HP-97 Battery

[Nick]

I just got hold of an HP-97. The battery pack looks brand new, but I suspect needs to be rebuilt. Can the HP-97 be run with a power cord only? And what size battery cells are those? They look like "c" cells, but I'm not sure.
Thanks.

[Erik Wahlin]

The HP-97 cannot be run with just the power cord. You can damage the unit if you do.
The battery uses sub-C batteries. These are available at radio shack for about $16 for four.

[Tony Duell]

No, the HP97 cannot be run off the AC adapter without a battery pack in place. It may even damage the calculator to try it (but I think the 9x series -- Topcats -- don't generally mind).
You can cut open the plastic housing and replace the cells -- of course replace all 4 at once, even if some still seem OK.
They are Sub-C NiCd calls, get ones with solder tags already fitted. A good electronics shop would be able to get them.
Make sure you fit the new cells with the right polarity, of course.

[Andrés C. Rodríguez (Argentina)]

See article by Katie Wassermann here in the Museum, in the Articles Forum. She did a great work describing batteries replacements for many models.

[Nick]

Well, I rebuilt battery pack and still a dead HP-97. The display will breifly come on, for about 5 seconds and then fade to nothing. I'm fairly sure the new batteries must have some charge in it, and I also have the power plugged in.
what is the next troubleshooting step?
Nick

[David Smith]

It could very well be your charger circuit. After charging the batteries overnight measure the pack voltage. A fully charged new nicad will measure around 1.42 volts/cell. Nominal cell voltages is 1.2 volts/cell. A nicad is fully discharged at 1.0 volts/cell.

I have had to repair numerous '97 charger circuits over the years. In one particularly nasty (and all too common) failure mode the big power resistor opens up forcing all the machine current through a small PNP transistor. The PNP then decapitates itself and the other 2-3 transistors in the charger circuit die of fright.

The charger circuit on the '97 is located on the keyboard printed circuit. The Museum CDROM set has the '97 service manual... highly recommended reading.

[Tony Duell]

This does sound like power supply or battery problems. Take off the bottom cover from the HP97, and put the battery pack in place.
Measure the voltage between the battery terminals (a good place to find these is on the front 2 pins of the rear right section of the PCB).
Turn the machine on. If this voltage drops much from 4.8V, then you have not got a charged battery pack.
If the battery voltage is OK, then check the internal power supply lines. On the ACT chip (the 22 pin DIL chip at the front left corner ofthe logic board), check for +6.2V on pin 1 and -12V on pin 2. Pin 12 is ground.
If the battery voltage is low, even with the charger plugged in, tben check the charger circuit on the keyboard PCB (you have to take the machine apart to get to this :-(.
In particular check the 2 2W resistors on this board. They sometimes fail.
I assume you've checked that the AC adapter is giving the right output (about 12V with no load, I think).

[Nick]

Tony:
You called it. The Charger I was using was giving me strange readings. The battery pack was dead. I had a spare charger, an hp 82059 and the calculator immediatly started purring. And now the calculator even works without the cord. I also appreciate everybody's effort to help me.
Nick

[Tony Duell]

Sometimes it's the simple things :-). It's good that the calculator itself is OK.
The charger is electrically very simple. A transformer. The primary winding goes to the mains, the secondary goes to the 2 pin connector that plugs into the calculator. The transformer, though, is a little special. It's designed to have a relatively high internal resistance
-- my measurements suggest about 11 Ohms (with an open-circuit voltage of about 12V).
It's worth doing a couple of tests on the charger -- measure the resistance of the windings (i.e. between the 2 pins of the mains plug for the primary and
between the 2 contacts of the calculator connector for the secondary, with the charger unplugged from the mains and the calculator). If either
tests opne-circuilt, crack open the charger case (this is not always easy...) and check the resistance between the connections at the end of the cable inside. Most of the time, the fault is
the cables, not the transformer. It's often possible to find the break or replace the entire cable.

[Steve (Australia)]

Check out the articles forum for the posting about zaping nicads to rejuvinate seemingly terminally dead nicads.

I have found that it works in about 90% of cases in the short term (say a few weeks) falling to about 60% in the longer term.

I have yet to have this process do any damage to anything (well, any worse than the pack not working in the first place).

If you think it may be a while before you can rebuild the pack, it may be worth a try just to tide you over for a while.