Hi, On Sun, 12 Mar 2017 14:09:47 +0100 Damian Stewart (ML) wrote:
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.
you surely have double-checked LED polarity but have you also looked into any difference where the 10cm wires were actually connected. Were both in exactly the same lands as the LED leads? Think about a cold junction or a hairline crack towards a via, a short to a neighboring pin from soldering. You can easily measure with a mulitmeter directly from the pin of the 74HC check the line to the 10k pre-resistor, measure the resistor value, the line to the anode pad and check if the cathode pad is really connected with ground. When in works 1:1 with the long wires the LED has to be ok and the fanout of that pin good enough to drive it.
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
nah, you pick up more noise when it's wired with long cables than when it is close on the PCB. There are dozens of such LED http://wiki.openmusiclabs.com/wiki/x0xb0x?action=AttachFile&do=view&target=m...
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?
Tomorrow Wendesday the MetaFunk folks have their informal meetup. I'm sure someone is willing to assist you to go through. You don't have to necessarily lure them with obscure noise and parasitic capacities ;-) cu, Peter