Panasonic tc-63R50N color tv

HOT ok. 106v at Collector.
no HV
no heater.
sound ok.
can change channels + adjust volume.

this set has a separate p/s board using an STR51213.
tested resistance on this ic, no pins shorted.

the chopper is making a squealing noise while turned on.

done the usual, esr caps, checked all the big resistors, transistors.

what should i look at next?