Help with HP 15c

[lyuka]

Hi, edwincool2

The story of 15C with your father is a good story,
but 15C that doesn't turn on seems sadly,
The inside is kept as beautiful as it was,
So let's bring it back to life to make another story.

First, to check for poor contact at the power terminal, insert the batteries in the correct orientation and measure the voltage across the 22u / 10V tantalum capacitor at the end of the PCB with a digital multimeter.

If the voltage is about the same as the battery voltage, the contact is normal and the processor or something may be defective. There's not much you can do in this case, but it's possible that the 180uH microinductor (marked as 181) is scratched and may stop oscillating due to a wire break in that inductor. To check this, measure the resistance across the inductor with the battery removed. Normally it should be around 17Ω, but in case of disconnection it should be several hundred kΩ or more.In this case, it is worth asking someone with rework skills to replace the microinductor with an equivalent.

If the voltage is near 0V, measure the current of the power supply current (although it is a little difficult to measure). If the current is almost 0mA, it is possible that the power supply terminal has poor contact. You may need to do something about the aluminum foil on the negative terminal.

If the current at power off is more than 100uA, it may be broken in the short circuit mode of the tantalum capacitor due to the reverse insertion of the battery in the past. To check this, measure the resistance at both ends of 22uF / 10V (marked as | 22 10V) with the battery removed. At this time, the + side on the PCB should be the plus side (usually the red lead wire) of the multimeter.

If the resistance across the tantalum capacitor is less than a few tens of Ω, then the tantalum capacitor is probably failing in short circuit mode. In that case, it may protect the processor as a result. In this case, it is worth asking someone with rework technology to replace the tantalum capacitor with an equivalent one.The inside of the 15C looks beautiful enough.

First, to check for poor contact at the power terminal, insert the batteries in the correct orientation and measure the voltage across the 22u / 10V tantalum capacitor at the end of the PCB with a digital multimeter.

If the voltage is about the same as the battery voltage, the contact is normal and the processor or something may be defective. There's not much you can do in this case, but it's possible that the 180uH microinductor (marked as 181) is scratched and may stop oscillating due to a wire break in that inductor. To check this, measure the resistance across the inductor with the battery removed. Normally it should be around 17Ω, but in case of disconnection it should be several hundred kΩ or more. In this case, it is worth asking someone with rework skills to replace the microinductor with an equivalent.

If the voltage is near 0V, measure the current of the power supply current (although it is a little difficult to measure). If the current is almost 0mA, it is possible that the power supply terminal has poor contact.

If the current at power off is more than 100uA, it may be broken in the short mode of the tantalum capacitor due to the reverse insertion of the battery in the past. To check this, measure the resistance at both ends of 22uF / 10V (marked as | 22 10V) with the battery removed. At this time, the + side on the PCB should be the plus side (usually the red lead wire) of the multimeter.

If the resistance across the tantalum capacitor is less than a few tens of Ω, then the tantalum capacitor is probably failing in short mode. In that case, it may have been protected the processor as a result. In this case, it is worth asking someone with rework skills to replace the tantalum capacitor with an equivalent one.

Cheers,
Lyuka

P.S.
About nominal current and power trace, see my 15C's page below.
HP-15C LC resonator replacement for triple speed 15C
Note : The replacement of a tantalum capacitor to MLCCs is NOT recommended for everyone. As it lose a sort of reverse voltage protection.