This topic seems to come up rather frequently now that it is hot!
Palo Alto Speedometer and Hollywood Speedometer are both rumored to repair these. It has also been rumored that the new unit is available for not too horrible of a price.

It is difficult to repair the bulb/tube at home (soldering with ether, whew!) so you can pretty much figure it not possible.

Another solution has been to entirely replace the guts of the meter with an electric one from a later Volvo (or pretty much any source) and use the electric sensor.

Cruise the threads and you will see.

Mike!