Re: RXTX Ensemble, lots of headaches / faulty board
- Hi Jess
We would often find that an etch short was where the pc trace changed direction. Look where the sharp point is at the edge of the trace where the trace does a zig or zag (please excuse the technical jargon) and points toward the shorted partner trace.
Sometimes the short is so fine it can't be seen under the solder mask.
We used to burn fine shorts out with a 9 volt battery if we knew it was really thin. We did that even with PCBs that were to be flight/space certified. Often that process would leave a little burn mark under the solder mask.
A thick short would burn a crater or actual hole through the board before it fused. In that case we would cut or drill and not burn/fuse.
--- In email@example.com, "jess_flyin_high" <jess@...> wrote:
> I built the entire softrock before testing anything (I have years of smd work under my belt so I was sure of my soldering skills)
> when I powered it up for the first time I was met with a very hot 5v regulator and ac74 (>200ma draw on 12v line)
> I checked and triple checked the soldering on the ac74 with no faults found, so assuming a faulty chip I ordered 2 new ones.
> while waiting for delivery of the new chips I turned the multipliers off on the si570 and linked the clock input for the ac74 through to pin 9 (one of the qsd lines) for the ac74, this allowed me to use the ensemble in RX only (with no image rejection).
> When the new chips arrived I checked that everything was as it should be where the ac74 mounts and found that the 2 qsd lines were shorted, followed it through to the rx mixer and sure enough pin 2 was shorted to pin 14 (there was no way that this was down to bad soldering, adjacent pins maybe but not when they are on opposite sides of the chip) I assumed the fs3253 was faulty and removed it from the board only to find that pis 2 and 14 were still shorted on the board with the chips removed.
> close examination of the tracks in question revealed no visible fault, so I resorted to track cutting, after cutting the 2 tracks just to the left of where they pass under R25 removed the short from the ac74 end, cutting the tracks to the right of R28 removed the short from the fst3253 end.
> after all the messing around I can say for certain that the tracks that pass under R25 - R28 are shorted together within the board (area shown in pc at http://126.96.36.199/shorted.jpg)
> does anyone know of a cheap supplier of the fst3253 chip in the uk since mine did not survive removal from the board?