The display on my HP 28s does not respond to the darken or lighten commands ("on" plus "+" or "-"). It is dim but readable at an oblique angle. Is there some other way to command the display function via machine code, etc? The unit operates perfectly otherwise.

Hi.

Sorry, I am not sure my post will be of great help !

I don't know any way using the line command or any system subroutine call to adjust the display contrast. This will not prouve that there not such a way.

Since the unit operates perfectly otherwise, I will assume that the key contact of the ON, + and - keys are correctly operating as well.
Unit turn OFF/ON easily, ON key interrupt programs, + and - oparate,...

In such a circumstance, low contrast display may only be interpreted as a consequence of a low batteries level (old batteries, one inverted battery or bad/dirty contacts).
Another cause may be low temperature of the unit, the LCD or the battery cells.

I already encounter such low contrast symptom after a software crash. Resetting the unit clear the defect.

If battery and temperature is OK and several resets don't help, then a defective LCD electronic have to be suspected.

Thanks for the advice. I have already tried new batteries, cleaning contacts, etc. The annunciator portion of the display is perfect, so I tend to think the problem lies elsewhere. I picked up a replacement on eBay for a reasonable price so perhaps this one will be put away for good. The one that works is older, made in the USA while the new one is of Singapore manufacture.