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Post subject: My turn !
Posted: Fri Apr 23, 2010 11:25 pm
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Joined: Fri Jun 26, 2009 6:53 pm
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Location: Harvard IL
Like I said, I'm really out of the amp repair business. Just do work for players and friends around here. So the doorbell rings, and here is the original drummer from our old Blues band(well over 25 yrs ago), hadn't seen this guy for 10yrs. "Hey Art, got this amp, I know you can fix it, paid less than $100 bucks for it, the only problem is I've spent $8 bucks on fuses, and the 7 amp fuse even blew. " Turns out, my pal Ray, handed me the ultimate example of the rusty-crusty Mid West garage kept amp I've ever seen. It's a '75 Bassman 100. The good news is that it is totally original(OEM GE dual getter 6L6s and RCAs in the pre amp). The bad news is that it is totally original, waxed board and all. After an initial check, the tubes and OT aren't the source of the short. So I'm at the PT. Getting too old and forgetful, so I might be checking in for advice. Art

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Post subject:
Posted: Fri Apr 23, 2010 11:56 pm
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Joined: Wed Mar 31, 2010 12:48 am
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Location: Tombstone Territory
Have you checked the diode stack, Art? If those test okay you might examine the bias/balance circuit -- it's possibile that RC network tied to the pot has gone south. Of course it could also be something simple, like a wonky polarity switch (watch out for that "death cap").

Here's the schematic BTW......

http://www.schematicheaven.com/fenderam ... _schem.pdf

Best of luck, HTH

Arjay


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Posted: Sat Apr 24, 2010 12:40 am
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Joined: Tue Mar 30, 2010 11:13 am
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Location: Australia, New South Wales
Hi Art

sweet pick up, love those sort of bargains.

you could disconnect power transformer slowly reconnecting it to determine what part of the circuit is causing it to narrow it down.

In mine it was a blown diode in the bridge rectifier (high tension) causing a short, possibly same problem here.

Test the diodes with an ammeter on both sides for resistance one side should show minimal resistance swap test other way around and should get off the scale resistance if it's the same as other direction the diode is dead.

the diodes of that time needed capacitors in parallel to prevent they from having a short life in amps. modern diodes don't need the capacitors in parallel.

anyway best of luck with it Art like to hear how it goes.

Stephen

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Post subject:
Posted: Sat Apr 24, 2010 4:31 pm
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Joined: Fri Jun 26, 2009 6:53 pm
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Location: Harvard IL
Thanks for the tips, you guys. I forgot how nasty that wax was to work around, what a pain. Oh well. Art

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