Powering a HP 67 without battery pack?

[Lode]

For powering a HP 67 without battery pack (and without willingness to use any batteries):

Question 1) I read in multiple threads that powering it with the HP DC adapter/charger can break the card reader. However, this one is already known to have a broken card reader. Is it safe to use in that case?
NOTE: I tried it *very briefly* and it worked, it turned on and could calculate.

Question 2) What is the +/- side of the battery terminals? The terminals in the photo in this thread (https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=219662) are on the opposite side as in my case, in my case they are on the left (with the top of the calculator at the top of course). So are + and - reversed (+ up) in my case?

Questions 3) Is it safe to power it with an adjustable DC power supply to the battery contacts (once knowing what the + and - are)? should it be set to 3.75 V or to 4.5V? How much milliamperes is it expected to draw?
NOTE: a very quick experiment did not work, unlike the HP adapter afterwards.

Question 4) Is it safe to power it with USB (by modifying a USB cable first), given that USB is 5V?

Thanks!

[rprosperi]

Q1 - I will let more knowledgeable folks reply on this one.

Q2 - [Corrected] As Tony pointed out below, and actually seen in the photo, that machine in the linked article is a 35. And though on 35/45 the tabs come up from the right, they are still in the same relative position, so the battery sits in the same orientation, with (-) terminal on top and (+) on bottom, so even though tabs are different, the polarity is the same. Sorry for earlier comment and thx to Tony for correction.

Q3 - No idea why that would not work; the battery pack used 3 NiCD cells, nominally 1.2V ea. (but often a bit higher when fully charged of course), so 3.6V certainly should work.

Q4 - Yes USB can be used, there is a special cable available which does just that, but note that this cable connects to the CHARGING pin, not the operation pin (see the connector pinout in that same link). I can't say if direct connection to the battery tabs is safe, but I would not attempt it until someone here absolutely and clearly confirms so, the risk of damage is not worth it.

[teenix]

(05-27-2019 06:17 PM)Lode Wrote:  For powering a HP 67 without battery pack (and without willingness to use any batteries):

Question 1) I read in multiple threads that powering it with the HP DC adapter/charger can break the card reader. However, this one is already known to have a broken card reader. Is it safe to use in that case?
NOTE: I tried it *very briefly* and it worked, it turned on and could calculate.

Question 2) What is the +/- side of the battery terminals? The terminals in the photo in this thread (https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=219662) are on the opposite side as in my case, in my case they are on the left (with the top of the calculator at the top of course). So are + and - reversed (+ up) in my case?

Questions 3) Is it safe to power it with an adjustable DC power supply to the battery contacts (once knowing what the + and - are)? should it be set to 3.75 V or to 4.5V? How much milliamperes is it expected to draw?
NOTE: a very quick experiment did not work, unlike the HP adapter afterwards.

Question 4) Is it safe to power it with USB (by modifying a USB cable first), given that USB is 5V?

Thanks!

The card reader chip is fed direct from the battery to drive the motor and from the internal power supply to power the rest of the chip. I haven't seen any specifications for this chip so I don't know the safe voltage levels. The logic power source is 6 volt, If it is damaged, I would unplug the reader circuit board from the main board in case it causes trouble. The calculator will still work.

Higher battery voltages may work but could stress the power supply circuit.

Isn't that a HP-35 in the picture? The battery is where the card motor is.
The HP-67 -ve terminal is closest to the calculator top.

Yes it is safe to use an external power supply. It would be best to set it to 3.7 volts where the calculator was designed to operate. If voltage gets below (around) 3.5 volts, the low battery LED will begin to light.

Just be careful with voltage and polarity as there is no internal power protection. An error could cause total calculator failure.

The calculator can draw up to 250mA from the card motor depending on the cam adjustment. In your case the digits on the display will determine the max current. The current will be in the range 150mA to 185mA.

USB should supply up to 500mA (or more) but the power depends on the source, ie. PC, Wall wart charger etc. It could supply a fixed voltage regulator circuit for the calculator.

cheers

Tony

[Lode]

I heard a brief motor sound when connecting a DC voltage supply with 3.7V to the + and - terminals (with the calculator switch *off*). It also draws as much current as given (I stopped at 0.5 amp). But the calculator does not power on.

With the HP power supply connected to the 3-wire input, the calculator does work (so phew, that's not broken!).

So is it possible that the broken card reader is affecting how the power to the calculator itself from the battery terminals works, but somehow using the external 3-wire input with the official power supply manages to get around that problem?

How does one disconnect the card reader board? Are the screws under the sticker on the back? Shame to have to remove it in that case...

[teenix]

(05-28-2019 11:56 AM)Lode Wrote:  I heard a brief motor sound when connecting a DC voltage supply with 3.7V to the + and - terminals (with the calculator switch *off*). It also draws as much current as given (I stopped at 0.5 amp). But the calculator does not power on.

With the HP power supply connected to the 3-wire input, the calculator does work (so phew, that's not broken!).

So is it possible that the broken card reader is affecting how the power to the calculator itself from the battery terminals works, but somehow using the external 3-wire input with the official power supply manages to get around that problem?

How does one disconnect the card reader board? Are the screws under the sticker on the back? Shame to have to remove it in that case...

Assuming the power leads were connected correctly, it sounds like the sense chip is faulty and probably has some sort of short. If so, the chip may get warm (hot) with power applied. The motor should not turn on unless instructed by the CRC chip. You could disconnect the battery (red) wire from the motor and see if has seized by applying 3.7 volts to it. I think the capacitor on the motor connectors is polarity sensitive too.

It might pay to check the internal wiring from the 3 pin plug to the card reader board. I'm guessing the battery charger won't deliver the same current as your external supply.

If desired, you can unplug the card reader board from the main cpu board and unsolder the 4 wires and remove the board entirely. The calculator still works but whether this will ultimately cause problems, I don't know. The RA RB inputs to the CRC chip will be floating, but that may not matter.

cheers

Tony

[AndiGer]

If it works from the power supply but not from the battery pins check that the connector where you put the power plug bridges the left and right pin by an ohm meter.
Don't forget to unplug the power plug when it is not connected to the mains when trying to operate from the battery (pins).
Andi