[Yaesu] VFO Lock-Up

[Robert Pack (NX5M)]

I have one of the older Yaesu FT-757 GXII rigs that I have rarely used over the last 10 years.
Had a guy wanting to buy it so I connected it up today to make sure it was working properly.......it was.
However, after dialing around for several minutes the vfo suddenly bacame hard to turn and eventually would not turn at all without a serious grip.
Do anyone know what would be the cause of this and how to possibly fix it?

Thanks.
Bob NX5M