HP-67 - no battery - Bad Idea?


Should I be buying a battery pack for the 67 I just obtained before applying power? I believe I have a dead HP-45 with an equally dead battery pack. If it's not good to power the 67 without a battery installed, would a dead pack do the trick?



I have no direct experience on such a case but, based on previous postings, you should have a GOOD (clean contacts AND charge keeping) battery pack, at least every time you use the card reader.

It should not be difficult to rebuilding the dead pack you have, using new AA cells. And check the contacts for oxide, corrosion, dirt, etc. which may prevent the cells to do their functions.


Now, this is cool.

Your mentioning of AA cells gave me a Blinding Flash of the Obvious: I had recently rebuilt a battery pack with three AA size NiCads for one of my HP-75s! I pulled out the defunct HP-45, and sure enough, the battery pack was identical to the one I had rebuilt for the 75. So the next step was to yank that working pack out of the 75 and put it in the 67. I did that, and it worked! I had a big grin on my face until I noticed the "property of" sticker on the bottom of the calculator. My "new" 67 was pristine! What the heck??

I had put the working battery into the "dead" HP-45! Hee hee!

Edited: 3 Sept 2005, 5:47 p.m.


at least every time you use the card reader.

Actually the possibility of damage to the card reader circuitry exists regardless of whether you are actually using the card reader. It's a matter of the unfiltered supply voltage from the adapter being wired directly to the sense amplifier chip.

The possibility of damage due to missing or bad battery packs is arguably about the dumbest thing HP did in their second-generation calculators. I can't imagine that it saved them more than $0.25 worth of components that would have prevented the damage, or $0.50 worth of components to make it actually work well without the battery.



The card reader circuitry can be damaged if you run the HP-67 with no battery pack, a bad battery pack, or a battery pack making poor contact (e.g., corrosion).


All the LED classics without card readers can safely be run with the wall charger only with no battery in place.

In the 65 and 67, the battery is directly connected to the card reader module to drive the motor. That also connects it to the sense amplifier chip. Without a battery, the open circuit voltage reaches 16 volts, more than enough to do some damage. There are cautions about this in the manuals.

The best solution is to test with a dc power supply capable of at least 1 amp. Actual operation is less than 300 ma but the c/r motor is a real current hog.

Many thanks to Tony Duell for pointing out this difference in a thread awhile back.


Well, the 67 worked fine off my rebuilt battery pack, but when I plugged in the AC, a indicator "dot" appeared in the lower right of the display. (Low battery indicator?) After I removed the battery pack and reinserted it, the calculator again worked fine on battery alone, but would not turn on at all with AC plugged in. Flexing the machine slightly brought back the old behavior of the machine up with warning indicator lit.

I plugged the AC transformer into the (suddenly resurrected) HP-45, and it worked fine. The battery pack also works fine in the HP-45. And the combination of transformer and battery pack also is just fine in the 45. So I suspect a poor/intermittant connection somewhere in the external power circuit(s) of the 67. There's no obvious sign of corrosion either in the battery compartment, or on the three external power pins.

There is no service manual for the 67 on the museum DVD. It would be nice to have some kind of road map before opening this one up. Does the HP-67 service manual exist, and if so, does anyone have it in digital form? If not the latter, is someone on the North American continent (or a very patient South American, European, African, Asian or Australian) willing to send me the manual so I can scan it? (Micronesians and Antarcticans shouldn't feel left out, either. 8)


I've never seen a service manual for the 67, although one must have existed at HP service centres. There are basically 3 sources of information :

1) The HP97 service manual on the MoHPC CD-ROMs. The machines are similar, although obviously the 67 has no printer or PIK chip, and the PSU circuit is quite different

2) My HP67 Notes on the articles forum here

3) The HPCC schematics CD-ROM contains a hand-drawn schematic for the 67.


Here's my take: a)your 67 works fine on battery power, b)you have two battery packs with two safe units to charge them in, and c)plugging an ac adapter into the 67 can be dangerous just because you don't know if a connection is good or bad at that moment.

