[Metalab] Help debugging a x0xb0x 74hc595 led issue

[Damian Stewart (ML)]

Hey list,

I have a x0xb0x (303 clone) that I build from a kit in the mid-90s.

I dug it out recently and started playing with it and noticed it has an odd LED issue - basically one of the LEDs doesn't light when it should, but if I unsolder the LED I'm seeing 5V across its pins at the right moment, and then it worked for a while when it was soldered via a pair of 10cm wires but stopped when I soldered it back onto the board. 

I don't really know what's wrong, I'm thinking there's noise or parasitic capacitance but I have no idea how to debug that. The LED's are driven by a bunch of chained 74HC595's. I took a look at what's going on with my cheap-ass USB logic analyzer (Saleae Logic) but it all looked fine.

Would anyone be interested in helping me debug it?

Thanks
Damian
--
damian stewart . digitaler fuzzi . vienna, austria
http://damianstewart.com . twitter @damian0815