Woodstock Display Problem

[teenix]

(02-14-2020 05:15 AM)BobVA Wrote:  Maybe a power supply problem? I'd check the batteries / connection (supply 2.5v directly to the battery terminals with a power supply if you can), then have a look at the 4.4v "Vdisp" voltage (you'll need to google up the schematic) and see if that's in the right ballpark. The next suspects would be one of the driver chips.

(The adjustable resistor indeed looks suspiciously like a drive current adjustment, but it actually sets the "low battery" warning level.)

Thanks Bob, I didn't know that :-)

The HP29C schematic is here if needed...

http://www.hpcc.org/cdroms/schematics5.0.../hp29c.pdf

Looking deeper, there is another resistor connected to pin #6 of the cathode driver that according to the Woodstock block circuit diagram, controls the display brightness. It may have gone high (or low) resistance or has a poor solder joint. The schematic lists it as 3K ohm.

cheers

Tony