Click to view a full size picture (opens in a new tab)

Upon arrival, the machine was non-functional. Most of the time, there was no visible response when powered on. Only occasionally, during testing, would it display either all zeros or all ones.

Tests conducted according to the service documentation pointed to a fault on PCB4. The service documentation advises replacing any PCB that fails these tests. While board-level repairs are covered in the documentation, schematics are not provided. Most of the semiconductor components are early Japanese MOS ICs, which are difficult, if not impossible, to source.

Fortunately, further investigation raised suspicion of PCB5, which contains several discrete components. A partially completed reverse engineering effort (see assets page) identified an astable multivibrator responsible for generating the main clock signal and two bistable multivibrators, AC1 and AC2, which are believed to handle the reset and clearing of the electronics during start-up. AC1 switches after a short delay, while AC2 is expected to switch immediately upon power-up. However, AC2 was found to be non-functional. Replacing a resistor restored its operation. As a result, the calculator now displays all zeros upon switching on. Full calculation functionality has not yet been achieved.

Troubleshooting was complicated by several design characteristics. For example, the electronic circuits on the PCBs are arranged from right to left, making navigation during reverse engineering less straightforward. Additionally, the circuits operate on -24V and ground, rather than the more conventional +24V and ground. While one can adjust to these design choices, it does require some time.

Below photo’s of the 5 PCB’s.  They are numbered 1 to 5, where 5 is the lowest board.

On clicking a picture a full-size view opens in a new tab.

PCB-1

PCB-2

PCB-3

PCB-4

PCB-5