Commodore 1701 monitor?

[VintagePC]

Anyone have experiences with a C64 1701 monitor?
I've got one that works partially- gives picture and sound on startup, then a few minutes after warming up, the picture disappears and I hear a "hissing" noise coming from the set somewhere. (NOT from the speaker).

I've got a service manual for the thing with various troubleshooting charts and such, but I figure I'd ask here first if anyone knows a good starting place to look for the problem. Nothing on the board is visibly burnt or broken/bad.