[GreenKeys] seeking info on ASR-33 interface board

[gil smith]

Hi Brian:

If you have a line/off/local switch, you have a "private-wire" CCU (the 
call-control-unit is the interface module in the right side).  Private wire 
CCUs are for 20-mA current loops (but can be strapped for 60-mA) -- they 
should have a blank cover plate, though many got hacked with lights and 
switches.

There is a 9-position terminal strip in the rear of this CCU, where you 
connect the typing-unit/punch receive (input) loop, and the keyboard/reader 
send (output) loop (or strap them in series for a single half-duplex 
loop).  Looking at the terminal strip from the rear of the chassis, the 
leftmost screw is terminal 1, and the rightmost is 9.

The send (output) loop is on terminals 3 and 4, and has no polarity.  The 
receive (input) loop is on terminals 6 and 7 -- note that terminal 7 must 
be positive and 6 negative, when you hook up to an externally-powered loop 
-- if you are connecting to the LV-OUT loop on tty-connect, the negative 
tip of the 3.5mm plug will connect to terminal 6, and the sleeve to terminal 7.

If the M33 is running open in local mode, the internal (local-only) loop 
supply is not providing current to the selector mag driver.  My first guess 
would be the sel-mag circuit board in the CCU, which you will recognize as 
a small board with several large power resistors, and possible burning of 
the fiberglass board under the resistors.  Many of these boards have 
unsoldered or cold solder joints at the power resistors, due to excess heat 
and a crappy pcb layout that allows the hot resistor leads (and vibration) 
to eventually compromise the solder joints.  I had a resistor flopping 
around, connecting intermittently on one of these boards.  Remove the board 
to check the solder joints of the power resistors, and look for cracked 
traces as well.  Before putting it back in, clean the board edge contacts 
with a white pencil eraser (not an abrasive red eraser).  If that does not 
fix it, then a bit of measurement will be needed to find the problem.  Have 
you tried it in line mode with an external loop supply?

As for the interface board you found, I'd say that it would definitely be 
the MITS folks (of Altair fame), given the 1976 date.  I don't know the 
Altair details, but other S-100 machines often had a port for a tty 20-mil 
loop available on an interface card.  In that case, they would connect to a 
standard private-wire CCU on a 33.  If your MITS board is inside the CCU 
area, I'd suspect it is a 232-to-20mA converter, that will wire to the loop 
connections on the 9-pin terminal strip inside the CCU, and, I presume, 
bring out a cable to a DB-25 for 232 connection to pins 2/3/7.

I'd love to see some pictures of the MITS board and the rest of the machine.

gil


At 09:34 PM 8/3/2004, you wrote:
>Hi Y'all,
>
>I just popped open an ASR-33 I picked up a few weeks ago. The tty ran
>OK in local mode in the store where I found it but in shipment to me
>something happened, it now free runs (runs free?). Maybe it's a loose
>connector.
>
>In the electronics bay on the right hand side, there's a printed
>circuit board that runs from front to back. It's definitely a power
>supply, and probably also an RS-232 to current loop converter. In
>front is the line/off/local switch, and in back are several large
>Molex connectors. I was surprised to see this printed on the circuit
>board:
>
>MITS INC
>TELETYPE INTERFACE BOARD
>REVISION 0
>COPY RIGHT 1976
>
>Does anybody know if this the Altair MITS? The date seems about
>right.
>
>Was it common to produce custom internals like this for teletypes?
>
>Anybody have the schematics for this board?
>
>Thanks,
>Brian
>
>=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
>_| _| _|  Brian Knittel
>_| _| _|  Quarterbyte Systems, Inc.
>_| _| _|  Tel:   1-510-559-7930
>_| _| _|  Fax: 1-510-525-6889
>_| _| _|  Email: brian at quarterbyte.com
>_| _| _|  http://www.quarterbyte.com
>
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