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Post subject: Eliminating hum from my Bronco 40
Posted: Tue Mar 05, 2013 10:07 am
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My new Bronco40 had an irritating hum when I first got it. To cut a long story short, I eventually fixed this problem by screening off the power supply using thin aluminum sheet. However it took a lot of trial and error to reach this simple solution (the obvious one with hindsight, really). My rather rambling story below may be useful to readers with a similar problem, and amusing to everyone else.

The amplifier's hum was acceptable in a noisy gig environment, but it was annoying in a quiet room at home. I had bought it online from a dealer several hundred miles away and sending it back wasn't a realistic option, so initially I just put up with it. After a while I decided to confirm that my house electricity supply was not at fault. For this I used an excellent little plug-in circuit tester (Martindale EZ150 E-ZE check Xtra) which told me everything was okay. (In fact it found a couple of minor faults with some other sockets in the house, which were easily fixed, so it wasn't a wasted purchase).

I then checked the voltages across all pins on the mains socket using a multimeter. There was a very small AC voltage (0.02v) between the neutral and earth pins, but a quick internet search told me that this was quite normal. I examined this voltage more closely using a freeware PC oscilloscope program (from http://www.zeitnitz.de/Christian/scope_de/). To do this I connected the neutral and earth to the MIC input of the PC soundcard (n.b. only suitable for low voltages < 1v). This displayed a square/saw wave, which when viewed with the spectrum analyser showed the main frequency as 50Hz with harmonics tailing off up through 100, 150, 200, etc. Maybe this was the cause of the hum problem.

I took my Bronco round to my neighbour's a few streets away, but it had the same hum there. Furthermore I have an old Yamaha B115 bass combo, and it produces no hum. So the problem definitely was within the Bronco, regardless of whether the mains is dirty or clean. I tried "lifting the ground" (disconnecting the earth wire in the mains plug) and this got rid of the hum. I briefly considered this as a solution, connecting via an RCD trip switch (a GFCI in U.S. parlance) for safety, but wasn't comfortable with this idea for safety reasons.

By now my warranty had nearly expired, so it was time to open the amplifier up and investigate inside. Here was a very neat layout, but the signal leads were not screened, which surprised me. But, no doubt the Fender designers and engineers knew what they were doing (or did they?). A lot of what I had read on the internet suggested that smoothing capacitors in the power supply are a likely failure point, so I decided to replace these, even thought they didn't bulge or appear faulty. This made no difference so I was no further on. (As an aside, in the process of doing this, I managed to blow the power supply fuse, which was a little PCB-mounted unit, which to me seems like a pointless idea. I replaced this with a conventional fuseholder which will make fuse replacement easier in future if needed. Maybe one day I will even drill a hole in the back panel and mount the fuseholder so it can be accessed from the outside.)

Finally I came across someone online who had solved a problem like mine by shielding the input section of his amplifier. Inspired by this I folded up some aluminium foil and carefully inserted it between the power supply and the other parts of the Bronco circuitry. Like magic the hum vanished! I then fashioned a more robust sheet of aluminium from an old beer can, folded it into an L-shape, with mounting tabs aligned to the screw holes on the power supply PCB. I covered this with duct tape to make sure it was insulated, and fitted it, and now at long last have a perfectly silent Bronco40, so I can now concentrate on the music.

The attached photograph shows an inner view of the amplifier with the power supply in the foreground surrounded by the L-shaped aluminium screen. Behind this are the pre-amp and effects PCBs, and to the left is the power amp stage with big heatsink. The big blue capacitors are the new ones I fitted, and you can also see the new black fuseholder.
Image

I hope someone finds this tale useful and/or interesting!
Paul.


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Post subject: Re: Eliminating hum from my Bronco 40
Posted: Thu Mar 07, 2013 8:27 am
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Joined: Fri Mar 28, 2008 3:19 pm
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Location: Vancouver, BC Canada
Interesting stuff ...thanks for sharing.. :)


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Post subject: Re: Eliminating hum from my Bronco 40
Posted: Mon Apr 01, 2013 9:26 pm
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Amazing post. What brand beer can? Light or regular?

Glad you sorted it. You'd think they would have figured out this issue in R&D. I don't know about replacing perfectly good caps though. How did you secure the aluminum shield? Is it grounded?

I wouldn't be happy with the ground lift as a long term solution either. Not wise.


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Post subject: Re: Eliminating hum from my Bronco 40
Posted: Tue Apr 02, 2013 3:14 am
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Thanks, Brotherdave.

The aluminium shield is secured by three little tabs at each corner, held down by the PCB mounting screws, which also provide the grounding. You can just about see two of these in the picture (front left and rear right). I doubled up the very thin metal and covered it with duct tape, which made it rigid enough when bent into an L-shape. (I think I used a Marston's Pedigree real ale can!)

You are right about replacing apparently good capacitors. It was a long shot which didn't make any difference, but didn't do any harm either, fortunately.

I like your website, by the way.

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