coaxial/spdif to usb converter

May I know what you want to accomplish with such converter?

I have a mp-md2 mk2 tube dac that is having problems playing through the coaxial input but have no problems with the usb input.

Such a converter would have to act as a USB "host," which likely means that it would have to either be a computer that could accept a S/PDIF input or a device designed to emulate such a computer. And as far as I am aware any such device would probably cost a good deal more than the kind of simple converter you appear to be looking for.

Converting in the opposite direction (USB to S/PDIF, using a device connected to the USB port of a computer) is technically a much simpler task, and many devices which can do that are available at relatively low cost. But converting S/PDIF to USB is a different story altogether.

Good luck. Regards,
-- Al

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.