[GreenKeys] Model 33 bell problem solved

[Jones, Douglas W]

On Dec 30, 2016, at 3:37 PM, I wrote:

> I've diagnosed the problem I'm having with my model 33 where the bell rings with every character received.  The log entry is here:
> -- http://www.cs.uiowa.edu/~jones/pdp8/UI-8/log2016.shtml#2016-12-30

The problem is now solved.  The log entry describing the fix is here:
-- http://www.cs.uiowa.edu/~jones/pdp8/UI-8/log.shtml#2017-01-03

Thanks, Wayne, for the hint about how to pull and replace the codebars, it was an easy fix.  The only annoying part is that you have to pull all the codebar springs and remove the spring plate from across the codebars in order to get the codebar out.  That means hooking all the springs back on in the end, which is very fiddly work best done with a very small screwdriver and sharp fingernails.

Here's a before photo:
-- http://www.cs.uiowa.edu/~jones/pdp8/UI-8/2016/12/30-TTYcodebars.JPG

Here's a photo of the work in progress:
-- http://www.cs.uiowa.edu/~jones/pdp8/UI-8/2017/01/03-TTYcodebarFix.JPG

And here's an after photo:
-- http://www.cs.uiowa.edu/~jones/pdp8/UI-8/2017/01/03-TTYcodebars.JPG

If you compare the before and after photos, you can see why it was so hard to diagnose the problem with the end of line bell always ringing.  The difference is very subtle (aside from the change in camera angle).  The key difference involves the position of the automatic codebar (the one with the arm reaching up to engage the bell crank that hangs down from the rear carriage rail).

In the before photo, you can see a clear space between the automatic codebar and codebar number 1, while there is very little space between the automatic codebar and the rear rail of the codebar assembly.  This is best judged in the vicinity of the arm that reaches up to engage the bellcrank.

In the after photo, the automati codebar is much closer to the number 1 codebar and correspondingly farther from the rear rail.

Next job -- re-adjust the depth-of-punching on the paper-tape punch (it's been on and off the TU and the depth adjustment has gone bad as a result).  Then, see if I can get the tape reader to work.  I think that's about it for fixing this TTY, leaving me with a PDP-8 and a Flexowriter to work on.

		Doug Jones
		jones at cs.uiowa.edu