Repairing fault-handling of the Burroughs Portable adding machine

With the subtract-function fixed and columns 7 & 8 re-activated, this Burroughs Portable performed all calculating functions flawlessly. What however did not work at all, was the catching of faulty, incorrect input conditions. (Because this doesn't impact the normal functioning of the machine, this a 'malfunction' that can remain completely unnoticed. The machine did not dis-engage and simply completed a cycle, with possibly incorrect results.)

This now also repaired and re-activated.

Looking back, this functionality was probably broken from a combination of wear to parts and from parts getting bent by handling without its case/base. And made worse from my injudicious handling of the keyboard-module during cleaning - learnt for a next time that the exposed brackets are 'soft' and critical to timing.

Most adding machines have interlocks to prevent wrong operation that could cause incorrect results or damage to the machine. E.g. the keyboard is blocked during a cycle or it's made impossible to reverse half-way through a cycle. The Burroughs Portable has plenty of such safety interlocks. It however also has two fault conditions that are not blocked, but that are prevented by dis-engaging the handle and in that way prevent completing a cycle.

The two conditions that should trigger the handle dis-engaging are trying to start a cycle with:

• Depressed total or sub-total and a value set on the keyboard.
• Depressed total or sub-total and the machine set to subtraction.

Reading user instructions confirmed that the machine should catch these fault conditionsa and then reset. (Scans of these original instructions very kindly provided by Mike Hancock who created the very informative Burroughs Info website.) 

Reading further in the very extensive US patent 1,853,050 by Mr Horton covering the Portable gave more background on these two exceptions, how they are caught and what should then happen. The two conditions have their own specific 'trigger' that both then lead into activating the same dis-engage & reset mechanism.

To start with, the mechanism to dis-engage the handle from the mechanism is shown in Fig. 3A of the patent (here below). This is a detail of the left-side of the machine shown in Fig. 3 that is mostly dealing with the electric drive arrangement. The axle actuated by the handle (91, orange) is inside another hollow axle that drives the machine. (Actually, it pulls the machine via springs with the dashpot limiting the speed; a lot of thought & experience went into this design.)

To give the short (...) summary: The handle drives the rod 91 that has mounted on it the hook 89, this pulls along the machine's mechanism by contact-surface 87 onto hook 83 that is fitted to the machine's mechanism. At the same time, the rod 191 moves rearward (locks the keyboard among other things), the stud 194 moving in the slot 210 then rotates the extra hook 212 out of the way. That's normal operation. When an exception is detected, the rod 191 does not move all the way rearward and the hook 212 is left in the path of face 86. This then smartly pulls hook 83 away from 89 and the handle is dis-engaged from the mechanism.

In an actual machine, without electric motor, the relevant parts can be seen much clearer.

This sixteenth of an inch (well, approximately) should make the difference between the stud of the subtract lever sliding past the upward-fork or catching and stopping it. This no longer worked, probably because I accidentally (clumsily) bent the rod 191 assembly during cleaning of the keyboard module - the brackets may all be fairly heavy-gauge, but are soft. To repair this, applied another kludge; an angle-piece shaped from a steel nail was added to the side of the forked-extension. This now makes it catch on the stud when a totals key is depressed in subtract-mode - and move free when no totals key is depressed.

The left image shows it clearing the stud, the right image shows it catching on the stud (and triggering the dis-engagement of the handle, the machine jumping back to start and clearing all keys).

To note though that there are some detail-differences with actual machines (e.g. the subtract-lever) and it was not written for a repairman. It e.g. glosses over some details that will be important when servicing. For example, what it states is true; the keyboard can be lifted off after removing two screws and sliding it rearward. But you also must unhook two springs and use a finger to keep rod 191 pulled all the way forward to take it off without breaking anything! (I've taken the keyboard off about a dozen times by now; that little spring that holds the reset-push-bar is easy to miss.)