Toshiba 34HF85, M3U01T Chassis

[pfm]

This is a large CRT set assembled in 2005 which recently suffered a power surge. The set switches on, produces a raster for a few seconds and shuts down.

The B+ is generated from an SMPS. The B+ is fed back to provide the reference voltage to a shunt regulator which controls an optoisolator. The opto circuit can turn off the switching transistor in the primary winding. The opto and 431 shunt regulator check OK.

This appears to be some type of SOPS.

The B+ OC (LOPT disconnected) is 139V with the supply making a hum at 595Hz (The trouble and strife has perfect pitch). With a light bulb as a load in place of the LOPT and HOT, the supply groans and shuts down.

Can anyone throw any light on how the frequency is controlled?

Thanks
Pfm