I believe there were 2 versions of the valhalla. One for 50hz and the other for 60 hz. I have some suggestions for you to check having gone through some problems with some of my previous decks.
1. Check the motor. I thought I had a bad power supply too but it turned out to be the motor. If you order a motor, you need to know if you have a 50 or 60 hz model. It depends on the serial number for your board. I havent had a valhalla in a long time so I dont know how to advise you.
2. Check the belt. If you get a replacement belt, once in a while there is a bad one. I had one that was too small and it definitely slowed down the table. I had a hard time accepting this at first, but it was true. Supplier ended up replacing it. It is possible, the dealer gave me the wrong belt.
3. Just because you dont see a speed control, you can adjust the speed on a Linn LP12. There are screws on the mount for the motor that tilts the motor. This sends the belt to a higher or lower position on the pulley. In this way the speed can be adjusted up or down.
4. I dont know how to advise you on where to get Linn parts. I think if you look for their distributors on their website, they may be able to help you, or email Linn directly. There are often Linn parts listed on ebay but I have no experience with any of those sellers so make sure you check them out. Also, check out some of the british audio mags like HiFi +, What Hi fi, and various others. They may be able to help you.
5. As suggested, check the motor. that was my problem. I had a Valhalla for years and years and years with no problem. But I did have motor problems so dont rule that out.
Hope this helped you.