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When I bought this Friden  it partly worked.  Operating the machine manually I noticed that multiplication didn’t function and that division blocked the machine.  I cleaned the machine as good as possible. That freed the multiplication parts, but not division, back transfer and product transfer.

On the internet there is quite some information about the STW10 which is not very different from the STB10.   John Wolff provided me with a user manual.  With the information I found  and a lot of patience I managed to figure out how each of these functions is  executed and managed to locate the points where parts were not moving freely due to old oil and grease.  

In particular the backtransfer function was difficult. Per digit a small hook hold in position by a spring is part of the transfer mechanism. One spring wasn’t properly attached anymore causing one digit not to take part in the transfer.  The transfer mechanism transfers the values in the upper register to the keyboard. A small aluminium component prevents that the keyboard input arms fly back during the transfer process. If these components are a bit sticky due to old oil, the transfer doesn’t work properly.

Like the carriage also the motor unit can easily be taken off, allowing easier access to parts that may need cleaning.

Backtransfer:
fly-back inhibit mechanism