HP32SII Repair

Inside the HP-32SII

For a trifle, I bought a defect HP-32SII calculator. The previous owner told me it stopped working after it dropped to the ground. I bought it to open it and have a look inside. Also to see if I was able to fix the calculator.

On the internet a few articles can be found that describe how to open these series. The calculator is not designed to be repairable. The housing can only be opened destructive, by means of a drill and a knife.

To open the housing a 4mm (1/8") drill is needed. The heads of the four heat-stakes beside the battery holder have to be removed. When removed the housing can be separated. The two housing halves are still connected by four heat-stakes located between the lower and second row of keys. I used a scalpel to cut the heat-stakes.

	This picture shows the keyboard half of the open HP 32SII. On the top the batteries holder is visible. The four drilled heat-stakes can also be observed. Almost all the electronics of this calculator is integrated into one chip. This chip is placed in a hole in the printed circuit board. This printed circuit board is kept in place by six twisted metal clamps. The keyboard is covered by a solid metal plate, which is kept in place by 41 heat-stakes with the front panel of the calculator. The four lower heat-stakes that were cut to open the calculator are visible in the second row for heat-stakes. To open this sandwich all heat-stack heads has been removed.

	This picture shows the inner backside of the HP -32SII. Eight holes on the edge can be clearly seen on this picture. The eight hole / pin construction is used to align the front and back halves. Also the four cut heat stakes are visible.

	The integrated circuit on the PCB is the Saturn sacajawea processor, fabricated by NEC. The chip is mounted in a 100 pins SO (Small Outline) package. This site of the PCB has no wiring, only a grid to reduce EMC .

	On the other side of the PCB we find the connectors to the LCD display and the keyboard. The display connector can be found on both sides of the chip. The 16 pins, wide pitch connector on the bottom is the keyboard connector. The calculator don't use real connectors. The foil of the display is clipped to the PCB. On the print a tantalum capacitor of 10 uF (top left) and a unidentified SMD component (left bottom) are soldered.

	The picture on the left shows the LCD screen from the back side. On the top and bottom of the screen the LCD contacts are connected with the printer circuit board by special foam strips. A lot of small conductive strips encapsulated in foam.

	The picture on the left shows the keyboard foil, visible when the metal plate with the glued LCD is removed. All heat stakes that kept this sandwich together are cut. On the top of the foil the connector to the PCB is visible. The keyboard is build from a sandwich of layers. The top layer, counted from the metal shielding plate, is a transparent foil with a circular hole pattern. See the picture on the left. This foil isolates the keyboard foil from the metal plate.


The flexible keyboard foil is dual sided printed with key contact points and wiring. The rake shapes are the contacts that will be shorted when a key is pressed. A pressed key result in a resistive contact of about 6000 Ohms. The pictures show both sides of this keyboard foil.

	The drawing on the left shows the electrical schematic of the keyboard. All keys, except for the on/off key, are arranged in a matrix structure. The matrix consists of 7 rows and 7 columns. The drawing on top shows the connections to the PCB. A more readable version of this drawing can be downloaded by clicking on the picture.
	The picture on the left shows the foil with the matrix of domes with small carbon buttons that short circuit the rake structures on the keyboard foil when a key is pressed. This dome foil has a three dimensional structure. These domes acts like a spring. It also gives the key its famous click feeling. This dome foil was very dirty. The contamination on this foil was the main problem for this calculator. The dirt between this foil and the keyboard foil created a resistive short on the rake of the key labeled 2 of about 25K ohms. The processor interpreted this resistance as a continuous pressed key. Between the dome foil and the real mechanical keys, a fourth transparent layer is placed. This layer should protect the domes for dirt.


The picture on the left shows a part of the backside of the keys of the keyboard and also the dirt around those keys. The hinges of the keys are hardly visible due to the grease. The right picture shows the same keys after a decent cleaning.

To repair the cut heat stakes I used the method described by Nicolas GILLES. This French article describes the repair of an HP-15C calculator but the same technique can also be used for calculators of the pioneer series.
	Small metal pins are pressed between the edge of the hole and the cut pin. These metal pins are fixed afterwards by a two component epoxy glue.
Rebuilding the calculator resulted in a full functional HP-32SII. The picture below shows the result of the self test.

Inside the HP-32SII

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