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Hi everyone,
I’d like some help concerning a current issue with batteries for my HP-35 and HP-67 calculators. I’ve been using battery packs marketed online by Mark Hoskins and been very pleased with their performance, but he has recommended me to this forum for tech help.

Some history, in case it’s important:

Both calculators use the same battery pack, so I bought one for each of them in August 2015 (usually good for at least 3 yrs, which has been found to be true). In case it matters, the 35 was bought new with its charger in 1974. The 67 was bought new with its charger in 1979 and was refurbished by HP when its card reader wore out. It’s been my experience that the 35 will function without the battery installed when used on the charger, but the 67 MUST have the battery installed even when using the charger

Thanks
(05-28-2020 02:42 AM)johnhaley Wrote: [ -> ]Hi everyone,
I’d like some help concerning a current issue with batteries for my HP-35 and HP-67 calculators. I’ve been using battery packs marketed online by Mark Hoskins and been very pleased with their performance, but he has recommended me to this forum for tech help.

Some history, in case it’s important:

Both calculators use the same battery pack, so I bought one for each of them in August 2015 (usually good for at least 3 yrs, which has been found to be true). In case it matters, the 35 was bought new with its charger in 1974. The 67 was bought new with its charger in 1979 and was refurbished by HP when its card reader wore out. It’s been my experience that the 35 will function without the battery installed when used on the charger, but the 67 MUST have the battery installed even when using the charger

Thanks

Is the behavior you reported using the same Power Adapter, or is each machine using its own separate Adapter? If the former, then the issue is in the 67 and other more knowledgeable folks will offer specific insights, but it will likely need surgery.

If each is using its own charger, then the 67 charger likely has a problem, or possibly a damaged cord. The classic chargers provide 2 types of power, one for operating the unit, the other just for charging the battery. It's possible the former is dead but the latter is still working, which is why the battery must be installed to operate.

If you have a multi-meter, check the voltage from the leads, they are marked on the connector, the center is (-). The (+) lead should be about 15V and the unlabeled lead about 5V.
(05-28-2020 02:42 AM)johnhaley Wrote: [ -> ]It’s been my experience that the 35 will function without the battery installed when used on the charger, but the 67 MUST have the battery installed even when using the charger

Thanks

Not sure if this is just experience or a question.

Page 312 of the HP67 user manual...

CAUTION
Operating the HP-67 from the ac line with the battery pack removed may result in damage to your calculator.

cheers

Tony
(05-29-2020 03:00 AM)teenix Wrote: [ -> ]
(05-28-2020 02:42 AM)johnhaley Wrote: [ -> ]It’s been my experience that the 35 will function without the battery installed when used on the charger, but the 67 MUST have the battery installed even when using the charger

Thanks

Not sure if this is just experience or a question.

Page 312 of the HP67 user manual...

CAUTION
Operating the HP-67 from the ac line with the battery pack removed may result in damage to your calculator.

cheers

Tony
It's not reccomendecd but it is "quite safe to operate the basic calculator from the AC adapter/charger with the battery removed" if you do not need to use the card reader.
Michael de Estrada explained it well in this old thread.
(05-30-2020 08:45 AM)aurelio Wrote: [ -> ]Michael de Estrada explained it well in this old thread.

A good read.

It seems to me that the VCC pin of the sense chip seems the sensitive one.
The HP-67 has about 3.7V connected, unless the charger is connected without the battery in which case it will have the charger supply and is maybe why the warning.

The HP-65 has the same setup as the 67 but as mentioned in the text has a different sense chip and maybe would serve as caution to those swapping this chip to fix a 67 and expect it to run happily on a charger only. The 65 manual has the same warning about charger operation without the battery.

The HP-97 appears to have a power on delay attached to this pin, but connected to battery through Q1. In which case if the battery is out when the charger is in would be fed with rectified AC from the charger. Not sure of effect, but maybe why the extra circuitry on this pin.

The HP-41 card reader has 6V for battery power (4xNN I think) and this appears on the sense chip VCC pin via 3 diode drops bringing it back to about 4V.

So battery power from these 4 setups ranges from 3.7 - 6V and normally the VCC pin max is about 5V, but would be higher and maybe damaging without a battery. The 5V max may suggest why the card reader is happy running on 3 alkaline cells.

For me, I would just heed the manufacturer warning :-)

cheers

Tony
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