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Post subject: Super Twin Reverb blowing fuses
Posted: Fri Jun 25, 2010 8:36 am
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I have a '78 Super Twin Reverb that played fine last week. This week I come to practice and power up the amp in standby to let it warm up. All seemed fine- the power light came on, but as soon as I take it out of standby it blows a fuse. So I disconnect the guitar input and line out and all the pedals and try again with a new fuse-same result-powers up okay but as soon as I take it out of standby it immediately blows a fuse. Does anybody have any ideas on where to start looking to solve this problem?
Thanks for your time!


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Posted: Fri Jun 25, 2010 9:11 am
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I'd check the power tubes. One could be shorted.

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Posted: Sat Jun 26, 2010 6:40 am
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If the filter caps are original that's the first place I would start. They can break down to the point that they can't handle the voltage spike when you throw the standby and short out, thus blowing the fuse.

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Posted: Sat Jun 26, 2010 8:40 am
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Absolutely, Mike.

Also, there are some caps (probably electrolytics as well) in the hum balance and output tubes matching circuits that may be due for replacement.

HTH

Arjay

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Posted: Sat Jun 26, 2010 1:50 pm
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IMO the power tubes have more chance to be the problem.

Or old fuse or wrong size fuse.

I don't think the caps could be the problem.

pdaling : remove all your tubes and try with new fuse.


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Posted: Mon Jun 28, 2010 5:21 am
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The caps are pretty old if it's never been recapped it should be. I just go for the easiest thing first. :D I'm usually pretty lazy.

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Posted: Mon Jun 28, 2010 7:05 am
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Absolutley power supply caps. They regulate the voltages throughout the preamp nodes and power tube node. If you have new PS caps then double check to see that they are installed correctly. I have seen amps where some one placed the first cap backwards not thinking about the first to caps being in series. Also the death cap if still on the ground switch can do this as well. Cut the cap off the switch and to the chassis


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Posted: Mon Jun 28, 2010 8:03 am
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There is no "death cap" on a Super Twin Reverb chassis.

Arjay

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Posted: Mon Jun 28, 2010 8:40 am
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Some did and some did not. Just giving all things to look for. Fender used all kinds of different values at times and not all amplifiers even follow the schematic completely.. I see this a lot and i can tell when a worker at the factory put in an old pre CBS part value instead of the one the schematic calls for.


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Posted: Mon Jun 28, 2010 8:51 am
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I could be wrong, but since 4 years I fix vintage amp and I never see caps causing a short / fuse blow.
But I see tubes causing short

Is there somebody on this forum see by himself a cap make a fuse blow ?

I want to learn more


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Posted: Mon Jun 28, 2010 9:09 am
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triodeamplification wrote:
i can tell when a worker at the factory put in an old pre CBS part value instead of the one the schematic calls for.


There is no "pre CBS part value" for a Super Twin Reverb. This amp was one of the first all-new FMI/CBS-designed platforms and as such, had no FEIC descendants.

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Posted: Mon Jun 28, 2010 9:50 am
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Death cap & PS cap that will blow the mains fuse after the PT starts to cook internally if left on under load.

Image

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Last edited by triodeamplification on Mon Jun 28, 2010 11:47 am, edited 4 times in total.

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Posted: Mon Jun 28, 2010 10:01 am
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That looks like a coupling or isolation cap to me.

Death caps are typically rated for 600V.

Arjay

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Posted: Mon Jun 28, 2010 10:11 am
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The ground switch comes off the mains VAC then to the .047 cap which goes to chassis ground. If the switch is in the cap to ground position and the cap shorts to the chassis it will knock the crap out of you if the guitar is in your hands

It will not most likley blow the mains fuse. But still some thing to be warry of in old amps. I cut them out first thing


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Posted: Mon Jun 28, 2010 10:25 am
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Death caps are intended to discharge the filter caps directly to chassis ground, most of which may contain voltages in the 460-490 volt range.

How's a cap rated for only 125 volts going to accomplish that without catastrophic damage to the component?

Removing the cap you've indicated on the STR schematic will render the amp inoperable.

But by all means, proceed as you will.

Arjay

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