HP-42S battery drain

[Marcovecchio]

I bought a second hand, perfectly working HP-42S, but I just noticed it has a huge battery drain problem. I measured 17mA, which will drain new batteries in hours.

I have a disassembled HP-17BII at hand, and I just measured it's current draw: less than 1mA on and idle, and a peak of 2mA when making calculations.

I know this has been discussed many times, and with several HP calculator models (I had an HP-28S that showed the same behavior many years ago), but I would like to insist a little more before giving up. I opened the calculator, removed the PCB from the case and removed every component I could from the PCB: 3 capacitors, the diode, the IR led, and even the SRAM and crystal! When plugging the calculator to a power source, I get the same 17mA. This is with only the processor still soldered to the PCB.

I then washed the pcb with isopropyl alcohol and a soft brush, in an attempt to remove contaminants that might be shorting the delicate processor pins, but it didn't work, still 17mA. I resoldered everything back into place. Calculator works, and still the same 17mA.

May I consider the processor is damaged forever? Is there anything else I can try? Is it possible that it's locked in a "power waste" state that even a reset won't clear? Is there any "stronger" reset than shorting - and + battery pins with no batteries installed?

And a last question: why this happens so much to HP calculator processors? I have read about a lot of cases like this over the years. How can a processor fail to the point it draws hundreds of times the current it used to, but keep working perfectly?

Thanks for your help!

Eduardo