or 32).

Also, this TWX machine uses a touch-tone dialer and dataset (modem). It
has the UCC-3 version of the 33's CCU (call-control unit), with a
touch-tone keypad, and connects to an external 101D dataset. I need to fix
the dataset. I have the standard 33 manuals, which have simplified
schematics and some theory on the UCC-3 CCU. But, full schematics would
let me check things properly.

Does anyone have a manual/schematic on the UCC-3?
Does anyone have a manual/schematic on the 101D?
How about the more-common 101C?
Does anyone know the difference between a 101D and a 101C?



; vaux electronics, inc. 480-354-5556
; (fax: 480-354-5558)
Received on Thu Feb 27 2003 - 14:48:00 GMT

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