Repairing vintage calculators

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Leviset
Posts: 4
Joined: Sat Jul 15, 2017 4:19 pm

Repairing vintage calculators

Post by Leviset » Sat Jul 15, 2017 9:48 pm

I'm a total beginner in Electronics and Electrical Engineering. I'm a 68 year old retired IT Manager and for some years I've collected mainly HP and TI vintage calculators which I also program.
My current project is very personal to me; it's an HP-97 calculator\computer which appears to be dead to the world. It takes a 6v AC input from an HP adaptor (which I've tested and is supplying current) which also recharges a built in 4.5v DC battery pack. I've tested both battery packs I have and neither are holding any charge. I'm led to believe this calculator will only work with the power adaptor IF a working battery pack is in place and will run just on a charged battery pack. I've ordered a new battery pack which should be here in 2 weeks time. I've decided to take an online course in Electronics in the hope I may be able to fault find these problems myself.
Are there any members living in Sheffield, UK please who might be able to help me troubleshoot this HP-97, I do have a PDF of the Service Manual which shows all the circuitry.
Thanks
Dennis Tuckerman

David
Posts: 216
Joined: Sun Oct 04, 2015 6:18 pm

Re: Repairing vintage calculators

Post by David » Sun Jul 16, 2017 11:03 am

Although I don't live anywhere near Sheffield, it would probably be of use if you could post a link to the pdf, or just the circuit diagram to the thread.

Leviset
Posts: 4
Joined: Sat Jul 15, 2017 4:19 pm

Re: Repairing vintage calculators

Post by Leviset » Fri Jul 21, 2017 11:08 pm

The HP-97 Service Manual can be downloaded at
http://www.nightfallcrew.com/wp-content ... manual.pdf

David
Posts: 216
Joined: Sun Oct 04, 2015 6:18 pm

Re: Repairing vintage calculators

Post by David » Tue Jul 25, 2017 6:38 pm

Thanks for posting the manual.
From section 2.41 (battery charging) the presence of the installed batteries is necessary, and indeed the text states that the calculator will not function correctly without having them installed.
Looking at section 2-22, each of the seven 10 ohm resistors which form the printing head draw 500 mA (as a pulse) when a mark is to be made on the heat-sensitive paper. Therefore the highest peak will be drawn when the head is printing a vertical line - some 3.5A (possibly a bit less, because the head moves while printing, so the vertical line may be tilted a bit to the right) and this way of working will be specifically designed to allow a high current to be drawn from the batteries during printing, rather than trying to draw the printing current in full from the charger.
Presumably you have now received the replacement batteries and all is OK?

Leviset
Posts: 4
Joined: Sat Jul 15, 2017 4:19 pm

Re: Repairing vintage calculators

Post by Leviset » Fri Jul 28, 2017 12:05 pm

Still not working. The battery pack for the HP-97 is the same as used in the external printer of the HP-41.
I've charged it and it maxs at about 5v DC.
With both the power adaptor and the battery pac installed I now get a single red dot on the LED screen which is new and I believe it to be the power charging light. No other keys do anything and I'm yet to get the red 0.00 screen

David
Posts: 216
Joined: Sun Oct 04, 2015 6:18 pm

Re: Repairing vintage calculators

Post by David » Fri Jul 28, 2017 7:35 pm

That's a pity. It is always very difficult to do fault-finding at a distance, but you could start with the power supply circuit as shown in Section 2.37. this should produce two outputs: +6.25v and -12.0v. Check that these are correct, and then follow the tracks across the boards. Also look for broken wires or loose connectors, and check the power-on reset circuit (sect. 2.43). That may help.

Obviously, if you have a faulty chip that may be the end of the matter - do you have enough test equipment (scope, function generator, etc?)

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