Ex-NOS 41, battery compartment modified & ROM?



Picked up an ex-NOS 41CV thru the Classifieds.

Unit is in excellent condition, however does not power up when batteries inserted. Upon closer inspection, when looking into battery compartment, a pair of wires (one white, one brown, copper core) have been routed thru the cavity on the left of the battery housing, in that space used for recharge leads. They are held in place by a small black plastic block and contact cement. Haven't opened her up yet, don't want to mess with it without a bit of direction. It appears as though it is a mod for an external power supply.

The unit came with a proprietary ROM labelled "C-9B P-S APAC WT. & BAL. 12/30/93 1/1" It has a keyboard overlay labelled with "CREW, ZERO, EMPTY, PAX" etc. My guess is that it was a weight and balance calculator for an aircraft dispatcher or weight and balance authority.

1. Can anyone help me with getting the battery power reconnected?

2. Does anyone have any user instructions or other documentation for the mystery ROM?

Nick Hayes Air Transport Pilot.


I also got one calc from this lot. Mine works without any problems. The small thing in the entrance to the battery compartment is a so-called speed-up. The 41 powers up in fast mode (Try BEEP to hear the difference, should be about twice as fast as the original). If you tip onto the small point on this block, you change the speed. The block is a switch.

Don't try to remove it, you can't switch back into normal mode if you do that. You can change the speed doring operation of the calculator which indicates that the speed up is well done, more than just a switch and a inductivity or capacitor.

Maybe you should clean the battery contacts and remove the ROM. My contacts look excellent, but I remember having problems by inserting the batteries the first time. My pattery pack is somewhat hard to put in and out of the compartment (this is caused by the switch) but fits well. I never had problems with it since that first time.

My suggestion: Remove the batteries and the ROM. Let all of these lie some time (maybe days, but mostly some hours should be enough) without the batteries and then try to reanimate it by putting in new batteries. This will unload the internal capacities in the calculator and cause a complete reset. If the calculator works without the ROM, insert it again to check if the ROM is faulty. I power the 41 with NICADs and I'm rather sure it's a contact problem with the batteries or the module.

I also don't have any information about the ROM and I'm also interested in knowing something about it. I saw that the ROM takes and sets the 41 into USER mode when the machine is powered up. This is a common practice for custom ROMs.

Does your calc also have a green point between USER and PRGM keys?


The ROM is particularly picky about where it is placed in the machine, it must be in the slot 1. If you reset the machine the ROM seems to autostart and set the user mode keys. There must be only one copy of the ROM in the machine or the machine will not start at all! As Reinhard comments, remove all ROMS, reapply power do a On-Backarrow and see what happens.


Guys, thanks for the tips.

Unit was shipped with no batteries, and took more than 4 days, so should have been completely powered down. Have now removed ROM and batteries for 24 hours, will try again tomorrow.

Power on with backarrow tried, unsuccessful. No life apparent. Battery contacts look fine, will clean with spirits and cotton swab. Batteries are new and power up my other 41 ok.

ROM tried in the other 41 also, but in slot 3 as I have the timer module in 1 -- remember hearing that a proprietary ROM should auto-exec in slot 1. Was able to CATALOGUE 2 the contents, but guessing as to the application. Looks interesting, but some user documentation would be needed.

Thanks again for the help -- will see if your sggestions get it going. If not, seller has offered to exchange it for a not so pristine 71. See how we go.

And yes, it does have a green dot where you said.


Reinhardt, how does it switch? is it toggled by jumpering the contacts?


It sounds like the same speed-up conversion that I installed in my HP-41CX 12 years ago. It is toggled using a momentary switch. That is, make contact to switch on (fast speed), make contact to switch off (normal speed). I verified this when the switch came off the wires, and I could toggle by touching the wires together.


Dan, you're completely right. If you look exactly onto the switch, you'll see a small "point" standing out of the switch, on the side where the battery cover should be.

You can tip onto this and your 41 is faster. Do this again and you have a slower 41. Test this with the BEEP command -> fast= higher tones.

The switch toggles a FlipFlop, which controls a CMOS switch with a parallel inductor, I think. I didn't open the 41, but it's the way I would have done this. I remember reading an article with a similar schematic somewhere.


Instead of waiting for days for the power to dissipate, short the two outside battery connections. Then press the On button, then Enter and Clx several times. This ususally does it! 805-469-6714

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