[GreenKeys] ASR-33 problem with clutch

[Aaron Nabil]

I spent some time digging into a junker chassis to see more closely
how it worked.  The baffling part was the character sensitivity, since
each selector cam has it's notch in a different place, I didn't see
where the mysterious detent force was coming from, they shouldn't be
additive with the notches being distributed across the cam.  But
having a cam to look at, they don't just have a notch like you see in
the manual, they also have a ridge, but the ridges are all aligned.
With the selector around 60 it stops in sort of in a metastable state
with the cam followers just on the slope on one side of the ridge.
With a symbol with lots of 1's you get enough force to slowly pull the
cam (in my case quickly since it doesn't bind at all) back into the
valley of the ridge, which slightly over-opens the clutch causing the
slight rubbing.

By turning the rangefinder the other way, you can make it always stop
on an entirely stable part of the cam.  Although advancing the
rangefinder the back to lower number sort of fixes it, mostly it just
moves the shoe lever out of the way, so it doesn't matter which side
of the ridge the cam settles on.  By advancing it to 80, you can make
it stop on a stable part of the cam so the force of the followers is
irrelevant, there isn't any slope for them to lever the cam out of
position.  Since I'm running this on a computer and I'm not expecting
any distortion, I don't think running at 80 is going to cause me any
grief.





On Mon, Jul 28, 2008 at 4:15 PM, Aaron Nabil <krellboy at gmail.com> wrote:
> On Mon, Jul 28, 2008 at 2:32 PM,  <Teletypeparts at aol.com> wrote:
>> It should not make any difference what character you type.  The problem is
>> probably the selector cam is dragging, needs some oil on the main shaft btween
>> the cam and the shaft.  Oil the main shaft on the far right side of the cam and
>> also in the small hole in the cam near the rangefinder bracket if you can
>> find it.
>
>
> I found the hole and shot in a bunch of oil.  The cams turn even more
> freely now.  Didn't help.  If anything, it's the fact that the cams do
> turn so freely that is the problem.  There is a detent-like force (I
> think it's from the SPACING LOCK LEVER CAM or the STRIPPER BAIL CAM)
> that is much stronger if the last character typed (more BLOCKING
> LEVERS up) is a rubout (vs a null) that tends to turn the cam slightly
> forward and over-open the clutch.  It's not a lot of force, but the
> cams turn so freely it doesn't take much to rotate it.
>
> What I did as a stop-gap measure was turn the rangefinder to 47 (was
> 64). The stop lug still stops in the same place, but now the shoe
> lever is stopped further along, so the clutch doesn't get over-opened.
>  I probably didn't need to do this as the drag is very slight and only
> happens on few characters (I would have been more concerned if CR or
> LF caused it to drag) and my TTY sees between little and no user, but
> at least now I don't have to worry about leaving it running.  I just
> ran a bunch of quick brown fox with my test set and it copies fine, is
> there any reason to worry about having the rangefinder advanced so
> far?
>
> I'm going to go look at another teletype for reference.  I'm wondering
> if something isn't resetting as the blocking levers stay up after a
> character and they move slightly down as the selector cam drifts
> (after it has stopped with the clutch open) maybe the STRIPPER BAIL
> CAM isn't working.  It's odd that it otherwise works perfectly, and
> that there are two that act like this (and that they are both
> virtually new units).
>
> Thanks and call anytime.
>
> Aaron
>