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Post subject: 2000ish Fender Bassman 400 Pro Combo Power amp dead?
Posted: Tue Aug 30, 2011 7:14 pm
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Hey all,

New member and long-time Fender user here. Got a problem with my ol' Fender Bassman 400 Pro 2x10 combo. After some time sitting, I turned it on and there's a serious problem; No guitar signal comes through the power section and at first it makes a low-mid buzzing, which turns into a really loud grinding buzz after a few seconds. It looks like DC voltage is going to the speaker, because the speaker sucks inward and stays. Volume controls make no difference and the effects send still passes signal (didn't have an XLR on hand to test the Line Out, but I'm assuming the same results). Any thoughts on what to do or look for inside? My father is a long-time electrician and has helped fix my amplifiers in the past, I intend to have him take a look at it. Thank you for any thoughts!

- Chuck


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Post subject: Re: 2000ish Fender Bassman 400 Pro Combo Power amp dead?
Posted: Fri Sep 02, 2011 12:41 pm
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Don't insist and deconnect the speaker, ask to your father to measure the output transistors just to see if there is a leakage or one failed.

Measure all supplies :+/-72V, +/-30V and +/-15V, check also solder joints.

http://support.fender.com/schematics/ba ... ematic.pdf

If it is too much complex; let it to a good tech.

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Post subject: Re: 2000ish Fender Bassman 400 Pro Combo Power amp dead?
Posted: Tue Sep 06, 2011 1:03 pm
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We're waiting on a dummy load before we turn it on without the speakers to measure anything, but I'll report back as soon as we do. He said that the problem sounds like a bad capacitor; in the way that it sits there humming for a few seconds then suddenly shoots up to a screaming loud grinding buzz. Said that he's seen bad caps build up energy and suddenly discharge just like this. Would you agree?


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Post subject: Re: 2000ish Fender Bassman 400 Pro Combo Power amp dead?
Posted: Thu Sep 08, 2011 12:25 pm
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Caps go bad sitting unused. So based on the fact the amp was fine, then went unused for quite a while and then failed, I'd tend to say that whatever is wrong now at least started as a capacitor issue but could now involve more components and I wouldn't be surprised to find some smoked resistors or power transistors for example.

Anytime an amp sits unused for a period of time the caps often go bad. Under average use if you get 12 years out of them then that was a good run.

Capacitor life can be significantly extended by frequently powering up the device and I do so about once per week. I have a late 70's Peavey Century 200 that has never had a screwdriver on it. I use it at the rehearsal space at least weekly or otherwise fire it up weekly for a while, even if I'm not going to play through it.

The worst thing you can do for an amp over 10 years old, especially a tube amp, is not use it and then after six months or a year or two years slam it with 120 Volts AC. I can almost guarantee that when you do that enough times you are going to get precisely the result you describe.


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