buzzing/tone loss on volume pot

[psilocyborg]

Hey all. New to these forums. Perhaps you guys could help me with a small problem?

I've got an Am. Deluxe strat (year '99, I think) on which the volume knob doesn't work quite right. It sounds fine when it's turned up all the way, but it seems to work as a sort of tone control when I roll back the volume. What's more, as I turn it down, a buzz/hum gets louder. This noise is particularly perplexing considering how the pickguard is pretty well shielded, especially when compared to a standard strat. Also, the pups are Fender's Noiseless series, so it's not your run of the mill single-coil hum. I've had this problem for as long as I've had my guitar, but it's never been much of a bother since I usually keep volume cranked up anyways. However, I'm about to drop some serious cash on an Analogman Sun Face (fuzz face clone, with nos NKT-275 germanium transistors), which I've read are very sensitive to volume/picking dynamics, so naturally I'd like to solve my problem in order to get the most out of my purchase.

Here are some diagrams from Fender's support site, which may be useful:
page 2
page 3

Fender must have slighty redesigned the electronics in more recent models, because my set up is a little different from the diagram. First, #19, a 680pf capacitor, is mysteriously absent from my guitar. Second, #20, a 220k resistor, is soldered a little differently from mine. Instead of jumping from the first contact to the second, mine goes from the first contact directly onto the pot itself. Also, I think my resistor is 1M (black-brown-green-gold), instead of 220k. I wonder if this has anything to do with the humming I've been experiencing?

Anyways, any help would be greatly appreciated.

[sumgai]

'borg,

First, to the forums!

Your hum problem is not easy to diagnose, but the first thing I'd do is re-wire it to what the diagram says should be there. That 1Meg resistor (you were correct on your color codes) is just sitting in paralllel with the pot, and doing you no real good. In essence, it is forcing the pot to a lower value, at a skewed rate (the taper, explained elsewhere on these forums). This is not what Fender intended, and I seriously doubt that it is a factory installation.

The 220K and 680pf parts were intended to reduce the effect of the volume control on the bright parts of the tone, whenever you turned the volume down. (BTW, the diagram looks like it has a period in front of the 220K designator, but that's obviously a misprint of some sort - the parts list has it correct.)

Restoring the two parts, and eliminating the 1Meg part, should go a long ways toward fixing your hum problem. But just in case, while the parts are disassembled from the pot, use a meter to make sure the pot is acting like it should. If it has any sudden jerks of the needle/readout, then it may also be on the fritz. Before you dive in, you should have a "spare" pot on hand, just in case.

HTH, and Good Luck!


sumgai

[psilocyborg]

Hey, thanks for the help! I've been doing some reading on kinman.com recently. Here's what Chris has to say about 1Meg volume pots:
"Several pickup makers recommend using higher values such as 500K or even 1Meg to reduce the choked, strangled, compressed and constricted feel in an attempt to bring their pickups to life. They try to increase the Air, presence, bite, output and dynamic range because their pickups lack these crucial elements of Fender tone. As many players have discovered, it's impossible to resurrect a pickup that was born dead in this manner as the extra brightness is artificial and the sound is shrill and empty with no real air and no real dynamic range .... Some problems with high value pots is that when you turn the volume control down a lot of hum/noise is introduced. This is because the amplifier input is becoming progressively unloaded and is effectively floating (like when the cable is not connected to your guitar) which results in hum/noise. 250K pots also exhibit this characteristic but not nearly to the same extent as 500K or 1Meg pots do. The higher the value the worse the problem becomes, it's part and parcel of high impedance circuitry."

So yeah, I guess Fender uses the 1Meg pots to brighten their Noiseless pups, despite the drawbacks. Anyways, I'm thinking I'm gonna hold off on the fuzz face for now and throw down the scratch for a set of Kinmans, which, though noiseless, are tweaked to work with 250k pots. Yeah, they're expensive, but I'm pretty sold on em. Besides, it's my birthday today! What better opportunity to splurge?