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HP35 problem with a calculator, maybe due to a faulty charger
03-22-2021, 04:55 AM
Post: #21
RE: HP35 problem with a calculator, maybe due to a faulty charger
The Woodstock and later chargers aren’t regulated, but I thought the classic ones were (yes, they can fail; so can a bench supply, although it is probably less likely and also easier to figure that out before it’s too late). Then again, no supply may resist a lightning strike…so, yes outside of the calculator is best. Tough for the 19c as the only thing that fits is an HP-10 which doesn’t have continuo us memory, but also won’t resist a lightning strike and is harder to fix/replace than the 19c.-kby

(03-21-2021 03:50 PM)mfleming Wrote:  
(03-21-2021 09:37 AM)ThomasF Wrote:  as chargers becomes more and more unreliable, do you know the best way to charge battery packs outside the calculator?

The simplest approach is to use a battery pack where the batteries can be removed for external charging. You can modify an existing pack by hand or buy a suitable pack from the previously mentioned supplier. I'd do what Maximilian suggests and toss all chargers in a box Wink
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03-24-2021, 08:55 PM (This post was last modified: 03-24-2021 08:57 PM by aurelio.)
Post: #22
RE: HP35 problem with a calculator, maybe due to a faulty charger
(03-21-2021 06:52 PM)teenix Wrote:  
(03-21-2021 04:43 PM)aurelio Wrote:  Well, if the voltage on test point is the one expected it means maybe that the PSU works properly, so what would be the next step in the troubleshooting?
What does it mean that if I use any of the numeric keyes, the calculator turns off, while clicking on the functions the display shows numbers that vary, from time to time, with different criteria?

I would try swapping a known good CPU. If you feel this is too risky, then Harald may still have replacement Classic CPU boards which you can try instead.

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It may be a faulty ROM.

When the keys are scanned, a key code is generated and this value is used as an address offset in ROM so that the processor can execute the code for that key press. Very efficient.

If the keycode is invalid, the Program Counter could be set to an incorrect ROM location causing the code to run in error.

If one or more ROMs have failed then code execution will fail also.

When a key is pressed, the display is turned off before processing begins. It is possible the code is still running but in a dodgy loop that makes the calculator look dead.

cheers

Tony
Thank-you Tony and thank- you Harald and congratulations for the HP Classic upgrade kit - Multi Emulator project .......I missed it when it was published, a few years ago Sad
I'll follow your advices and meanwhile I'm "playing" with the faulty board with the presumption of understanding something more.
Yes maybe when i press a digit key "the display is turned off [....] it is possible the code is still running but in a dodgy loop that makes the calculator look dead.", I checked the test points and found alll the values of the calculator "on" and this maybe is a confirm that there's not been a power reset...
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03-24-2021, 11:08 PM (This post was last modified: 03-24-2021 11:08 PM by teenix.)
Post: #23
RE: HP35 problem with a calculator, maybe due to a faulty charger
This is the HP-35 reset circuit.

It would difficult to test without an oscilloscope.

cheers

Tony


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04-01-2021, 06:23 AM
Post: #24
RE: HP35 problem with a calculator, maybe due to a faulty charger
Related to this, and with my fear of using the old chargers in my calculators, I modified and build my own charging pack.
I followed the information on this page:
https://davidreaton.com/hp-calculators/h...-chargers/
Replaced the resistor (and the capacitor) to lower the charging current from the charger, and built my own external charger with led feedback.

   

Right now I just built it on a prototype board, but will make a better box for it when I get the time ... Big Grin
So now I can slowly charge the NiMH batterypacks outside the calculator in a safe way Wink

Cheers,
Thomas

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04-01-2021, 05:50 PM
Post: #25
RE: HP35 problem with a calculator, maybe due to a faulty charger
(04-01-2021 06:23 AM)ThomasF Wrote:  Related to this, and with my fear of using the old chargers in my calculators, I modified and build my own charging pack.
I followed the information on this page:
https://davidreaton.com/hp-calculators/h...-chargers/
Replaced the resistor (and the capacitor) to lower the charging current from the charger, and built my own external charger with led feedback.



Right now I just built it on a prototype board, but will make a better box for it when I get the time ... Big Grin
So now I can slowly charge the NiMH batterypacks outside the calculator in a safe way Wink

Cheers,
Thomas
thank-you Thomas I know that interesting site that iis the same Ivisited and linked in a previous thread The circuit has been rebuilt using modern transistors nowadays available
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04-16-2021, 01:34 PM
Post: #26
RE: HP35 problem with a calculator, maybe due to a faulty charger
Hi all, it is definitely a CPU board problem, I will investigate forward, when I will reach the skill to do it
I enjoy for now the Tony-Harald's emulator which brought my calculator back to a new normal life Smile

The normal life which we all dream, and will be after this pandemic sufference period Smile
thank-you very much
Aurelio
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