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PostPosted: Mon Feb 04, 2013 3:33 pm 
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Bottle Rocket

Joined: Mon Dec 03, 2012 8:45 pm
Posts: 3
Image

I have posted an image of the Schematic that Mesa sent me. My issue is I keep blowing my 4 amp fuse. When I disconnect my wire at point A labeled on the drawing with a red A, the fuse doesn't blow. I have 345vDC at point red point A but it does seem to fluctuate a bit. All of this is done with all the Rectifer Tubes removed. When I reconnect that point, the fuse blows. When I disconnect the caps at the points labled with a red B, the fuse doesn't blow. All of this is done with the Standby switch in standby, therefore open. The only parts in the circuit should be the 4 caps, 2 resistors and a filter cap. I've checked the resistance of my resistors and they are correct. Again, if I take the 4 caps out of the circuit, with the standby switch open, the fuse doesn't blow. With the caps back in the circuit, with the standby switch still open, the fuse does blow. This leads me to think that I'm having some shoot through in the caps. I've taken the caps out and checked them, and the reading with my Fluke 87 on cap setting reads 220uF for each cap. I meggered the caps with 500 volts and they get to around 380v with .25M ohms resistance. The voltage still climbs but it is very very slow after around 360v. Can anyone give me advice? I'm about 95% certain some of the caps are bad, but checking them individually leaves me uncertain. Please help if you can. Thanks in advance for your assistance.

Shawn Spano
260-442-1146
spanstar2010@gmail.com


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PostPosted: Tue Feb 05, 2013 2:10 am 
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Single Recto

Joined: Fri Jun 20, 2008 3:31 am
Posts: 1023
Location: California
I know a little bit about electronics, but not as much as you. I can read the schematic, but I lack the real world understanding that you seem to possess. With that being said, does the fuse blow when using the solid state rectifier, too?

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PostPosted: Tue Feb 05, 2013 9:45 am 
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Bottle Rocket

Joined: Mon Dec 03, 2012 8:45 pm
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Yes sir. My fuse blows in both Solid State and Tube mode. Again, I have the tubes removed and it still does this. Standby switch is for sure open. Not really sure if my relays in the top part of the schematic are working or not.


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PostPosted: Tue Feb 05, 2013 12:43 pm 
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Single Recto

Joined: Fri Jun 20, 2008 3:31 am
Posts: 1023
Location: California
Man, I wish I was more knowledgeable about this stuff. I say if your instincts tell you the problem lies in the capacitors, then replace them.

It was really cool of Mesa to share that schematic with you!

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Guitars --> Pedals --> Amps

Past/Present Boogies
Express 5:50+ 1x12"
Stiletto Ace 1x12"
Electra Dyne Head
Lone Star Special 1x12"
Stiletto Ace Head
Express 5:25 1x10"
Mark IVb Widebody 1x12"


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PostPosted: Mon Jul 27, 2015 3:13 pm 
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Bottle Rocket

Joined: Mon Jul 27, 2015 3:09 pm
Posts: 3
Two years too late..

But for anyone following the trail, the caps measure OK at voltmeter voltage levels (probably a 9V battery) but are breaking down and shorting at operating voltage, thus blowing the fuse.

Hard to spot which one it would be. Easiest to replace them all..


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PostPosted: Mon Jul 27, 2015 4:03 pm 
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Bottle Rocket

Joined: Mon Dec 03, 2012 8:45 pm
Posts: 3
The caps were measured with a Meggar, not a Fluke. 500Vdc, not a 9 volt battery.


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