[GreenKeys] Seeking Teletype Part 1882241

[Jim Cooper]

On 5 Jun 2021 at 15:23, Eric Moore wrote:

> There appears to be a ground fault
> with the teletype that is preventing
> the system bus from being used when
> the teletype itself is energized. It
> is in fact setting a bit in one of
> the computers registers. The
> teletype itself can be bypassed
> luckily with an adapter, which is
> what I have been doing up until now. 

not sure what you mean by a 'ground fault' ... 
related to the AC power?
related to the loop current? 

tread carefully in either case. 
 
> After basic maintenance I was hoping
> to get to expected behavior when in
> local, then was going to use an
> RS232 adapter to test online mode
> with the teletype off its pedestal,
> and use that opportunity to try and
> find the ground fault. 

get the monster to work properly 
OFF-line, on just a local DC loop -- 
60ma or 20ma, depending on how the 
selector magnets are wired. 

don't even mess with an 'adapter' or 
terminal unit until the printer works 
properly on the local loop, with the 
keyboard in series with the loop. You 
should be able to type on the keyboard 
and see exactly what you typed show up 
on the paper ! 

> Is maintenance easier to perform
> with the teletype removed from its
> base? 

depends on whether you are talking about 
mechanical maintenance or electrical loop 
testing.   For mechanical work, yes -- off 
the base, you can still turn the shafts by 
hand and watch how everything responds. 

when you are happy with the mechanical 
machinations, then put it back on the base 
so the motor gear is engaged and you can 
turn the motor MANUALLY (without power) 
to be sure everything still follows along as 
it should. 

THEN connect up the dc loop and set the 
current correctly (60ma or 20ma, as noted 
above).  If you don't have a local loop (instead 
of an adapter or TU), MAKE ONE!!   not hard 
to do ...  60ma is not a lot of current or power, 
you could even use two 6.3v filament transformers 
back-to-back to get the isolated 120vDC !! 

YES, you want 120vDC ... important ... because 
with the high voltage, you need a high-ohm 
resistor (nominally 2000 ohms) and you need a 
high ohm resistor to assure good 'time constant' 
for the square wave signal.    The time constant 
is based on  L / R ...  inductance of the selector 
magnets divided by the Resistance in the loop ... 
big R, low time contant, squarer square waves ! 

Once the mechanics are good and the typing unit 
is back on the base and turning the motor manually 
makes everything go smoothly, then you can connect 
the local loop and turn on the motor AC.  At that 
point, everything should be quiet and steady aside 
from the sound of the motor whirring. 

Press R on the keyboard - hopefully see R type on paper; 
Press Y on the keyboard - hopefully see Y type on paper. 

If both happens, you are probably successful.   :)) 

(Note that R and Y have opposite Baudot code bits, 
that's why RYRYRYRYRY is used for testing 5-unit 
teleprinters -- it makes everything in the selection 
mechanics go from one extreme setting to the opposite). 

Hope that helps. 

w2jc 

btw: for those reading, I have a few more M-14/15/19 
AC synchronous motors if anyone is in need ... contact me.