HP 45 problem


Hello, I just turned on my dad's old hp 45 and the screen flashes zeros across, without coming to stability. What do you think is wrrong ? The rechargeable battery is probably bad, does not work at all without being plugged in. So, what do you think ? thank you, Kevin



it's just as you wrote, the battery is dead.

The HP-45, unlike newer models,

can be used without the battery, just on the AC adapter.

If that doesn't work, it's likely to be an internal contact problem.





I own an HP55, and it seems to me they are similar (if I am not wrong, all classics use the same AC adapter/charger).

Kevin wrote:

The rechargeable battery is probably bad, does not work at all without being plugged in.

If you do not plug the charger in an AC outlet and connect it to the calculator, the calculator won't work. The charger itself feeds the calculator with properly voltage while charging. If the charger/adapter is not connected to the AC outlet, connecting the cable to the calculator will simply diconnect the batteries terminals.

If possible and you have the proper means, try testing and charging the battery pack externally. Be carefull choosing an adequate charger based on battery characteristics: I'd use a maximum of 50mA to 60mA of charging current, considerign the batteries are not new and are able to supply at least 700mA. In this case, 60mA would charge them in about 12 hours. In my HP55 I use a thirdy-part battery pack, in fact it is three NiCad cells used in one commercial walkie-talkie.

Hope this helps a bit more.


Luiz (Brazil)

Edited: 26 Jan 2005, 3:12 a.m.


If you search on ebay for hp45 battery you will find someone selling them. They are good and work out cheaper than rebuilding the pack. I am just a very satisfied customer using my hp45 right now.



Quite possibly dirty ON/OFF switch contacts. Very simple to fix. Remove the six screws holding the back on, then remove the 8 screws that hold the keyboard backplate, take everything apart, clean the contacts and the slider, lubricate with silicone lubricant from Radio Shack and reassemble.


I vote for Jeff O's suggestion - at least as the first/easiest thing to try to fix.

My HP-35 often shows the flashing zeroes when I turn it on. Playing with the ON/OFF switch seems to be the fix. I haven't bothered trying to clean the switch yet.


The battery is not the problem. The 'classic series' machines without a card reader (35, 45, 55, 70, 80) will work fine from the power unit without a battery in place (and there's no risk of this damaging the calculator).

The fault could be just about anything from bad contacts to a defective (custom) chip. If you are happy doing electronic troubleshooting and have a multimeter and 'scope, then start by downloading the 'HP45 notes' from this site. Take the bottom case off the calculator (6 screws, 2 under the label, 2 under the front feet, 2 in the battery compartment I think) and you'll see the logic PCB. At the right hand side of that board as you look at it are 12 testpoints. The first check should be on the Vss (+6.2V) and Vgg (-12V) supplies. Check them both with a voltmeter _and the 'scope_ to look for ripple/noise.

If that's OK, I can tell you what to expect on the other testpoints, and what the likely chip failure is if one of them is wrong.


If you get flashing digits when the unit is plugged into the AC adapter, the problem is very likely to be a bad electrolytic filter capacitor in the power plug. The plug is held together with four screws (either Phillips or a small Torx, or even small security Torx). The bad cap is the large 330-470uF one... usually orange.


Great information, all of you. I will try the switch fix first, I can handle that, but electronics, well my only experience is college physics and wiring my house. I want to sell it but wondering if I can recoup my repairs and how much it is worth repaired. Thank you all for your help. I will let you know how it comes out. Kevin


Where would I get one of these (330-470uF one... usually orange) for the bad electrolytic filter ? does not sound too tough to put a new one in. thank you, Kevin


Try Radio Shack. Just about any capacitor in that range should work. Voltage rating should be 15V or higher. Just make sure it's physical size is not too big. Since it is an electrolytic cap, it is polarity sensitive. Make sure you get it in the right way.

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