IIb effects loop "fixes"

[dalepsmith]

So I'd really like to have a better effects loop in my IIa.

From the schematics, it looks like I can convert it to a IIb style loop without too much difficulty. But I've heard the initial IIb loop had problems and there are fixes for it. Do the usual schematics for the IIb show the original or the "fixed" effects loop?
Does anyone know what the actual fix is? Is it just some component changes? Or something much more involved, like replacing it with a IIc(+) circuit board and components?

Thanks,
-Dale