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This programmable calculator was a bit dirty but furthermore in near mint condition and fully operational running via the AC adapter as well as on alkaline batteries.

In the past a small modification has been made by adding a LED indicating “power-on” of the calculator.  Furthermore an on/off switch was added to the AC adapter.  However that switch is broken.

The calculator has one small problem:  The key contacts are a bit unreliable which sometimes results in missing entry and sometimes in double entries. It is a problem which was already noticed in reviews in 1972/1973.  Generally it isn’t a big problem, but it does become a problem when entering a program into one of the two program memories. There is no way to repair a faulty input, making it almost impossible to correctly enter a program.  So  I decided to clean the keyboard PCB.  That fully solved the problem.

To clean the keyboard PCB:

• Open the calculator case by removing the six screws which hold the two shells together.  Two screws can be reached from the battery compartment, two are hidden under the rubber feet.
• Disconnect the DC-power wires
• Take the PCB-assembly out of the calculator casing.
• Disconnect the flat cable from the keyboard PCB
• Put the keyboard PCB upside down on a soft surface and remove the 9 screws which hold the keyboard assembly together
• Now the PCB can be taken of while the buttons and metal domes stay in place.
  When re-assembling the keyboard take care that the metal domes are properly positioned.

Furthermore when I received the calculator the power switch situated in the back of the machine was bridged.  A check of the switch revealed that when closed its resistance changed between 0 and 10 Ohm.  So apparently it didn’t function well.

The switch can easily be opened. Some cleaning and readjustment did the job: 0 Ohm with each measurement.

For more details have a look in the 300-series service manual (See the assets page)

If needed you will find much more information regarding disassembly of this machine on the Internet.

AC adapter

Battery compartment

Power connector position

Keyboard

Battery compartment after repair of the power switch