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Odd sign problem with HP-97 - Printable Version

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Odd sign problem with HP-97 - Dan W - 07-15-2009

I have an odd problem with the CHS key on my HP-97 I have never seen before.

When I enter a number and hit the CHS key, a minus sign shows twice, at the left where expected, and at the right where it is used for the EE component. However the number is still correct (a negative number). The extra minus sign stays after hitting ENTER.
If I do a calculation that results in a negative number, both sign segments show.

When I enter a number with an exponent and try to change the sign of the exponent, no minus sign shows, although the number is entered correctly. However even though the number has a negative exponent, the minus sign does not show.

Also if I do a calculation that results in a number with a minus exponent, the minus does not show, but the number is correct.

The problem is with the LED display only, the numbers print with the correct signs and the math is right. It passes the HP97 diagnostics program.

So it seems the problem might be with the LED display drivers. Any ideas on how to debug this, and determine if it is an IC or a problem with a discrete component of the PCB? Anyone else ever seen such a problem?

-- Dan