Troubleshooting - lowest five bits of PC lit?

[Martin Reilly]

I finally got back to my started-but-not-finished PiDP8 recently. Originally gave up because it was too tricky with the rubbish soldering tools I owned at the time - now upgraded.

I have all the components in place other than the switches, as recommended in one of the build guides I want to test before I solder those in place. I can't see any bad joints or solder bridges and I'm 99% sure all the components are orientated correctly. I've loaded the image on an SD card and the Pi (a Zero 2 W) boots up OK connected to a monitor.

With the Pi installed on the PiDP board as soon as I apply power the lowest five bits of the PC row light up - dimly, but clearly lit. It's immediate, long before the Pi has time to boot. Nothing changes when the Pi has had plenty of time to start up.

I've read the troubleshooting document and I think I get the row/column select logic but I don't have a whole row or column misbehaving.

Does anybody have any suggestions where I should look for what I assume will be my mistake or poor soldering?