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Post subject: PA 135
Posted: Thu Jul 22, 2010 8:46 pm
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Hi, I have an old Fender PA 135, don't know what year. Everytime I plug it in, it blows a fuse. Any ideas on what is doing this? Thanks, John


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Post subject:
Posted: Fri Jul 23, 2010 4:09 pm
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Hi there John and welcome to the forum. It's not clear, to me what's happening. with the amp plugged into the wall outlet and the amp switched "off" it's OK, right ? Then what happens ? Don't mean to sound condescending, but a step by step account is about the only way to do this on a board. Art

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Post subject: Blowing fuses
Posted: Fri Jul 23, 2010 8:31 pm
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Not knowing much else, this is what I do in that situation:

1.) Buy a bunch of correct amperage fuses.

2.) Replace use. Pull ALL tubes. Have amp plugged into speakers. No volume, no guitar plugged in.

3.) Turn on. Fuse blow? Then it's either your diode recitifier bridge or the power tranny.

4.) Unsolder diode bridge. Replace fuse. Turn on. Fuse pop? Tranny suspect. Unsolder the secondary leads. New fuse. Pops? Then it's either the primary side or the wiring leading from the outlet to the primary side. You'll need to check all connections for shorts. And pull the power tranny and check primary and secondary for shorts.

5.) No fuse pops with no tubes? Then plug in ONLY the power tubes. Be sure that the bias is turned down (overbiased). Turn on. Fuse pops? Bad power tube(s). Need to have them tested.

6.) Gain stage tubes shorting rarely will lead to fuse popping. Just doesn't draw as much current. But, worth a try if the power tubes don't pop the fuse.

My bet is that you have a bad power tube, bad rectifier stage, bad power filter stage (bad electrolytics). In any case unless you are good (& comfortable) with a soldering iron & DVM, best to have a tech look at the amp.

Good luck with that!


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Post subject: Edits... edits.... edits
Posted: Sat Jul 24, 2010 4:28 am
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2.) replace FUSE.

3.) if fuse pops with ALL tubes pulled, first suspect the filter caps in power supply. Most amps with 'lytic caps older than 15-20 years old have bad caps. Then, suspect diode bridge. Then, power tranny.

WHEW X 2!!!


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Post subject:
Posted: Mon Jul 26, 2010 11:00 pm
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I would suspect a power tube first. If that does not do it then start with the other components mentioned.


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