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An HP-22 repaired

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This HP-22 needed a thorough internal cleaning due to leaking batteries. Opening a woodstock series calculator is rather simple. See link below. To open it two screws under the upper feet must be removed. The next picture shows what I came across.

HP22

The fish paper isolation was affected by the potassium hydroxide from the leaking batteries. Also the printed traces were affected but not interrupted. The hydroxide also entered the keyboard, below the key labeled 4. The space within the dome of this key was completely filled so this key couldn't even be pressed anymore.

The opened calculator gave me the opportunity to make some pictures from its internal.

All electronics is build on two PCB's. One PCB is the keyboard and the other contains all active components. The PCB's are connected to each other by two rows of gold plated contacts. The component PCB has a big hole in the middle. This print fits around the battery holder.

HP22 internal HP22 print

On the top of the printed circuit board we find the LED display module. More information about this display can be found here.

 

HP22 Display

 

This display module can be removed easily. Below this module the row and column driver circuits are located. The two big pins in the middle is the back side of the charger connections.

HP22 Display drivers

 

On the left side of the print, the power supply circuit is located. On the right side a large resistor is mounted. This resistor is used as current limiter for charging the batteries. The picture below shows a view on the power supply circuit.

HP22 Power supply

 

On the bottom the ACT, RAM and ROM circuits are situated. This picture shows also the battery connections.

HP22 CPU

This HP-22 calculator uses the following chip set:

1818-0163 ROM and Anode driver
1818-0164 ROM (Read Only Memory)
1820-1382 Cathode driver
1820-1523 ACT (Arithmetic, Control and Timing)
1820-1564 RAM (Random Access Memory)

The keyboard was fixed by dripping vinegar with a needle in the hole corresponding with the key labeled 4. The vinegar washed away the potassium hydroxide. The vinegar was washed down with water and finally the hole was cleaned with IPA (IsoPropyl Alcohol). Now the key click is back to normal and the key is registered fine.

The battery contacts have to be replaced. In this condition they don't make reliable contact with the batteries.

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HP-22 inside related links:

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