|Re: HP-41 CX dead overnight|
Message #13 Posted by Randy on 25 Feb 2006, 9:19 p.m.,
in response to message #1 by Geir Isene
In general, putting batteries in backwards or shorting the terminals in a 41 doesn't clear memory. Of course your mileage may vary.
It depends entirely upon the leakage current of the reverse polarity protection diode(s). HP used some good diodes there and as a result these methods usually meet with little or no success.
If you want to do a little experiment, remove the battery and measure the voltage present on the outer calculator terminals, the two center points are only a jumper. Leftmost is (-), right is (+) with the display at top. If the voltage is 4 to 6 volts, that's good because it means the unit is holding voltage as it should and holding memory . If it is zero-ish volts, that's bad and will require further diagnostics to find out is wrong.
For the purposes of the thread, the game is now to get that 4 to 6 volt memory maintaining level to zero to accomplish a master reset.
Short the terminals for a few seconds, now measure the voltage and see that it doesn't change.
Reverse the batteries, insert for a few seconds, remove and check. See that the level may have dropped a few hundred millivolts. Then watch it rise back up all on it's own. I'll leave it to reader to figure out why that is...
Reinsert the reversed batteries and leave it for awhile. Remove and measure. You'll find the level never goes below about 0.6 volts. Never (*see disclaimer below). It can't because you're dropping the voltage across a silicon diode, the very diode that prevents reverse current flow and the resultant damage. Because of that diode, chances are slim to none that you'll ever clear memory.
* Never disclaimer: Chances are the one unit you test will have a leaky diode and reset just fine when shorted :) Consider yourself unlucky and go buy a lottery ticket since no doubt your luck will change for the better.
** Never Never disclaimer. If that diode is leaky, reversing the polarity of the batteries MIGHT do some damage. Consider yourself sufficiently warned.
Now, back to the chase: Is the unit in question a Fullnut or Halfnut? BIG difference internally...