Welcome to the wonderful world of the old-ass 122
There's the full service manual here if you don't have it: TASCAM 122 SM Service Manual free download, schematics, eeprom, repair info for electronics
That's an awesome book even if you don't have that model. I actually carry the paper one around in my car and read it sometimes when I get coffee. I've got the regular owners manual here too, and could have sworn I had a PDF of that as well, but can't find it in my disk catalog. I think the same stuff may be in that full service manual anyhow.
BTW- There's a similar Teac model ("C" something?), but it may have been missing the dual speeds or something.
That thing, and my 234, are definitely beasts. Everything got smaller and lighter after that generation (even the service manuals