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Joplin blown fuse

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Posted on October 6, 2009 at 02:46:42
jeff mai
Audiophile

Posts: 632
Joined: February 6, 2001
 Hi,

I've got a loaned Joplin amp here and after a few hours use it has blown a fuse. Half a second before it blew, I heard a brief, load hum and may have seen one of the valves flash or brighten out of the corner of my eye.

None of the valves has a short that shows up with a multimeter. Filaments are all fine. I've slipped off the bottom cover and I believe it's a factory build (red paint on all the screw threads) and it looks like a self bias circuit.

Can someone give me a tip on what to check? Cathode bypass caps and coupling caps are the obvious culprits to check. What else should I look at?

Cheers,

Jeff

RE: Joplin blown fuse, posted on October 6, 2009 at 14:45:06
bcherry
Manufacturer

Posts: 2216
Joined: March 11, 2001
Contributor
  Since: June 1, 2001
 check the DCR of all secondaries. If one is open or shorted it will pinpoint which section of which channel has the problem and you can test all the parts down stream from there. Look for any overheated parts.
A tube tester will be needed to check the tubes.
Bad tube, loss of bias (check tube sockets as broken 'c' spring can cause it), and coupling cap failures are typical failure modes.
regards
Brian
DIY - Done Right!
RE: Joplin blown fuse, posted on October 8, 2009 at 12:40:43
jeff mai
Audiophile

Posts: 632
Joined: February 6, 2001
 I'm measuring about 80Kohms from the power transformer primary winding to ground. That can't be good.

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