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HP41CY - Replacing the internal +3v lithium cell

Posted by Christoph Klug on 27 Feb 2006, 4:09 a.m.

HP41CY - Replacing the internal +3V Lithium Cell

The HP41CY, an upgrade of the HP41CX halfnut model, created by W&W Software Products (Germany) includes a turbo speed mode plus an internally 64KByte RAM-Box. Like the W&W RAM-Box which is placed in the card reader shell, the internally RAM chips are buffered by a small +3V CR3025 lithium cell. This additionally +3V supply voltage is only needed, when the four batteries of the HP41CY are removed or when dropped down to low voltage levels.

The RAM consist of several 4KByte pages, needed for the storage of individual user applications like mcode programs, user code programs, data, or images of existing ROM solutions. Furthermore the belonging operating system of the RAM-Box is stored in the RAM itself. Unfortunately from this hardware architecture results a long time problem : When the +6V battery voltage of HP41CY falls down for long time, or when the HP41CY is stored without batteries, the internal +3V lithium cell buffers the RAM contents. When the +3V lithium cell itself drops down, the complete RAM contents would be cleared, than also the operating system is lost. The CR3025 lithium cell would have a typically lifetime duration of 10 years, may be up to 20 years, depending on the main battery state of the HP41CY. Therefore for today HP41CY systems exist a high risk to loose the internal operating system needed for the internal RAM-Box.

Reloading the operating system seems to be a really complex procedure, and special knowledge and plug in tools are needed. Therefore do not wait until your RAM-Box operating system is lost and your HP41CY is a worthless machine. Alternatively it is much easier to replace the +3V lithium cell by a new battery for saving the valuable operating system for much more years :

First switch OFF the handheld than remove the battery block. Now open the HP41CY case by removing the four glued rubber feet and the four screws. Put the face of the forty one down to the workbench and lift up the lower case from the frame. Attention : Do not ignore the thin wires form beeper and turbo switch, may be you have to rotate the lower case. Now you see the internals of the HP41CY, the small bridge board = Clock and X-Memory, below this the additionally RAM-Box board with +3V lithium cell and below this the halfnut main board.

Have a look to the upper region of the halfnut main board (below the display driver), here you find the golden contact area which is needed for interfacing the battery & port assembly. Contact 3 from the left side is ground potential.. Contact 11 from the left side is the +6V potential from the battery. For your orientation - on the left edge of the main board is also placed the big electrolyth condenser. Follow the pcb leads from the two golden contacts 3 and 11 to the next via. Here you are able to solder two wires, for powering the HP41CY by an external +6V DC power supply unit. Check the correct polarity of the external +6V DC power supply unit, than switch ON the HP41CY. Now the handheld works without the lower case and without the belonging battery block Switch OFF the handheld.

Now you are able to remove the old +3V lithium cell, because now the RAM is buffered by the external +6V DC power supply unit. During re-soldering the old cell avoid any shortcut between the +/- contacts and to the RAM-Box hardware. Solder short wires to the connectors of a new CR3025 lithium cell and insulate the metal housing with plastic tape. Solder the new cell to the RAM-Box board with correct polarity and with the correct mechanical position. Again avoid any shortcuts. Execute a short function test by switching ON the handheld and running CAT 2 : If your work is correct CAT 2 lists –RAMBOX 64a or -RAMBOX 64b, the operating system was not lost during the lithium cell replacement process. Switch OFF the handheld, than remove the wires to the external +6V DC power supply.

Place the lower case shell to the correct mechanical position on the frame above the upper housing – this would be only possible when the lithium cell is in a correct position. Insert the four screws and have lot attention during fixing them to the old screw flight : Do not crack the sensitive screw posts of the upper housing shell. Last insert the battery block and switch ON the HP41CY for an final function test : By executing PG<> you toggle form one 32KByte page to the other 32KByte page of the RAM-Box. Generally operate and store your HP41CY only with a set of new batteries. This will help to extend the lifetime of the replaced +3V lithium cell…

Long live your HP41CY – Christoph Klug


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