yamaha psre 413 power input problem

[cameronmc]

I bought a psre 413 for home use after trying one in the school where I work. After a while the power input socket became loose causing intermittent cut outs and crackling. I opened it up to resolder the socket and found what looked like a botched repair job had already been done with a large blob of dirty looking solder! Since then I have repaired 3 units in the school with exactly the same problem, purchased from a different supplier. Anyone else come across a similar problem?
Thanks,
cameronmc