Inductor on HP 25C board

[jebem]

(09-21-2015 08:50 PM)Harald Wrote:  All the registers are working and retain their contents even when the calculator is turned off.
LastX in deed always returns zero. So I guess it is that RAM chip that is bad. Which one is it? Unfortunately the IC markings aren't legible any more.

Hi, Harald,

Looking to the posted picture, we can see a lot of battery acid corrosion around the IC's. A few things could be done before concluding it is a defective IC.

1.- Wash the PCB with warm water and dishwashing soap using a soft brush; try to go under the IC's on the component side by using a thin medium soft brush; then let it dry for a while before proceeding with step 2.

2.-Wash the PCB with abundant isopropilic alcohol using a medium soft bush around the IC pins and under the IC's; then let it dry at room temperature for a few hours.

3.- Probably one or more of those IC pins are not making good electrical contact with the PCB holes anymore. This next step is more critical to do as it requires good soldering skills. We need to undo those soldering joints and apply good fresh solder.

As a side note, professionally we start from step 3 to remove the suspected IC's, then proceed with the cleaning, and at last place back the IC's (same ones if there are no stock available).
But in this case, it is much easier to do it as suggested.

Good luck with your new machine, and please keep us posted here about your findings.