Hi all.
This may not interest anyone but...(at least it's on topic...)
Vassilis is indeed correct in that the ext. power connector goes straight to the V+ regulator input. Wrong polarity will kill the calc.
I got a really messed up 41c recently...
For experimental purposes I soldered wires direct to the pads on the keyboard pcb after removing the gold balls and springs in my 1980 vintage 41c from Ebay. The wires exit the holes where the springs and balls were. I run the darn thing from a battery pack of four AA cells. I am going to fix the hopelessly destroyed battery connector with conductive silver from a pen applicator when I feel the need. THERE'S a HOT TIP for d-i-y repairers for free...
A switching regulator using a reference zener diode does step-up regulation and one could tweak this diode down a little to ease interfacing to 5 volt logic if one desired. The factory part is a 6.2 volt zener and I should imagine (given the incredibly generous margins in the hardware) that a 5.1 volt diode would be fine.
The BAT annunciator kicks in at about 3.6 volts on the battery buss.
When picking up from ebay/wherever you may occasionally encounter really messed up machines. Suggestion: If you get them, just use a current limited power supply to check them first as that way you won't cook the switching regulator. Some of these machines come with all sorts of highly conductive contamination in addition to the usual corrosion problems.
If running the machine from a low current supply with say a clonix module or very certainly a card reader but even a timer module (remember I have very old c/cv stuff, guys) you will find the ssytem won't boot due to voltage dips on the power buss due to transient current demands in the tens of milliamp range or hundreds for the card reader. You may get very strange displays from it though...
Anyone interested in more info please feel free to write me.
donwallace63@yahoo.com.au
Edited: 27 Oct 2005, 3:53 a.m. after one or more responses were posted