Marantz ST-450 Tuner

Thermal fuse failure

The Marantz ST-450L is a beautiful mid-range tuner of the 70s. This one ceised to work at all, suggesting an easy to fix problem in the power supply. A quick measurement revealed no AC on the secondary side of the transformer, although I was able to measure 160 ohms in each secondary coil. What is going on here?

The fix

The schematics show a thermal fuse, which turned out to have blown. Since the fuse is buried deep within the transformer, I shorted it as indicated by a red trace in the schematics, by simply connecting the black and blue wires. Sometimes thermal fuses are accessible after removing some of the protective tape around the transformer - I'd recommend to try replacement first before shortening the fuse. The transformer gets hot, but not alarmingly so. I'll report if the tuner goes up in flames.