ST-400 finally has a problem after 35 years

It has been a long time since i spent any time in electronics. I built the ST-400 about 35 years ago and it finally has a problem.

The Dynaguard kicked in on the 40 Watt setting and instead of tapering off the volume it just dropped to zero with a slight fizzeling sound. Shortly after the high temp light came on and would not go out. I shout everything down and let it cool but I did not hear the relay click out.

I tried turning it on a few times but the relay would not click in and the Dynaguard lights would flash on and off with no volume. After a few seconds the high temp light would come back on so I would shut it down. This happened regardless of the Dynaguard setting, even in the off position and the volume controls on both the ST-400 and the PAT-4 set to zero.

After letting it completely cool to room temperature for a few hours I tried again and this time R215 on the PC28 board went up in smoke and on of the fuses above the relay blew. R215 kept cooking even after the one fuse blew.

I am guessing I blew an output transistor and maybe welded the relay partly closed.

Any thoughts on where to start?

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Topic - ST-400 finally has a problem after 35 years - mike.burns@theedge.ca 19:52:58 05/05/15 (2)

RE: ST-400 finally has a problem after 35 years - petercapo 06:45:48 05/06/15 (1)

RE: ST-400 finally has a problem after 35 years - mike.burns@theedge.ca 17:12:32 05/06/15 (0)

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