coaxial/spdif to usb converter

What do you mean by "affordable"?  I have a friend whose whole system cost $600; another spent multiples of that on a power cord.

A Mutec MC-3+USB would do the job. If you can find one, they run about US $650 used. But if you want to try before buying (and I would advise that), a used one is about $1200.

Yeah, how much can you afford? Affordability is relative.

A new musical paradise mp-md2 cost $900 to $1500 DAC depend on option, as Al (almarg) pointed out, any such device that OP looking for would probably cost a good deal of money. IMO, OP's best option is either repair or replace the DAC.