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Annoying buzz when patch cord is inserted. [message #12665]

Thu, 14 January 2010 19:09

ironman
Messages: 12
Registered: January 2010
Location: Chicago

Junior Member

HI,
I'm new to the forum, although I've scanned it for years. I'm repairing a vintage K100-2. My first mod straight off was installing a new three prong grounded plug. Other than that the caps are not leaking but look original (I've found replacements but will wait until they are completely necessary). The amp idles rather quiet with no patch cord installed, a hiss is audible only when the amp is cranked. As soon as I plug in my bass or guitar it begins to make a annoying hum like it's not grounded. I can control the hum with the treble knob and almost make it go away if I roll it off all the way. I've switched multiple instruments to make sure it wasn't the pickups. Any suggestions of where I should start? Could this just be something with the input jacks? Any help you all can provide would be very appreciated.
Dave

Oh I also clipped the .022mfd 600v cap on the back of the amp. No noticeable difference. May even be a tad worse? Amp now squeals (feedback) when I crank the treble when in the High input.

[Updated on: Thu, 14 January 2010 20:33]

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Re: Annoying buzz when patch cord is inserted. [message #12667 is a reply to message #12665]

Fri, 15 January 2010 07:37

stevem
Messages: 4746
Registered: June 2004
Location: NY

Senior Member

The first thing to check/fix is the feedback problem.
If this still happens with a guitar plugged in but with its volume(s) control turned down than that channel you are plugged into has two possible problems, and I will start with the easy one first and this fix may even take care of the hum/buzz problem!

Each of the 4 input jacks have a built in switch that shorts them to ground when no in use. In this way if a channel is left with its controls turned up but no in use, that channel will produce no hiss/ hum or buzz to the output signal.

If you look close at any input jack you will see that inside the long tag that brings the tip connection of your guitar cord to the base of the jack these is another shorter flat section of metal. This is connected to ground so when no cord is pluged in the tip and sleve of the jack are shorted, which shorts out the first input stage of that channel and hence no noise!
To clean these sectionsI use some contack cleaner/lube sprayed onto a thin but strong section of blank cardboard( like a greeting card or match book)I than bend that tip connection back justenough with my finger to open that switch and load the card board into it and than pull the card board back and forth to clean off the two points of contack in that switch.

If doing this does not stop the feedback than more than likly one of the small Tantalum type caps on that channel have gone bad and will need to be tracked down and replaced.

The unforunate result of making a amp safe A/C shock wise by adding a three prong cord will be a new buzz noise at times.
If with the feedback problem fixed and your guitar plugged in, but with its volume(s) down you do not get the buzz, this is why.
The best fix for this, though costing some 60 bucks is to buy a isolation transformer to power the amp off of.
Let us know how you make out.

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Re: Annoying buzz when patch cord is inserted. [message #12668 is a reply to message #12667]

Fri, 15 January 2010 09:05

ironman
Messages: 12
Registered: January 2010
Location: Chicago

Junior Member

Thanks this is a good direction for me to go. I'm pretty sure the input jack is ok, but I'll clean it again with the note card trick.
On second inspection it's clear that the High input jack had not returned to it's grounded position. I've been able to bend it back to resting position, but I'm going to switch the jacks out so this doesn't happen again.
This has stopped the feedback but not the noise once a guitar or bass is plugged in. I'm going to get some cap values and switch some of them out. I had wanted to avoid taking out the pc board. In for a penny in for a pound I guess. If I'm getting noise that is adjustable by the Treble knob, does it make sense to start replacing them in that area? Or could this be a transistor issue? I was picking up some good am radio stations the other night. Shocked

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Re: Annoying buzz when patch cord is inserted. [message #12669 is a reply to message #12665]

Fri, 15 January 2010 11:51

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Welcome to the board. Glad to see another Chicagoan here.

First off try this, plug in your bass and turn down the volume on the bass to zero. Does the amp still buzz?

If it does, then you will need to look into cleaning up all of the grounding in the amp. Look for loose input jacks, pc mounting screws, etc.

If the buzz goes away, then the noise is being generated by the bass and is just being amplified by the amp.

I would not recommend tearing into the amp until you have localized the problem, as you may do more harm than good by randomly replacing components.

Let us know what you find out.

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Re: Annoying buzz when patch cord is inserted. [message #12670 is a reply to message #12665]

Fri, 15 January 2010 12:16

stevem
Messages: 4746
Registered: June 2004
Location: NY

Senior Member

Good to hear that the feedback was just that jack being open, also the radio station being picked up was due to this also, the reason? The preamp design of these amps places two geain stages before the volume control, so if a jack is switched open with nothing plugged in the high band width of these amps will pick up radio frequencys.
In regards to the buzz trouble you have I try this first before diving into trouble shooting the circuit board. I would pick up a ground lift adapter from a hardware store and see if that kills it so you do not have to unsolder or unbolt the ground wire from the new a/c cord.
If your am radio problem crops up again, email me as their is a fix for that problemt hat Kustom issued in late 68 that will cost all of 6 bucks in new parts.

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Re: Annoying buzz when patch cord is inserted. [message #12671 is a reply to message #12670]

Fri, 15 January 2010 17:25

ironman
Messages: 12
Registered: January 2010
Location: Chicago

Junior Member

Thanks
Yes agreed I always like to take a less is more solution. First let me say that I have a single channel amp with Trem and Reverb. I just plugged in my SG guitar and my P Bass and I'm getting some minor buzz while the pickups are down. Also a very noticeable grounding out when I touch my strings. With pickup volume turned up there is a lot of noise mostly controlled by the amps treble knob. Radio station is only audible when the amp is past 75% volume(could be worth addressing later).

Problem-In either input at about 50% volume when I crank the treble I get loud squealing feedback. Obviously not normal. I'm going to focus on this issue for now. Any thoughts?

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