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HP48G Hardware Troubleshooting Questions/Tips
10-11-2019, 04:07 PM (This post was last modified: 10-11-2019 08:49 PM by Phenixg.)
Post: #1
HP48G Hardware Troubleshooting Questions/Tips
Hello Guys,

I would like to know if any of you have some tips to determine if a processor is dead. My HP48G stopped working a few weeks ago and I tried all the well know tips to revive it but none of them worked for me.
Here are what I have done so far :
* The keyboard foam have been replaced and the connector cleaned with isopropyl alcohol
* Zebra connectors cleaned
* Tried the warm reset, the pinhole reset, the capacitor short and the battery connectors short.
* I left it without battery for 2 weeks.

Here are what I can see:
* There is no sign of life on the display. Nothing when I just connect the batteries, nothing when the "ON" button is pressed and after.
* The "ON" key seems to work: I measured voltage between ground and the ''ON" connector, I get 4.x volts when the on button is pressed; I get 0.0 volt when released. I also measured the resistance, when the on is pressed; the value seems to vary.

Q1: Is there any specific voltage or resistance(or whatever parameter) in some specific spots on the board that would confirm that the processor cannot wake-up or being prevented to wake-up by some other components failure?

Q2: Is there a way to test that display is receiving signals from the processor.

Q3: What are your thoughts?

Thank you Guys.
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10-12-2019, 01:24 AM
Post: #2
RE: HP48G Hardware Troubleshooting Questions/Tips
A crude approach I applied recently is to swap cpu boards with a known good 48s|g calculator. You can also find schematics here at hpcalc.org:

https://www.hpcalc.org/details/3543

Those may provide help finding a clock signal, address line, etc. to see if the cpu is functioning.

Who decides?
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Yesterday, 03:54 PM
Post: #3
RE: HP48G Hardware Troubleshooting Questions/Tips
Thank you mfleming. I am working on it.
I wish there were as much detailed information about this landmark device as there are about the venerable 41. I really envy the 41 people; It is such a regrettable thing that some of us discarded our beloved toy just because a banal resistor become open or common transistor shorted.
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