Hp91 with very dim display

[teenix]

(08-02-2020 04:30 PM)James Linder Wrote:  Thanks for your help! I’m going to take voltage readings with my multimeter working from the battery to the power supply and beyond to see where the voltage first drops and replace or re-solder the component where the problem begins. I expect it will be within the power supply but will also focus on the press/pull connectors between boards since they are obvious potential problem sources. Since it appears this 91 had never been opened up before I got it last week, it’s not likely a connector unless it was dropped and this doesn’t appear to be the case.

It could be the power supply, but I would suspect something else, otherwise the calculator overall would probably not work. The drive for the LED display is not part of the power supply, as a clever design kept it separate. My starting point would be the 4 ohm the resistor feeding Q1. Only one of the 106 LED segments is on at a time, and the logic circuits appear to work ok, so a higher resistance, or other faulty component around there, would easily dim the entire display.

Be careful with the display ribbon cable, it could be damaged removing and re-inserting it.

The printer working, but not printing, could be a failed battery connection in the ribbon cable or connectors. This power is also cleverly isolated from the logic supply and most of the analog smarts are on the circuit board covered in the maroon coloured coating. As you mention, it may also be just a dodgy circuit board pin connection. Also, please be careful if you feel the need to bend these pins, they break easily.

All just ideas of course :-)

cheers

Tony