Belt Slipping - Husqvarna Viking 1

Is the belt too large and slipping? Get proper-sized belt.
Did the center part of the handwheel get loosened? Tighten it.

Probably the belt is slipping. It could need to be tightened, but if it is old and frayed, replacement may be the best answer.

Is there an adjustment to tighten the new belt? Wrong size belt?

Very possible the belt has stretched so it is slipping. Usually, there is a way of exposing the pulley on the belt. The bolt holding the pulley can be loosened, then pulled to increase tension on the belt, then re-tighten the bolt. You may find more information by searching the information for your make/model machine.

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Could be several things. The bobbin winder is not deactivated. The belt is damaged, slipping, or broken. The machine has seized due to lack of cleaning and oiling (consult the owner's manual).

On a treadle sewing machine usually there is no adjustment on the tightness of the belt so you have to shorten it. There will be a metal clip holding the belt together and you can remove and cut the belt shorter and then put it back together. Be very careful to not cut too much off. Probably be a good idea to trim it a very little at a time.

talk to the guy where you buy the belt

Here's a You Tube video that should help. https://youtu.be/8Voq7f5vT6E

check machine for oil hole locations and oil sparingly. NEVER oil a drive belt. The belt could be slipping which would cause a squeal.