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Post subject: Mustang V Head (V1) Ground Hum Problem
Posted: Tue Mar 31, 2015 2:01 pm
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Hey fellow Fender gurus! this issue is plaguing me with my Fender Mustang V head. I've tried a bunch of different cables, and 10 different guitars but the problem still purists. Before you blame my house grounding-- know that I have 3 other amps (all tube) which are silent and issue free. Know that I have also tested the grounding , and it's all testing out A-OK. My guitars are all shielded, and again no issues with the other amps.

Basically, I hear a 60hz? hum even on the clean settings . If I touch anything metal, the guitar / or the ground of my guitar cable jack, it immediately disappears. I know what you're thinking -- a cable, or the guitar. Nope. Tried a zillion different cables, as well as guitars with singles and with hum buckers and they all do the same darn thing. The sound even comes through the headphone jack, so I know it's not the "fizz" problem others have referred to over the years.

My other amps are a Fender Super Sonic 60, a Bogner Alchemist, and a Marshall Plexi. No issues at all with my other amps, same guitars and cables used. I bought the Mustang as just a practice rig because I can plug headphones into it ; and I have to admit they sound great.

Opened up the amp, checked the jack and the solder points... all looks good. Tried grounding my jack from the earth ground to no avail... the only thing that stops the hum, is me touching the strings, bridge, or cable ends. Any advice would be helpful. Thanks guys. :D


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Post subject: Re: Mustang V Head (V1) Ground Hum Problem
Posted: Tue Mar 31, 2015 4:04 pm
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Yea, getting rid of hum can be a B***h.
So we'll start with some basic questions
Did it always hum since you bought it or did it just start?
Does it hum with nothing plugged into the input?
Have you tried it in different locations away from your home?
Does the volume control change the level of the hum?
Is it a pure hum or a buzz of higher frequencies?
Do different presets change the level or character of the hum?
Have you tightened the nut on the input and speaker jacks?
This may help narrow it down for us.


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Post subject: Re: Mustang V Head (V1) Ground Hum Problem
Posted: Tue Mar 31, 2015 4:25 pm
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Thanks Tim,

#1. Yeah, it's always hummed I guess.... perhaps I never noticed? because I rarely put the guitar down when i'm playing and I just picked this unit up for practice.
#2. Doesn't hum with nothing plugged in.
#3. Tried it in 3 different locations in the house, same slight hum on every setting and hum goes away with touching anything on the amp, or guitar connected to amp.
#4. The gain seems to effect the ground hum more...
#5. It's more of a buzz sound... but like I said goes away instantly when touching the amp chassis, etc. which makes me think it's a ground hum.
#6. Yes presets either make it louder, or softer. I can hear it through headphones on all of my settings
#7. I removed, cleaned and tightened all of the input / output jacks. They had a black residue on all of them, which I removed with de-ox-it.
#8. Inspected the underside of the board with DSP / POTs... all pots test and function normally.
#9. When it was open on the bench, I could just touch the inside of the chassis ; and the hum would go away, instantly.


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Post subject: Re: Mustang V Head (V1) Ground Hum Problem
Posted: Wed Apr 01, 2015 6:38 am
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capthook wrote:
Opened up the amp, checked the jack and the solder points...

...voided the warranty. :?

Definitely sounds like a ground issue (not implying it's your house wiring). Unfortunately I do not know how to fix this. I would have taken it back under warranty.

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Post subject: Re: Mustang V Head (V1) Ground Hum Problem
Posted: Wed Apr 01, 2015 9:34 am
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If you can't take it back under warranty for a replacement, your options for repair are limited. These amps are not built to be very servicable.
Well, it sounds like it's not a power supply issue.
The buzz you describe cleans up when you touch it because the capacitance of your body absorbs the interference signal.
Since it has always hummed, its tough to say if its normal or not. Comparing to another Mustang may settle that question. If it is normal, it may be hard to reduce it.
Since there is no technical info available for these amps, we’ll have to try a few things to narrow down the cause.
Two things can cause hum, internally or externally introduced noise.
The amp may mask internally generated noise with a muting system. Some Fender amps mute when there is nothing plugged in.
To test for a muting circuit, get a ¼” plug with no wire or very short wires on it and short out the tip and ground.
Insert this shorted plug into the input. If there is a muting circuit, the background noise and hum should appear. If your hum appears when unmuted, the amp itself is generating the hum. The switching power supply is unshielded and may be radiating the noise.
It will be radiating that noise into the sensitive input circuits.
Shielding the power supply or the input are possible fixes.
You can fashion a shield from the metal wrapper from a 9 volt battery. Pry back the folded tabs on the top and bottom of the battery. Then, at the crease down the length of it, insert a small screwdriver and unfold the crease. Slide the cells out and flatten out the sheet.
This thin metal makes good shielding material. It can be cut with scissors and is solderable. Solder a wire to it and ground the wire to the chassis. Then cover the metal shield with tape to insulate it.
Open up the amp and insert the shorting plug to unmute it. Move the shield around the amp circuitry and see if it reduces the hum. First, cover the input jack and its circuitry then go over to the power supply and cover the transformer and rectifier to see if you can reduce the noise there.
You will probably wind up building a shield over the input jack and circuits.

IF there is no muting circuit and the noise is externally generated, the shorting plug will probably reduce the hum as there is no antenna effect from a long guitar cable.
First thing is to go to a different location, more than a mile from your home. This eliminates the possibility of local interference. If the hum is gone, go back home and try to track it down. Your guitar makes a good antenna . An AM portable radio can also be useful.
If the noise can’t be tracked down, you can try a custom noise canceling input cable. They don’t make em, you have to build one. Take a two conductor shielded mike cable and replace the XLRs with ¼” plugs. Connect the tips with one of the center wires. Connect the grounds to the shield. Connect the second center wire to ground on one end and nothing on the other end. This second wire acts as an antenna and injects the interference in an opposite polarity to ground. It rejects hum, noise and radio stations.
The only thing is that you need to connect the grounded second conductor to the amp
So mark the ungrounded end as guitar and the grounded end as amp.


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