Inside the HP-41CV halfnut


For a small amount I brought an HP-41CV. The calculator was not used for a long time and the seller told me it was stored in working condition. I seize this opportunity and bought myself an HP-41CV model. This also gave me the opportunity to open an HP-41 series calculator and study the interior. The calculator turned out to a halfnut version made in 1987.

On the web lots of articles can be found how to disassemble this type of calculator.

To open the housing a crosshead screwdriver is needed. The four screws are hidden behind the rubber feet. After removing the screws the housing can be opened.

HP41CV_repair_1 HP41CV_repair_1

The pictures above show the interior of the HP-41 CV halfnut version. The picture on the left shows the large printed circuit board which contains all electronics. The picture on the right e shows the back side without battery holder and the connector which connects the batteries and interface modules with the main printed circuit board. The buzzer is glued to the back. To separate the two halves, the wiring to the buzzer were de-soldered.

The calculator was working but stopped a short time later. I opened the calculator again see if I could fix it.

When a supply voltage is applied the calculator starts in sleep mode, with an increased current. When the calculator is turned on, the supply current increases but the LCD screen stays blank.
It is not possible to switch off the calculator with the ON key. The only way to switch the calculator off is by removing the supply voltage and wait.

The first action taken was cleaning the contacts and replacing the capacitors. This didn't solve the problem. I saw some blue copper sulfate near the LCD display module inside the print trough holes. To check if this sulfate has damaged PCB tracks on the front side of the PCB I decided to remove the print by cutting all 36 heat stakes.

HP41CV inside

The photo on the left shows the front side of the PCB. The LCD screen is removed, the driver PCB is visible. The LCD panel is contacted with two foam strips with the upper and lower contact strips on the brown module pcb.

This photo also shows the metal domes of the keys. The metal domes are glued to the print by a layer of transparent adhesive tape. The photo below shows the copper tracks under the dome.

HP41CV keyboard

The traces of the copper sulfate were removed and no damage to the tracks was observed.

The photo below shows the keys with there hinges construction.

HP41CV keys

I measured that the supply boost converter is function and a clock is running on the LCD module.

As soon as I found service documentation about this model calculator i will continue trying to fix the calculator.



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