HP 48G
hp48g.jpg hp48g-pcb.jpg
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Repairing intermittent key of the HP 48G, and memory upgrade

Takayuki HOSODA
Sep. 28, 2011 : Repair and RAM upgrade to 128KB.
Oct. 15, 2011 : Additional 128KB RAM and memory backup.
hp-48g-connector.jpg Fix the intermitant ontact of the keypad connector.

The problem was fixed by appling the double coated polyethylene foam tape with 0.8mm thickness (3M 4462W) at 2.5mm width onto the foam rubber that had been deformed under the connector.
hp-48g-orig.jpg PCB of the HP 48G

32KB RAM and 1000μF aluminum electrolytic capacitor
is going to be replaced.
hp-48g-remove.jpg The RAM and the electrolytic capacitor is removed.

Though the capacitor (Nippon chemi-con, ESME6R3E102MUC5S 2000h@85°C) is stale, its capacitance was measured to be 1060μF with leakage of 1.0μA(@ 5.0V,25°C).
It's good enough characteristics for an 18 years old electrolytic capacitor.
hp-48g-replace.jpg Replace the RAM and the electrolytic capacitor

RAM: Cypress, CY62128ELL-45SKI, 1MBIT(128K×8) 45NS 4.5-5.5V
Cap.: Panasonic, EEUFR0J122, 1200μF 6.3V 6000h@105°C
hp-48g-iram-ok-128k.jpg Self-test

hp-48g-iram-ok-128k.jpg Add one more RAM and memory backup circuit as to be port 1 RAM card.

The battery holder is being insulated using the polyimide tape and glued to the PCB.

Note. The accumulative charge current to the Lithium coin battery by the reverce leakage current of the diode should be less than 3% of the CR2032 battery capacity of 220mAh. Assuming the memory backup period to be five years, the average leakage current should be less than 0.03 * 220mAh / (5 years * 365 days * 24 hours) ≈ 150nA. Therefor BAR43CFILM is chosen by its characteristics of Irtyp≈100nA (@ Ta=35°C, Vr=2.5V) as well as its low forward voltage characteristics.

Actual charging current to the CR2032 was measured to be 60~70nA (@ Ta=27°C, Vcc=5V, Vbattery=3.1V).
hp-48g-replace.jpg Partial schematic

RAM: Cypress, CY62128ELL-45SKI, 1MBIT(128K×8) 45NS 4.5-5.5V
MLCC: Murata, RPEF11W104Z2K1A01B, 100nF 50V, F
Diode: STM, BAR43CFILM, SiSB, 30V 100mA, Vf=0.45Vmax@If=15mA, Ir=500nAmax@Vr=25V
Battery: Maxell, CR2032, 3V 220mAh

Note. RX2:330Ω is a series protective resistor. In case of short mode failure of the diode DX1, it limits Vcc to the CR2032 current to the safe level of less than 10mA which is thoght not to cause the battery explosion. This series protective resitor is requierd by UL safety standards in the United States.
hp-48g-replace.jpg Marging the port1 RAM with port0 to check the memory and its size.

Mem Available :
256324 bytes

Seems good so far :-)


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