HP-33C strange power / weak battery indicator problem

[btreguier]

Hi everyone,

I'm currently repairing my second HP-33C, and I'm confronted with a strange problem: the "weak battery" indicator is constantly on even with fully charged batteries, and the calculator also has an erratic behaviour.

I then replaced the batteries with a lab power supply, and gradually went up with the voltage, starting at 2.6V, to see at which point the indicator would vanish, and it did, but only around 3.08V, at which point, when the "STO ENTER" test is attempted (and all the display segments are on), the power consumption is about 150mA. The calculator behaves normally at this point.

As I had a few spare parts, I replaced some components one by one to see whether I could fix the problem, so here is what I replaced:
- the power board
- the processor (the one in the 33C was a 1820-2105-B, which I replaced with a 1820-2105-A from a 33E)
- the LED display

All to no avail... :-(

In fact, the only active component that I did not replace (as I had no spare part for that one) is the little 8-pin chip, which I believe is the memory (but not sure about that). So it sure might be that last component which is faulty, but I may have missed a few points, so if anyone out there has an idea about what else can cause such a problem, I'd be interested.

Thanks in advance!

Bruno