Why not just run it on battery only? Save peeling up the (pristine?) back plate and taking apart a calc that works just to run it on ac; one unlucky speck of dust on the contacts and that can kill it even if you fix the present problem. I'm assuming that you don't need to do a gummy wheel repair anyway.

A note: I was having trouble with the battery connection on a 67 and the contacts looked clean but i lightly scotch brited both the ones on the calc and the battery anyway. It works fine now. I guess there was some invisible poop there.


Good thoughts, thanks.

However, the card reader does need the gummy wheel repair. It isn't urgent, however. I'll leave it be for now. I've just ordered to extra batteries so I can run two 75's, the 45 and the 67 on batteries. And I can charge in the 45. I will give the power posts a quick sanding and see if that helps. Thanks for the suggestion.


Hi folks,

the Classics HP wall charger (three pins) has a voltage regulator circuit which will absolutely prevent any damaging circuit EVER to appear on the calculator - as long as the regulator circuit has no failed component by itself.

The HP67, despite of being a Woodstock, uses a Classics wall charger.

So it is safe to use the HP67 without battery, running from its HP provided wall charger. When the calc is switched OFF, the output voltage is around five Volts - it will certainly NOT damage any chips.

The only problem is that the wall charger is too weak to run the card reader motor. From my reverse engineering work on these calcs I do see a remote possibility that the attempt to use the card reader without inserted battery might overload and damage the wall charger itself. And then, overvoltage might occur which damages the calculator.

But just using the HP67 (without attempting to use the card reader) from the wall charger - even without inserted battery - is safe.

On all of the other Woodstocks - HP21, 25, 27, 29 - with the two pin wall chargers, any attempt to run them without properly working battery and good battery contacts leads to overvoltage and killed chips in the calc.

best regards,



There is a reason why HP warns of use without a good battery in place in both the 65 and 67 manuals.

Please see http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv015.cgi?read=71828#71828 for a recent discussion and explaination of the dangers.

(Sorry for the 1970's USA TV reference in the title)

Edited: 4 Sept 2005, 9:57 p.m.


Hi Randy,

as far as I have guessed out the 1826-0322 innards, the unswitched battery voltage (which indeed may go to +15V if no battery is inserted and the wall charger used) just goes to a series power transistor which is part of the card reader motor speed control circuit. As long as this transistor is able to withstand the +15V, no damage will be caused. Typical 1970's bipolar process technologies were able to handle that (see the +/- 15V supply voltage on opamps of this time), and it is fair to assume HP engineers knew what they did when they designed the 1826-0322 and took the required precautions.

IMHO the danger of damage lurks in the charger circuit itself. They have a current source circuit which limits the charging current (and hence, the motor isn't able to pull a card through, as it is current starved), but the activated motor acts much like a short circuit. Then, the series transistor in the charger's current source circuit has to sustain significantly more power loss (heat) than during charging, where the battery consumes much of the power. It is known that overheated bipolar transistors may break through, and then the current limiting gets lost, and whoever knows which component will give up next.

So there is certainly a danger involved when running a HP67 from a charger without any battery inserted, but as long as no attempt is made to use the card reader, IMHO it should be safe. I ran my own HP67 for years just on the charger without battery (and did not use the card reader) when I was fed up to pay HP's exaggerated prices for replacement batteries, and used it until I bought a HP15C years later. A while ago I rediscovered my HP67 (and some other LED HPs I used during school and university) and after fixing the gummy wheel problem the card reader still worked.

However, there is a proverb which says "Better safe than sorry". If anyone who has read my post feels uncomfortable about the alleged or presumed risk of blowing out the 1826-0322, then go the safe way and always use a battery.

I prefer to keep these acid - spitting beasts out of my valuable HPs whenever possible and charge them only in my least valuable (worn / corroded) calculators, and never in the good ones.

For the HP65, I can't give any testemonials as to the danger of damaging its card reader sense amplifier / motor control chip, as it is different from the chip used in the HP67 and I don't have a HP65 to experiment with.

best regards,


Sorry to butt in, but "Lost in Space," from which you quoted, was USA 1960s.


