HP-67 curiosities


Hello all.

Yesterday, I just got my HP-67. BUT, as excited as I was, I took for granted that batteries were already in the calc. So, I plugged in recharger. Figuring that the batteries needed a full charge cycle, I left the 67 charging for 14 hours. While it was charging, I was able to try some calculations.

Well, this morning, I unplugged the 67. I turned it back on and NO display. So, I went to look at the batteries! WHOA! No batteries were in the 67 to begin with. YIKES!! And then I flashed back to the message in the manual stating that plugging in the calc with no batteries installed could damage the 67. And, I'd left it charging for 14 hours!! OUCH!

But, when I put in a battery pack, the 67 behaved and I did see the red 'low battery' light. So, I am now charging the 67. As it did from yesterday, the 67 operates with the charger plugged in WITH the batteries inside.

BUT, could that 14 hour mishap have tormented my 67? Please let me know.


Edited: 15 Mar 2012, 12:27 p.m.


Try the card reader and you will know.



The HP67 uses a three-pin adapter which allows you to operate the calculator while charging. Fact is that the charger itself provides a regulated DC voltage to the calculator circuitry while an unregulated voltage is used for charging the batteries. See the schematics here (search for HP 82010A).

There is an issue but I am not completely sure about it. While the calculator circuits are 'protected' by the use of the regulated voltage, I remember reading that the charging circuit (edited to add) inside the calculator, fed by unregulated voltage, uses the battery voltage as a reference. And because of the current amount needed to drive the card reader motor, this same charging circuit feeds the motor itself. So, using the card reader when charging it while without batteries is not wise - I mean, it is actually unwise. You may burn the small motor and possibly its control circuit.

I think this is what it is.


Luiz (Brazil)

Edited: 15 Mar 2012, 12:52 p.m.


Well, can this be fixed?


Yep, it can.

Anyway, first you must be completely sure about it. Is this the same 67 you mentioned in the other thread? If so, do not take 'desperate measures' so quickly!!

Make some tests, first. It is fairly possible that the pulling gear needs specific maintenance. If the motor still works (you can hear it, right?) but the card is not pulled in, it does not mean the motor is the problem.

Let's just get another step, and unfortunately some skills are needed. I'm 50 YO, I still repair almost everything I can. I even solder SMD-packed IC's with a regular, fine-tip soldering iron.. without magnifiers!

And I am completely aware of the fact that many others closer to you can do that too. So, you are not alone... Help is available!

Cheers. And success!

Luiz (Brazil)


Well, thanks Luiz. So, if as you say, the motor is probably not the glitch, what do you suggest could be the glitch? Secondly, when you say 'And I am completely aware of the fact that many others closer to you can do that too. So, you are not alone... Help is available,' do you mean someone here at MoHPC could help?


The roller and/or the elastic coupling between the motor shaft and the roller definitely stop working over the years. If the card (or a sheet of stiff paper) gets covered with some goo, the roller has disintegrated. If you can sweep the card through without any mechanical resistance the roller has even completely vanished and you'll find the remains as a blob somewhere in the case.

The elastic coupler is just a short piece of tubing joining the roller axis and the motor shaft. Over time this dries out and no longer transmits enough force from the motor to the roller and you will see no card movement but hear the motor running.

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