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HP-80 display problem.
10-22-2021, 08:13 AM (This post was last modified: 10-22-2021 08:19 AM by teenix.)
Post: #2
RE: HP-80 display problem.
The resistor is to set an internal reference for the battery low indication (all decimal points being lit) I don't know why the resistor is different for the Classic models as the anode driver chip has the same part number. Maybe different battery loads ??

The anode driver is wired so that the decimal point for all digits should light in a low battery condition. If it is not lit on the actual decimal point digit under low battery, then that is strange, even stranger if the c segment does not light on that digit.

In normal operation, is the C segment lit regardless of what is on the display and does it also light where the decimal point is?

The fault is not on the CPU board, so check pin 1 of the anode driver and see if it is Logic 1 all the time under normal operating conditions. If it is this will be decoded inside the anode driver to display segment C all the time.

It should stay low all the time if only 2's are on the display.

If you can clearly see that this pin is changing when changing the display digits, then the fault could either be a faulty display - maybe an internal short, a short on the display board pcb tracks, (corrosion etc) or the anode driver has failed and is not decoding the incoming data properly.

Under certain conditions, an anode driver output can be destroyed by inductive voltages created by the display coils. It could be internally shorted for the C segment.

More display info can be found in my Classic Notes PDF file if you are interested.


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Messages In This Thread
HP-80 display problem. - jlw - 10-22-2021, 06:15 AM
RE: HP-80 display problem. - teenix - 10-22-2021 08:13 AM
RE: HP-80 display problem. - jlw - 10-22-2021, 11:49 PM
RE: HP-80 display problem. - teenix - 10-23-2021, 12:51 AM
RE: HP-80 display problem. - jlw - 10-23-2021, 06:04 AM

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