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Post subject: HR Deville Hum
Posted: Mon Feb 18, 2013 8:32 pm
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I own a 2007 Fender Hot Rod DeVille 212. I recently noticed a relatively low hum in standby as well as full power. It is a low frequency, probably between 60 and 180hz.

I am a long time guitar player as well as electronic repair tech and am well versed in troubleshooting electronic problems.

Things that this isn't: Ground loop. Bad Caps. Mechanical Resonance.

What I have done:To try to isolate the hum and since the problem exists in both; standby where the heaters are energized but no b+ is supplied to the anodes of the tubes, and full power. I reasoned that it might be a HK short in one of any of the tubes. So I just removed them from the amp. This had no effect on the problem, the hum remains.

Next, I pulled the chassis and made sure that solder connections on all of the mounts for the pots was solid, this is where most of the ground paths are located. All good here.

While I had the thing torn down I went ahead and replaced the gain controls in the clean and boost channels. They were a tige touchy. (Highly recommended!).

Next, I pulled tested and subbed all power supply capacitors, including those in the bias supply. No improvement.

Next, I reasoned that if this was an electronic problem and not chassis resonance which has been posed, I should not be able to clear the issue by disconnecting the speakers from the amp. Resonance is a mechanical issue. But when I pulled the speaker plug, the hum quit. Reconnecting the speakers re-engaged the hum. SO! The problem is not mechanical resonance.

I also noticed that when I touched the center conductor of the speaker plug to ground, the hum cleared. This seems to indicate that the hum is electro-mechanically created or induced.

I pulled out the big guns. An Oscilloscope. And here is the part that confounded me. The scope showed nothing in the audible range. While measuring everywhere from the power supply all the way directly to the speaker terminals I saw no low frequency signals which might drive the speakers.

So what next?

I swapped the speaker terminals. This puts the speakers into what is called "Reverse". What this means is; that when the speaker should be pushing out, it is mechanically pulling. What is the effect of this? Basically, it cuts the low frequencies.

No surprise. The hum quit. Again confirmation that this is not a mechanical problem.

So what does this leave?

Not much. The problem is likely the location of the output transformer. And I am considering pulling the amp from the cab and following the instructions that I found in an old audiophile news groups. Apparently, there is a way to custom place the output transformer on a chassis, using headphones, where it is effected as little as possible by other emf signals.

Comments?


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Post subject: Re: HR Deville Hum
Posted: Mon Feb 18, 2013 10:17 pm
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giluxis wrote:
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Seems like a lot of effort chasing a "relatively low hum" that is probably perfectly normal.

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Post subject: Re: HR Deville Hum
Posted: Mon Feb 18, 2013 10:44 pm
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bluesky636 wrote:
giluxis wrote:
Comments?


Seems like a lot of effort chasing a "relatively low hum" that is probably perfectly normal.


Sorry, but I don't consider this to be 'Perfectly normal'. Neither do I consider it to be "straining at gnats." It is an irritant and is audible on an open mic.


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Post subject: Re: HR Deville Hum
Posted: Tue Feb 19, 2013 1:12 am
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giluxis wrote:
bluesky636 wrote:
giluxis wrote:
Comments?


Seems like a lot of effort chasing a "relatively low hum" that is probably perfectly normal.


Sorry, but I don't consider this to be 'Perfectly normal'. Neither do I consider it to be "straining at gnats." It is an irritant and is audible on an open mic.


Your words: "relatively low hum". Relative to what? All guitar amps make some noise. Provide a recording so that we can hear what it sounds like.

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Post subject: Re: HR Deville Hum
Posted: Tue Feb 19, 2013 6:40 am
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Your words: "relatively low hum". Relative to what? All guitar amps make some noise. Provide a recording so that we can hear what it sounds like.[/quote]


I appreciate your need to try to get me to justify my complaint. But, unless you have something relevant to add I would prefer not to get into a semantic debate.

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Post subject: Re: HR Deville Hum
Posted: Tue Feb 19, 2013 10:28 am
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Location: Natural Bridge, Virginia
giluxis wrote:
I appreciate your need to try to get me to justify my complaint. But, unless you have something relevant to add I would prefer not to get into a semantic debate.

Cheers


Not debating sematics at all. You asked for help diagnosing an unknown "relatively low hum". Whether you care to admit it or not, most amps make some sort of hum or hiss in normal operation. In order to determine if the noise is normal or not, a recording of the noise is a reasonable request. The type of noise generated may easily lead someone who has heard such a noise before to readily identify its source. If you are unwilling to provide the requested recording, that is your choice. Take note however, that I have been the only person to respond to your request for help. Don't be surprised if your refusal to respond to a request for further information results in a lack of interest on the part of others here to provide help. Good luck resolving your problem.

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