Help needed: HP 67 repair

[teenix]

Does it power up with 0.00 displayed? If not there could be issues with the ACT and one or more ROMs. If numbers can be entered but displayed wrong, it may be PCB, Cathode/Anode driver or ROM 0 causing the issue.

Only segment G should light on both the sign digits. They have 4 transistors decoding and driving them from segments E and G. Because segment C is lit on the exponent sign, it may suggest something is "crossed" on these segments somewhere.

If the cathode driver is playing up, it should be easy to spot with a scope.

If you want to download this, I have some info on instruction decoding for Classics towards the end of the document which may help.

http://www.teenix.org/ClassicNotes.pdf

cheers

Tony