The SD2 is my favorite yellow BOSS pedal.

[Ptak]

I am 50/50 if this writing is my own. It may be slightly better so I guess I didn't mod that pedal I don't think I ran that resistor/cap in parallel but everything else are the same stuff I had on hand while I was modding those. I've done a lot of mods on this pedal, mostly tinkering for myself on my last amp around 6 years ago. The mod I had on hand was done by Keeley and was basically cap upgrades and a diode swap then notes from Wampler.

Biggest thing everyone needs to know is the older/newer versions sound drastically different. From my memory the older is darker/less clarity while newer has more clarity and I believe is much brighter.

Laurie, you should def pick one up and fix the green channel. Most everyone seem to dislike it. I never could tailor it to my liking personally and got a new amp. Still like the red channel though.

[zentropa]

You can see that a lot of the electrolytic caps were changed out, a resistor or cap value change here and there and some diodes replaced some LEDs. There is also a cap/resistor parallel piggyback happening right where the two chips meet at the bottom of the board.
If you look closely, the older/modded one is a different circuit from the stock one. The first thing I noticed on the modded one was R100 (located at the left of that chip pair on the bottom of the board. This component location is not present on the newer, stock board. When I looked at the traces on the back, there were a couple small variances, too. From what I have gathered, the SD2 hit the market around 04/1993, so the modded one is the original design, and the stock one is a revision of some sort.

Out of curiosity I opened up my SD-2. It does not have a spot for R100 on the board and is from January 1994. Trying to narrow it down to find out when exactly the change happened, but seems will have to dig into 1993 only to find out.

[zentropa]

Just got one from August 1993 and it also does not have R100.

[zentropa]

Pulled the service notes for the SD-2 with a date of May 1993 and those do not have the R100 either. I think I'm going to abandon this one from the variant hunt. Ended up overpaying (IMO) for a super clean August 1993 w/ box/manual, only to have it not be the variation and also lost the tone shootout to my beat up January 1994. Oh well.

[zentropa]

Ongoing saga... it is the later year SD-2s that have R100, C100, etc. I managed to get a 1998 in a trade.

I also seem to remember having one a long time ago that had a component soldered to the traces side of the board that appeared to be factory done.

I was able to find some old threads on various forums talking about this as well, but unfortunately all of the image links were no longer working.