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Post subject: Fender 75 master volume buzz and No gain when volume on 10?
Posted: Fri May 03, 2013 8:59 am
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Heya guys, its Feels again.

I was hoping to gain some knowledge on this point.

You see, the master volume on my Fender is just adding in buzz as far i as i turn it up. I tried switching out all the preamps (not the power tubes), cleaned out all the pots, the preamp volume on 0 and it still makes a loud buzz what ever i do.

I can bare with it until about 4 - 5 on the MV. After that i cant stand it anymore.

What is causing this? Is there a solution?

To the second question..
I got no gain whatsoever with the preamp volume on 10.
Im not bashing that, it sounds really awesome when you turn the MV up. (Besides the buzz.)

But what i would like to know is, has it been modified?

I cant read schematics that good, i know how to solder and drain caps.

Any help is appreciated!
Feels


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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Fri May 03, 2013 10:30 am
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I'd try rotating known, good new tubes in the input, gain, tone section, and phase inverter sections.

Buy a new 12AX7 and 12AT7. Rotate one-by-one the new tube into each appropriate socket.

Hopefully, this will work.


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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Sat May 04, 2013 5:00 am
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BMW2002Ti wrote:
I'd try rotating known, good new tubes in the input, gain, tone section, and phase inverter sections.

Buy a new 12AX7 and 12AT7. Rotate one-by-one the new tube into each appropriate socket.

Hopefully, this will work.


Tried it without luck..
There is some wimpy looking JJ 6l6's in there, when i put the amp on standby the hum is gone.

Would replacing them help?


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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Sat May 04, 2013 9:40 am
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You can try swapping out the 6L6GCs. But, the symptoms kinda sound more like gain stage issues and not output stage ones.

Good luck! Keep us posted.


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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Sat May 04, 2013 10:56 am
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Feelyfeels wrote:
BMW2002Ti wrote:
I'd try rotating known, good new tubes in the input, gain, tone section, and phase inverter sections.

Buy a new 12AX7 and 12AT7. Rotate one-by-one the new tube into each appropriate socket.

Hopefully, this will work.


Tried it without luck..
There is some wimpy looking JJ 6l6's in there, when i put the amp on standby the hum is gone.

Would replacing them help?


Putting the amp in standby shuts off the high voltage to all the tubes. No high voltage, no sound, period. JJ 6L6GCs are definitely not "wimpy".

Follow BMW's advice. If you end up replacing the power tubes, make sure they are biased properly.

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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Sat May 04, 2013 11:02 am
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BMW2002Ti wrote:
You can try swapping out the 6L6GCs. But, the symptoms kinda sound more like gain stage issues and not output stage ones.

Good luck! Keep us posted.


No luck again.. It did clear up the sound alot with the new PTs tho.
But the buzz is still there...

You think there is a ground problem?


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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Sat May 04, 2013 11:30 am
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You probably really need to become familiar with the schematic and start tracing through the amp looking for bad connections. loose wires, etc. Troubleshooting via Internet, particularly hum problems can be tough. You've done all the easy stuff, you're now going to have to take a deeper look. Start with the input jacks. Check the ground switch on the power supply (that ground switch and cap makes me nervous) and look at power supply grounding.

http://ampwares.com/schematics/fender_75.pdf

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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Sun May 05, 2013 1:20 pm
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bluesky636 wrote:
You probably really need to become familiar with the schematic and start tracing through the amp looking for bad connections. loose wires, etc. Troubleshooting via Internet, particularly hum problems can be tough. You've done all the easy stuff, you're now going to have to take a deeper look. Start with the input jacks. Check the ground switch on the power supply (that ground switch and cap makes me nervous) and look at power supply grounding.

http://ampwares.com/schematics/fender_75.pdf


Hey again,
I am afraid (not sure tho). That this amp is overfiltered.
The ground is ok.

There is some USA made Mallory caps in there. And i have heard that they are found in Marshall?
Take a look!

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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Sun May 05, 2013 2:12 pm
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Can't read the values on most of the caps. I suggest you compare the caps that are in the amp now with the schematic and see if they match or are at least close. The fact that some of the caps "are found in Marshall" amps is irrelevant.

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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Sun May 05, 2013 2:17 pm
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bluesky636 wrote:
The fact that some of the caps "are found in Marshall" amps is irrelevant.


All right!
Ok, thanks for your help anyways!

I think i'll bring her to a tech instead to get it over with.
But i do appreciate all your help, Bluesky!

Cheers,


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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Sun May 05, 2013 2:41 pm
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You're welcome. Good luck.

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Post subject: Re: Fender 75 master volume buzz and No gain when volume on
Posted: Sun May 05, 2013 3:38 pm
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Feelyfeels,

Looks like there maybe some (at least one for sure) pair of caps wired in parallel. This combination effectively doubles the capacitance. Whomever replaced the original caps prolly didn't have the correct value caps for the job.

Plus, the wiring under the dognouse is not OEM. Maybe indicates a short, at one point, which may have lead to overheated components.

The resistors in the power rail seem to be the wrong wattage. They look like 1/2-watt resistors. You should have 1-watt or 2-watt resistors.

That resistor by the two caps in parallel is definetly a non-stock item. What maybe bad is the leads seem to stick out a bit. And may contact the cover. This would not be good.

Maybe best to gut the present setup and replace everything with correct valued components.

Goo luck with your amp!


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