[Metalab] Help debugging a x0xb0x 74hc595 led issue

[Peter Kuhm]

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=mainboard2.png

> 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