HP Forum Archive 06

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HP 67 without battery Message #1 Posted by Edwin Morales on 2 July 2001, 4:10 p.m. It is safe to run an Hp 67 only with AC, no battery pack inside? Thanks.

Re: HP 67 without battery Message #2 Posted by Viktor Toth on 2 July 2001, 5:59 p.m., in response to message #1 by Edwin Morales The HP-67 will run without a battery pack (the power supply is regulated) BUT... you won't be able to use the card reader. The wall adapter doesn't supply sufficient power in the absence of a battery pack with some charge in it. I've also heard that in some cases, attempting to use the card reader without a battery pack caused chip damage, but I cannot confirm this from personal experience. In other words, it's best to have a working battery pack installed, or else, just hook up a regulated 3.75VDC power supply (that can deliver a couple of amps of current) directly to the battery terminals. Viktor

Re: HP 67 without battery Message #3 Posted by Viktor Toth on 2 July 2001, 9:12 p.m., in response to message #2 by Viktor Toth I stand corrected. Tony Duell is absolutely right: the card reader PCB is connected directly to the battery terminal, so when the adapter is plugged in without a battery pack present, it indeed receives the full idle voltage of the charger circuit, which I think was something like 17 Volts or so. In other words, even if you're not using the card reader, the sense amplifier chip is exposed to this excessive voltage. Viktor

17 Volts -- Ouch Message #4 Posted by Tony Duell on 3 July 2001, 6:06 p.m., in response to message #3 by Viktor Toth I didn't think the open-circuit voltage of the 'classic' charger was as high as that, but I've just measured it, and you're right. 17V between the 2 terminals used for battery charging with no load connected. I am not suprised this can damage the card reader sense amplifier chip.

Re: 17 Volts -- Ouch Message #5 Posted by Viktor Toth on 4 July 2001, 1:51 p.m., in response to message #4 by Tony Duell Tony, Yes, I was quite surprised when I measured the idle voltage myself, perhaps a few months ago. However, I didn't realize until I read your message here that the sense amplifier chip is actually exposed to this full voltage when no battery pack is present. Scary. I wonder why HP did this. The sense amplifier in the HP-65/67, all circuits in Woodstock calculators (but especially in C models), readily friable by HP's own charger, sometimes all that's needed is for the battery to pop out by accident. It'd have been so easy to prevent this (correct me if I'm wrong but I think a simple Zener protection circuit could have done the trick, at a cost that's negligible considering that the machines themselves still cost hundreds of dollars.) By the way, have you actually seen a dead sense amplifier? I have, occasionally, plugged in HP-67s without a battery pack, but I have yet to see a fried sense amplifier chip. Perhaps this one chip can actually survive the high voltage, or at least some limited exposure to it? (Wish the same could be said about Woodstock ROMs and CPUs...) Viktor

Re: 17 Volts -- Ouch Message #6 Posted by Tony Duell on 4 July 2001, 4:09 p.m., in response to message #5 by Viktor Toth My HP65 user manual actually warns you that it's possible t odamage the calculator by connecting the charger without a battery pack in place. I assume the part that can be damaged is the sense amplifier chip. I've not checked the 67 user manual recently, but I doubt that the sense amplifier would like 17V. On the other hand, no I've never seen a damaged sense amplifier chip due to this cause. The sense amplifer chips in the 65 and all the other machines (67, 97, 41 card reader) have different part numbers. The 65 uses an 1826-0158, the other machines use an 1826-0322. They have the same pinout and seem to be pretty similar. I have no idea what the difference really is -- maybe the later chip is (better) protected against overvoltage. I've thought about soldering a zener diode across the batter pack contacts (say a 4.7V one for the 65/67, a 3.9V one for the Woodstocks and Spices) to limit the voltage if the battery goes open-circuit. Since the charger is, of course, current-limited, a zener should work fine here, and I do wonder why HP didn't include one. But I am not going to risk a calculator by seeing if it protects the chips if the battery pack does fall out :-). Actually, I tend to charge my battery packs outside the machine for those machines which can be damaged if the battery goes open-circuit. For the classics, I either use one of the those reserve power pack units (I was given one :-)), or something like an 45 or 35 which are not damaged by open-circuit batteries. For the spice series I take the battery pack apart and put the AA cells into a normal charger. I don't have any continuous memory woodstocks. And the later machines (75, IL peripherals, etc) are not damaged if the battery goes open-circuit.

And HP says dont' do it Message #7 Posted by Tony Duell on 5 July 2001, 2:00 p.m., in response to message #6 by Tony Duell In a previous message in this thread I mentioned that the HP65 user manual said not to connect the charger without a battery pack in the calculator. I looked in my HP67 user manual today. It contains the same warning. It says something to the effect that the calculator may be damaged if it's connected to the charger without a battery pack in place. So I think it's _very_ unwise to do that, since the most likely part to fail is an HP custom chip (the card reader sense amplifier), for reasons given in this thread.

Re: And WHAT ABOUT THE CHARGER Message #8 Posted by Edwin Morales on 12 July 2001, 7:26 p.m., in response to message #7 by Tony Duell and...Can I use the hp 67 charger in a hp 65 with battery inside?

Re: And WHAT ABOUT THE CHARGER Message #9 Posted by Tony Duell on 12 July 2001, 7:44 p.m., in response to message #8 by Edwin Morales If you're asking if the HP65 and HP67 chargers are the same, yes, they are. The same charger (3 pin output connector) is used with the HP35, 45, 55, 65, 67, 70, 80 and the reserve power pack (battery holder thing) for the above machines. Many HP enthusiasts (including myself) have one or two of each type of charger that they use for various machines as required. HP were normally sensible. If something (designed to be handled by the user rather than a service engineer) will physically fit, then it's safe to try it. There are exceptions to this, but they're not common.

Re: HP 67 without battery Message #10 Posted by Tony Duell on 2 July 2001, 7:01 p.m., in response to message #1 by Edwin Morales NO! It is possible to damage the card reader sense amplifier chip if you connect the charger without a good battery pack in place. This applies to the HP65 as well. A little explanation. The 'Classic' series charger is electrically 2 units. One is a constant-current battery charger, the other is a constant voltage power supply for the calculator's electronics. When you plug the charger into the machine you disconnect the battery pack from the calculator (that's what the gold contact on the side of the plug in the machine is for), and connect the battery charger to the battery pack and the power supply to the logic. That's why it _is_ safe to run a 35, 45, 55, 70 or 80 (the models without the card reader) without the battery pack -- as soon as you plug the charger in, you disconnect the battery pack anyway. But on the models with card readers, the motor supply is taken straight from the battery pack. If you connect a charger to one of those without a battery in place, this supply rises to a rather high voltage. If you're unlucky, it burns out the sense amplifier chip (which also contains the motor control circuitry).

Re: HP 67 without battery Message #11 Posted by Edwin Morales on 2 July 2001, 8:22 p.m., in response to message #10 by Tony Duell Thanks....Interesting

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