Good luck and keep this site posted with how you get on. The other interesting thing is that, according to Geofex, the op-amps in those TS-pedals were truly blimmin’ awful. Much better op-amps are made now but, ironically, they don’t produce the TS-“sound”. If you ask the Japanese to make those lousy op-amps again, they’ll:

a) Look at you like you’re barking mad
b) Shrug and say, “OK, how many million do you want?”

It’s exactly the same problem with carbon comp resistors in old valve amps. Horrible things, drift nastily, ridiculous tolerances. They make way better resistors these days, but they don’t “sound” right. Again, nobody wants to make a few thousand rubbish resistors.

No joy on an OD-820 schematic. According to the Maxon site’s “vintage” page, the 820 was a modified 808. As you probably know, the Maxon OD-808 and the Ibanez TS-808 are identical and from the same factory. The TS-808 is itself very similar to the TS-9.

Your nearest bet is to have a look at http://www.geofex.com and search for Ibanez. The TS series schems and commentary are comprehensively covered there. Sorry I can’t point you to the OD-820, but this is al I could find. Incidentally, the geofex site makes the interesting point that the TS-10 can be picked up for very little money and can be converted to TS-9 by a few simple changes.