Monday, September 05, 2011

What follows is my tactic for repetitive rework, such as is apt to be found in mixing consoles, multitracks and really bad production runs. As far as recaps go, this approach makes rather high part count projects only a little more difficult than a simple tube amp recap. In not necessarily the most accurate order:

1st, I plot out the position of all parts to be replaced and mark the PCB so I can desolder everything on my bench (say, two or three channels) in a single pass. My preferred method of desoldering is to heat the joint with the iron and use a 7874B type pump to remove the bulk of material. Once most of the solder has been removed on everything at hand I will go through everything while the iron is still hot and twist each part looking for movement of both legs. If a leg is still soldered in, it will be heated while twisting to free it up before moving on to the next one. Once all parts are loose, I'll snip the bent ends if they don't pop out freely, and drop them out. This has resulted in minimization of pulled traces.

2nd, each identical channel has the same parts count, this unit per channel count has been marked on the bags containing the parts.

3rd, I begin with the lowest quantity per channel parts (typically "1") and load those all first, no higher part count parts are removed from the bags until it is their turn. Only the unique or super low part count parts are removed from the bags (and ALL of said quantity are removed prior to stuffing a single part). Pictured here is the 330uf, 100uf, 33uf and 4.7uf loaded into the "active" circuit, while an already stuffed part reference circuit (very important to this particular process, though after a few passes the example gets less and less attention) sits atop a bowl full of pull jobs. In the foreground are five 10uf that are to be stuffed as soon as the camera is set down, leaving all remaining seven slots for the 47 uf caps.

Observe polarity. This particular board is designed such that all "east-west" positioned caps had the ground lead to the left, and all "north-south" caps had the ground lead facing me, from the position that I was working on them. This made perpetual polarity checks less of an issue, but it's certainly not a feature one should count on.

Once the lot is stuffed it will be checked for shorts (a couple of the pads are very close to one another), cleaned, then reassembled.

160 some odd caps, minus a few known to have fled from the bench.

Channels are all done, next up is the summing, metering and PSU sections. Then a proper clean up & burn in.

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