HP-80 with bugs ... ?

[teenix]

It sounds like it is not resetting properly if it works properly after a keypress. It could be the reset hardware or a failing ROM. There is not much going on during initialization, registers are set to zero and just some formatting for the display take place. It is hard to tell without analyzing the code flow at start up.

The reset circuit is based on a couple of transistors, resistors and a capacitor, shown in the image. You would have to check the reset pulse using an oscilloscope or test these components to determine if this is the problem.

The C register may be corrupt at start up due to the reset problem and that is why the dodgy display. Pressing CLx would clear the C register and things may work properly, while other keys may try to process a function based on the invalid C data leading to other problems.

If it fails completely, Harald from the Forum may still have replacement CPU boards for the Classics.

cheers

Tony