True John! That's where those DNA splices ultimately came from too!
(From those psychic lab rats...)



God, was it that long ago?


I disagree with you! There is a danger.

The classic-series (3 pin) adapter/charger has 2 outputs. One is a constant voltage of about 4.2V. The other is a constant current (about 50mA) to charge the NiCd cells. The voltage of that second output is unregulated and will rise to 16V or so if there's no load.

The 3 pins on the classic calculator connector are ground (in the middle), the +ve side of the NiCd battery, and the +ve supply to the on/off switch and thence to the logic. The last 2 pins are connected by a spring contact in the plug on the calculator, allowing the battery to supply power to the calculator. When you plug the charger in, the spring contact is forced aside, the battery pack is charged from the constant-current supply, the constant voltage supply feeds the logic.

_BUT_ on the card reader models -- the 65 and 67, the card reader sense amplifier chip, the one on the card reader PCB itself, is powered from the battery pack directly. When you plug the charger in, it's effectively fed from the constant current supply. If the battery pack is in place, the battery effectively clamps that voltage to about 4.2V. If the battery is missing, dead, open circuit, etc, then the card reader sense amplifier gets 16V across it.

The sense amplifier was redesigned between the 65 and 67 (the latter chip is also used in the 97 and 82104 (HP41 card reader). The 2 chips have the same pinouts, and seem to be interchangeable. I conjecture that one of the changes was to make the chip less likely to be damaged if the battery back is missing (the 65 manual has a large warning about this, the 67 manual doesn't). However, I don't like risking it. This is a custom chip, spares are very hard to find.

There is no danger in running the 35, 45, 55, 70, 80 from the charger with no battery in place.


For what it's worth I definitely agree with Randy.

I stayed out of the thread as it has been 25 years plus since I heard these warnings and I dodn't have time to look at any documentation I MAY have (I just rxed a DVD on HP stuff but haven't looked at it much yet; so many chips to solder, so little time...)

Randys warnings all sound perfectly logical to me. They match my recollections of what an HP65 (67?) owner I knew found out the hard

Nasty design flaw if you ask me... Oh well, nobody's perfect.



Randy and Tony are right.

Be warned, Howard. (Long live your hp65 /67, whatever...)



Based on the discussion here, I have refrained from running the machine without a good battery installed.

Possibly Related Threads...
Thread Author Replies Views Last Post
  Bad Flash in HP Prime Han 11 1,431 09-27-2013, 12:38 PM
Last Post: Han
  Stupid idea of the day: 41-compatible watch bhtooefr 0 381 03-31-2013, 08:23 AM
Last Post: bhtooefr
  Cheap hp32sii in bad condition on sale on Taobao Waon Shinyoe (China) 1 470 03-01-2013, 08:35 PM
Last Post: Raymond Del Tondo
  Idea for a 41C rom of "big" programs Gene Wright 7 954 10-05-2012, 04:29 AM
Last Post: Mike (Stgt)
  39GII -> 50G idea bluesun08 14 1,283 08-19-2012, 12:03 AM
Last Post: Dave Shaffer (Arizona)
  HP Xpander. A good idea that could've worked? Matt Agajanian 1 381 05-24-2012, 05:01 PM
Last Post: Luiz C. Vieira (Brazil)
  HP 67 Calculator Battery Terminal plus/minus Steve E. 6 789 04-30-2012, 03:29 PM
Last Post: Nick R
  aOT--Battery/battery contact care advice needed Matt Agajanian 0 405 04-24-2012, 07:35 PM
Last Post: Matt Agajanian
  HP-35S..Not so bad Matt Agajanian 19 1,701 04-23-2012, 02:00 AM
Last Post: Valentino Ducati
  95C--Seemed like a good idea at the time Matt Agajanian 7 795 04-14-2012, 04:47 PM
Last Post: Matt Agajanian

Forum Jump: