Inside the HP-41CV fullnut


For a fair pricet I brought a HP-41CV, sold as defect. The calculator was not used for a long time and the seller had no batteries to test it. I grep this change to buy myself a HP-41CV model and give it a try to repair. This also gave me the opportunity to open an HP-41 series calculator and study the interior. The calculator is a fullnut version made in 1981.

On the web a lot of articles can be found how to open the calculator and lists of known problems. The main problems are connector damage caused by battery leakage and broken screw posts and bad contacts of the internal connector.

To open the housing a cross-head screwdriver is needed. The four screws are hidden below the rubber feet. After removing the screws the housing can be taken apart easily.

HP41CV_inside_1 HP41CV_inside_2

The pictures above show the interior of the HP-41 CV fullnut version. The left picture shows the small printed circuit board which contains the control logic. The right picture shows the batteries and the connector which connects the batteries and interface modules with the main printed circuit board.


When the small printed circuit board and the black isolation foil is removed, the complete main board becomes visible.

At the top, the backside of the display printed circuit board is visible. The chips that control the LCD display are covered with a blob of epoxy. This printed circuit board is connected to the main board by means of 16 solder bridges.

The keyboard is completely covered by the main board. This board is fixed by a lot of melt joints with the front of the calculator. The main board can not be removed without damaging the calculator.

At the bottom the gold plated connector is visible which connects the main circuit board with the logic circuit board.


This picture shows the logic circuit board. This board contains all electronics of this calculator except for the LCD drivers.

The chips on this board are memory (RAM or ROM), a processor and a switch mode supply controller. Hoover over the picture for more detail.

This circuit board is connected with the main circuit board by means of a gold plated connector.

On top of the processor chip, a piezo buzzer is glued.

To download the schematic of the calculator, click here.


Close-up of the gold plated connector that connects the logic circuit board with the main circuit board. The connector consists of a many isolated wires that connect the top side with the bottom side. See inset. It is a kind of Z-Carbon connectors, which is used for LCD connections.


A special connector is used to connect the batteries and plug in modules with the main circuit board.

The picture on the left shows the 21 connections to the main circuit board.

This picture also shows that this complex connector is build from one piece of black plastic with a gold plated foil wrapped around it.


On the left the contacts to the plug in modules are shown.


This picture shows the battery contacts. The most left and the third battery contact are worn out. These contacts will no longer make a reliable contact with the batteries. Repair is needed.

This damage is often the result of battery leakage.


I folded pieces of copper foil over the battery contacts and fixed them with a drip of epoxy glue.

The batteries make good contact with this fixed connector and the calculator is functional again.



HP41 repair related links: